systems to cells Flashcards
1
Q
what is ‘systems to cells’?
A
study of key molecular and cellular mechanisms that operate across various tissues.
-maintenance of physiological homeostasis
-disruption can cause disease
roadmap to investigate, understand and ultimately treat complex diseases
2
Q
what is the first law of thermodynamics?
A
energy can be transformed from one form to another but cannot be created or destroyed
3
Q
what is energy needed for?
A
cell growth and division
building new molecules/replacing old ones
movement (muscle contraction is ATP-dependent)
breathing, thinking, speaking etc
4
Q
what is the energy currency in biology and how is it formed?
A
ATP - adenosine (adenine + ribose) triphosphate
formed by substrate-level oxidative phosphorylation
*the (chemical) energy is stored in the last phosphate group.
high energy bond between last phosphate (gamma phosphate) and other phosphates.
hydrolysis of this bond releases energy (-7.3kcal/mol or -30.5 KJ/mol)
5
Q
where in ATP is the chemical energy stored and how much energy is released when the bond is broken?
A
the (chemical) energy is stored in the last phosphate group.
high energy bond between last phosphate (gamma phosphate) and other phosphates.
hydrolysis of this bond releases energy (-7.3kcal/mol or -30.5 KJ/mol)
6
Q
how much ATP is in the average human body?
A
100-250g ATP, daily requirement is 50-75kg
7
Q
how often is ATP re-formed from ADP each day?
A
~1000x
8
Q
where does ATP come from ?
A
oxidative phosphorylation of glucose.
ATP is also formed from the process of cellular respiration in the mitochondria of a cell.
9
Q
what is ΔG?
A
change in Gibb’s free energy.
measure of how spontaneous a process like a chemical reaction is.
-ve ΔG reactions release energy (require no energy input & are spontaneous)
+ve ΔG reactions require energy input (non-spontaneous)
10
Q
discuss glucose in terms of a fuel source and the TCA (tricarboxylic acid cycle).
A
glucose is an excellent fuel.
complete oxidation ΔG= -2840 KJ/mol
broken down to pyruvate by glycolysis.
aerobic (with O2) conditions the pyruvate is converted to acetyl-CoA and this enters the TCA/krebs cycle
anaerobic (no O2) conditions, its converted to lactate.
can be efficiently stored (starch; glycogen)
glucose is a key energy source; brain and nerves have an absolute requirement for glucose for energy (so do erythrocyte, testes, and kidney medulla)
11
Q
what is glycolysis?
A
Glycolysis ultimately splits glucose into two pyruvate molecules. One can think of glycolysis as having two phases that occur in the cytosol of cells. The first phase is the “investment” phase due to its usage of two ATP molecules, and the second is the “payoff” phase. Oxygen isn’t required for glycolysis.
12
Q
how is whole body glucose homeostasis controlled?
A
blood sugar levels kept constant by a range of homeostatic mechanisms.
when in excess, glucose stored as glycogen (liver/muscle) or triglycerides (adipose).
when levels low, these tissues become net exporters of glycose/fatty acids
hyperglycaemia-high blood glucose
hypoglycaemia- low blood glucose
13
Q
hyperglycaemia vs hypoglycaemia
A
hyper- high blood glucose
hypo- low blood glucose
14
Q
what are the multiple levels in which metabolic pathways involved in glucose metabolism are organised?
A
system
e.g. human, migrating bird, hibernating brown bear
tissue/organ
e.g. brain, liver, git
cellular
e.g. liver and muscle respond differently to high/low glucose
subcellular
e.g. mitochondria, lipid droplet, cytosol
genetic
cells/tissues can change patterns of gene expression in response to nutritional status
15
Q
what are monosaccharides and disaccharides?
A
Glucose, galactose, and fructose are common monosaccharides, whereas common disaccharides include lactose, maltose, and sucrose. Starch and glycogen, examples of polysaccharides, are the storage forms of glucose in plants and animals, respectively. The long polysaccharide chains may be branched or unbranched
16
Q
how is blood glucose controlled?
A
insulin- released from pancreatic β-ells when blood glucose increases
glucagon- released from pancreatic α-cells when blood glucose levels fall
**insulin and glucagon are hormones
insulin; increased activity of glycogen synthase, reduced activity of glycogen phosphorylase= net store of glycogen
glucagon; decreased activity synthase, increased activity of glycogen phosphorylase
= net breakdown of glycogen.
17
Q
what is gluconeogenesis?
A
a metabolic pathway that results in the generation of glucose from the non-carbohydrate carbon substrates such as lactate/amino acids.
18
Q
what does hormone insulin do?
A
increases glucose uptake into fat and muscle;
increases glycogen synthesis in the liver;
inhibits gluconeogenesis in liver;
insulin signals the fed state and the removal of glucose from the blood
insulin turns enzymes glycogen synthase on and glycogen phosphorylase off. (glycogen synthase converts glucose-1-phospahte to glycogen; glycogen phosphorylase does the opposite)
^both reactions are energetically favourable (spontaneous)…reciprocal regulation of enzymes needed; allow the system to quickly react to changes in the blood sugar levels
*gluconeogenesis = a metabolic pathway that results in the generation of glucose from the non-carbohydrate carbon substrates such as lactate/amino acids.
19
Q
what does hormone glucagon do?
A
stimulates gluconeogenesis;
inhibits glycogen synthesis in the liver;
triggers lipid breakdown;
glucagon signals the release of glucose into the blood
*gluconeogenesis = a metabolic pathway that results in the generation of glucose from the non-carbohydrate carbon substrates such as lactate/amino acids.
20
Q
how is glucose stored?
A
sugar stored as glucose subunits in the polymer glycogen (mainly in liver and muscle cells)
synthesis and degradation of glycogen is rapidly regulated by need.
Excess glucose is stored in the body as glycogen, a glucose polymer, utilized during fasting. In addition, glucose can be produced through gluconeogenesis, a process involving the breakdown of fats and proteins
21
Q
what is the fed state?
A
The fed state occurs within the first few hours after eating as your body digests and absorbs nutrients from food. During this period, your blood sugar levels increase, and higher amounts of insulin are secreted. Insulin is the hormone responsible for transporting sugar from your bloodstream into your cells
22
Q
describe the metabolic pathway of glucose conversion to glycogen.
A
Glucose converted to glucode-6-phosphate by hexokinase.
Glucose-6-phosphate converted to glucose-1-phosphate by phosphoglucomutase in reversible reaction.
Glucose-1-phosphate converted to glycogen by glycogen synthase in reaction which is irreversible.
*controlling the activity of these key enzymes allows careful integration of metabolism
23
Q
Describe the metabolic pathway of glycogen conversion to glucose.
A
Glycogen converted back to glucose-1-phosphate by glycogen phosphorylase in reaction which is irreversible.
Glucose-1-phosphate converted back to glucose-6-phospahte by phosphoglucomutase.
Glucose-6-phospahte converted back to glucose by glucose-6-phosphatase.
*controlling the activity of these key enzymes allows careful integration of metabolism
24
Q
what is reciprocal regulation of enzymes?
A
Reciprocal regulation using allostery- Binding of the same regulator to the enzymes of opposing reactions has the opposite effect (activation for one enzyme, inhibition for the other)
25
what are the types of mechanisms in which mammalian enzymes are regulated?
changing rate of biosynthesis/degradation LEVELS.
changing ACTIVITY.
changing LOCATION.
26
what is the most common way in which mammalian enzymes are regulated?
changing ACTIVITY.
a common way to regulate enzyme activity in response to a signal e.g. in response to a hormone, is to use something called reversible covalent modification (the most common is PHOSPHORYLATION)
27
what is phosphorylation?
Phosphorylation is the addition of a phosphoryl (PO3) group to a molecule. In biological systems, this reaction is vital for the cellular storage and transfer of free energy using energy carrier molecules.
phosphate added to protein by a kinase, transferred from ATP; the removal of phosphate is catalysed by phosphatases. this process can turn enzymes 'on' and 'off'.
>alters the 3D conformation of the target protein because of the high charge density of the protein-bound phosphoryl group, -2 at physiological pH. these often make salt bridges with nearby arginine or lysine residues (positively charged)
phosphorylation involved the covalent addition of a phosphate, transferred from ATO by the action of a class of enzymes called KINASES.
this is reversible, and the removal of the phosphate is catalysed by a group of enzymes called PHOSPHATASES.
28
how can changes to the enzyme induced by phosphorylation be reversed?
by virtue of the kinase/phosphatase system
29
what are the two main classes of kinase?
those that phosphorylate TYROSINE residues and those that phosphorylate SERINE/THREONINE residues
30
what word describes the way in which insulin and glucagon regulate carbohydrate metabolism?
reciprocal.
insulin and glucagon regulate carbohydrate metabolism reciprocally.
when one process is highly active, the other one is inhibited.
*do so by coordinating the flux through metabolic pathways
31
glycogen synthesis vs degradation
The formation of glycogen from glucose is known as glycogenesis, and the breakdown of glycogen to form glucose is called glycogen metabolism or glycogenolysis. Increased cyclic adenosine monophosphate (cAMP) catalyses the breakdown of glycogen (glycogenolysis).
Glucagon promotes degradation. Turns on protein kinase A (PKA). PKA phosphorylates glycogen synthase (turning it off) and phosphorylates glycogen phosphorylase (turning it on).
>example of hormone causing phosphorylation of both enzymes; but it turns one 'off' and one 'on'
Insulin promotes glycogen synthesis. It turns on glycogen synthase and turns off glycogen phosphorylase. It does this by switching on protein phosphatase-1, which dephosphorylates both proteins.
32
mechanisms of insulin vs glucagon
insulin
>increased activity of glycogen synthase, reduced activity of glycogen phosphorylase
= net store of glycogen
>gluconeogenesis in the liver is suppressed
> turns ON glycolysis; turns OFF gluconeogenesis
> turns ON glycolytic enzyme gene expression, turns OFF gluconeogenic enzyme gene expression
> blood glucose levels fall
glucagon
>decreased activity of glycogen synthase, increased activity of glycogen phosphorylase
=net breakdown of glycogen
>gluconeogenesis in liver is increased
>turns OFF glycolysis; turns ON gluconeogenesis
>turns OFF glycolytic enzyme gene expression, turns ON gluconeogenic enzyme gene expression
> blood glucose levels rise
33
what are biosynthetic and degradative pathways?
almost always distinct; this means that both pathways can be thermodynamically favourable
the rates of metabolic pathways are governed by the activities of key enzymes (not by mass action)
glycogen formation/breakdown is a perfect example of this.
34
what is an example of allosteric modulation of enzyme activity?
reversible modification
35
what is allosteric modulation of enzyme activity
Allosteric modulation is the mechanism by which the binding of an allosteric modulator slows or enhances the binding of other substrates to the active site of the allosteric enzyme. Overall, allosteric modulation affects the interaction between a ligand binding to the active site of the enzyme.
36
what effect do hormones have on cells/tissues?
specific hormones induce specific event in cells/tissues
37
what happens in cases where the direction of a metabolic pathway has to be reversed?
the pathway is controlled by an irreversible step
38
what do enzymes do to the energy landscape of a reaction?
The energy required to reach the transition state (the activation energy) constitutes a barrier to the progress of the reaction, limiting the rate of the reaction. Enzymes (and other catalysts) act by reducing the activation energy, thereby increasing the rate of reaction.
>enzymes lower the activation energy for reactions
39
what is the rate determining step?
the slowest step of a chemical reaction that determines the speed (rate) at which the overall reaction proceeds.
(the rate determining step can be compared to the neck of a funnel)
>this is true in metabolic pathways; often represents a key control point (logical)
A+B+C -> ABC (2 steps)
A+B -> AB (slow)
AB+C -> ABC (fast)
40
what is the activation energy?
Activation energy is the minimum energy required to cause a process (such as a chemical reaction) to occur.
41
what are the rate limiting steps in the glycolytic pathway?
1. phosphorylation of glucose by hexokinase
2. the phosphorylation of fructose-6-phosphate to form fructose-1-6-bisphosphate by fructose-6-phospahte kinase
the generation of fructose-1,6-busphosphate by phosphofructokinase-1 is a key regulatory pathway and is also the rate-limiting step.
> the reaction is coupled to the hydrolysis of ATP and is essentially irreversible
> hence a different pathway must be used to do the reverse conversion during gluconeogenesis
42
what else apart from phosphorylation can control the activity on an enzyme?
>enzymes can be controlled by allosteric interactions with other molecules
>often some of the other molecules and intermediates in the downstream pathway
>molecules which potentiate one direction (glycolysis) are often negative regulators of the other direction (gluconeogenesis)
>allosteric regulation can be used to superimpose other control pathways on top of a metabolic pathway
43
what type of hormones are insulin and glucagon and where are they released from?
polypeptide hormones released from the pancreas.
44
where do insulin and glucagon bind to?
to specific receptors enriched in muscle, liver and fat cells but have opposing actions
45
summarise glucose homeostasis
blood glucose maintained in normal range about 90mg/100ml
high blood sugar; sensed by pancreatic β cells; β cells release insulin; insulin promotes uptake of glucose by cells; promotes uptake into liver cells, where it is converted to glycogen; this lowers blood glycose.
as blood sugar decreases, the signal triggering the release of insulin decreases, so insulin levels also decrease.
46
explain the global epidemic of diabetes
diabetes is a growing and massive silent epidemic that has the potential to cripple health services in all parts of the world.
~415 million sufferers worldwide from diabetes (>6x estimates of 10yrs ago)
this figure is likely to more than double by 2050.
>4 million deaths (9% global total) can be attributed to diabetes each year.
diabetes is a risk factor for other diseases (COVID)
DYSREGULATION OF GLUCOSE HOMEOSTASIS
47
what are symptoms of diabetes?
macular degeneration
kidney failure
stroke
fatty liver disease
atherosclerosis
foot ulcers
inflammation
diet and exercise have a role; fewer people have jobs involving physical labour; genetics is a big component
48
type 1 vs type 2 diabetes
type 1;
β-cell destruction
autoimmune/idiopathic (unknown aetiology)
insulin is not produced
type-2;
defect in insulin action- insulin resistance
β-cell dysfunction
49
what are risk factors of diabetes mellitus?
obesity
sedentary lifestyle
age
diet
GENETICS
50
discuss the influence of genetics on diabetes
half of all south Asian, black African and African Caribbean people in the UK will develop type-2 diabetes by age of 80. for Europeans, the figure is 20%.
South Asian men are typically 5 years younger on diagnosis and have increased risk of all complications compared to other ethnic groups.
51
what is the role of leptin and what do mutations cause?
leptin (hormone) signals satiety
mutations cause hyperphagia and hyperglycaemia
hyperphagia =a feeling of extreme, insatiable hunger.
hyperglycaemia = where the level of sugar in your blood is too high
52
pros and cons of using mouse models in diabetes research
Mice are small and relatively economical to maintain, making them the ideal laboratory animal model. Thousands of laboratory mouse strains are now available, so scientists can therefore choose the ideal mouse model to study different diseases and disease processes.
53
discuss hibernation in terms of glucose homeostasis
hibernation - a state of altered metabolism
>hibernating animals store massive amounts of fat each autumn
>these serve as the main source of metabolic fuel over winter
>fats are energy rich and have the advantage of generating metabolic water as they are catabolised
>this is entrained by the length of daylight
bears have cycles of high calorie intake (20,000/day), obesity, and long periods with no exercise [humans would predispose to T2D]
-bears can turn on/off insulin resistance to maintain blood sugar. Insulin and glucose levels in the blood remain stable all year.
-gene expression changes responsible. 8 key genes identified (Akt), some not previously associated with glucose homeostasis.
54
discuss migratory birds in terms of glucose homeostasis
-some birds fly huge distances during their annual migration, often over water requiring constant flight for up to 60h or more at speeds approaching 40kph.
-this is made possible by accumulation of large fat deposits that then are efficiently and selectively mobilised during the flight.
-humming birds have a high sugar diet and high blood sugars- remain 'healthy'
-some birds accumulate up to 0.15g of triglyceride/day/g body weight (for a typical human this would represent 10kg/day)
-the birds are obese prior to migration; but flight muscles increase in size markedly too
*studying metabolic profile may help us to understand metabolic disease and diabetes in humans
55
where is the pancreas located?
adjacent to small intestine
56
what are acinar cells?
Acinar cells are highly polarized pyramidal epithelial cells with microvilli, and their cytoplasm shows strong eosinophilic staining.
found in the pancreas.
secrete digestive enzymes and discrete islets of cells (islets of Langerhans) made of up alpha and beta cells; they are highly vascularised.
57
what are islets of langerhans?
Islets of Langerhans are islands of endocrine cells scattered throughout the pancreas.
alpha cells secrete glucagon
beta cells secrete insulin
delta cells secrete somatostatin
(A number of new studies have pointed to the potential for conversion of non-β islet cells in to insulin-producing β-cells to replenish β-cell mass as a means to treat diabetes.)
58
pancreatic duct vs bile duct
The pancreatic duct or duct of Wirsung (also, the major pancreatic duct due to the existence of an accessory pancreatic duct) is a duct joining the pancreas to the common bile duct. This supplies it with pancreatic juice from the exocrine pancreas, which aids in digestion.
A tube that carries bile from the liver and gallbladder, through the pancreas, and into the small intestine. The common bile duct starts where the ducts from the liver and gallbladder join and ends at the small intestine.
The bile ducts carry bile from the liver and gall bladder to the small intestine to help with digestion after a meal. The pancreatic ducts carry digestive enzymes and fluids from the pancreas to the small intestine. The biliary and pancreatic ducts usually join together just before emptying into the small intestine.
59
what cells in the pancreas secrete insulin?
beta cells
60
where in the body detect blood glucose levels?
The glucose in the blood is therefore stored in liver and muscle cells in the form of a larger molecule called glycogen. The body is able to detect blood glucose levels via an organ called the pancreas. More specifically, it is detected by areas within the pancreas called islets of Langerhans.
61
what is the complex polypeptide called that mature mRNA encodes for from the insulin gene?
preproinsulin
mRNA translated into protein and protein inserted into lumen of endoplasmic reticulum; this preproinsulin undergoes proteolytic sequential cleavage
*insulin was the first protein sequenced
62
what is proteolytic cleavage
Proteolytic cleavage. Proteolytic cleavage is basically the process of breaking the peptide bonds between amino acids in proteins. This process is carried out by enzymes called peptidases, proteases or proteolytic cleavage enzymes.
63
what links polypeptide strands?
disulphide bonds (covalent link)
64
where are soluble proteins (like insulin) released into on the endoplasmic reticulum?
into the lumen of the ER
65
what the the steps of translation of preproinsulin?
1. Ribosome sees mRNA and starts to read along
2. Sees signal sequence – tells ribosome to pause. Translational pause signal.
3. Pause point is just as signal sequence is just poking out of ribosome exit tunnel. So this is insulin polypeptide in red.
4. Signal sequence is recognised by SRP – signal recognition particle. It grabs the signal sequence really tight.
5. By SRP receptor, ribosome passed to complex of proteins called Sec61 translocon complex
6. Imagine these to be a gateway into the lumen of the ER
7. Want to get insulin into ER lumen
8. And that point, translation carried on
9. Insulin peptide is co-translationally passed through membrane and into cytosol of ER
10. Signal sequence cleaved off by signal peptidase complex
66
what generates the stability of insulin?
the disulphide bonds
67
explain the importance of oxidative folding and disulphide bond stability in relation to insulin
>disulphide bonds play an important role in the stability of some proteins (usually proteins secreted to the extracellular medium).
>since most cellular compartments are reducing environments in general, disulphide bonds are unstable in the cytosol
>this is important because the newly synthesised insulin will only fold and be oxidised in the lumen of the ER.
>the formation of the disulphide bonds is crucial for the folding of the molecule into its biologically active form
> the cell needs to separate insulin from the cytosol for insulin to adopt the proper biologically active conformation
>co-translational insertion of proteins into the ER is a common theme
68
is proinsulin biologically active?
no- additional processing steps needed (removal of the c-peptide)
69
through which transporter does glucose from blood diffuse into beta cells in pancreas?
Glucose from blood transported into the beta cell by facilitated diffusion through a glucose transporter GLUT2.
70
summarise the secretory pathway of macromolecules via vesicular transport
1. Made in ER
2. Trafficked through ER/golgi intermediate compartment (ERGIC)
3. Traffic to golgi and through golgi until they reach the trans-golgi network
4.This is a sorting point.
5. Proteins can go in different places
>Secretory granules
>Consitutively released from cell surface
>Endosomes
>Lysosomes
6. Well conserved pathway – known to take place in every organism (eukaryote)
7. Secretory protein, packaged in vessels, sent to cell surface for secretion
8. This can be constitutive (going on all the time) or – as in the case of insulin – it can be regulated secretion
9. You have the package of insulin, waiting in cell, waiting for the signal to be sent to the membrane to dock and fuse with cell surface
71
how is all communication between membrane bound compartments mediated?
by vesicular transport
(by budding and fusing)
72
what are the steps of preproinsulin to insulin?
preproinsulin; a biologically inactive precursor to insulin that is produced in the beta cells of the pancreatic islets.
PREPROINSULIN -> cleavage of signal sequence and disulphide bond formation -> PROINSULIN -> removal of connecting polypeptide -> INSULIN
preproinsulin:
N terminis- signal sequence- peptide chain B- connecting polypeptide chain A - C terminus
>cleavage of signal sequence and disulphide bond formation
=
proinsulin:
A and B peptide chains parallel and connected by disulphide bonds; still connected by connecting polypeptide
>removal of connecting polypeptide
=
insulin:
A and B chains parallel, connected only by disulphide bonds (R-SS-R)
73
where is preproinsulin translated and inserted?
translated on ribosomes and inserted across the ER membrane.
74
what happens to insulin sequence inside the ER and at golgi apparatus?
processing begins; the pre-sequence is cleaved by signal peptidase.
disulphide bonds form between the A and B chains which stabilises the 3D structure
proinsulin traffics through the Golgi, before being packaged into secretory granules.
proinsulin is cleaved into insulin and C-peptide by enzymes called pro hormone convertase PC1/3 and PC2.
carboxypeptidase E removes basic residues at the C-terminus
high levels of insulin packages into secretory vesicles by making a crystalline complex with Zn2+ ions selectively pumped into the secretory vesicles.
net result: VERY high insulin concentrations in specialised packages.
in response to a suitable signal, secretory granules fuse with the plasma membrane and release their content (insulin and C-peptide)
75
summarise vesicular transport of insulin through different membrane compartments to sectretion.
1. Preproinsulin gene is transcribed to mRNA
2. Preproinsulin translated on ribosomes and is inserted across the ER membrane
3. Inside the ER, processing begins, the pre-sequence is cleaved by signal peptidase. Disulphide bonds form between the A and B chains which stabilises the 3D structure.
4. Proinsulin traffics to Golgi via vesicular transport
5. Proinsulin traffics through the Golgi, before being packaged into secretory granules.
6. Proinsulin is cleaved into insulin and C-peptide by enzymes called pro hormone convertase PC1/3 and PC2. Carboxypeptidase E removes basic residues at the C-terminus.
7. High levels of insulin packaged into secretory vesicles by making a crystalline complex with Zn2+ ions selectively pumped into the secretory vesicles. Net Result: VERY high insulin concentrations in specialised packages.
8. In response to a suitable signal, secretory granules fuse with the plasma membrane and release their content (insulin and C-peptide)
76
why is the process of insulin secretion so complex?
if even a small amount of insulin is released during low blood sugar it is disastrous.
different compartments mediate different functions to correctly process insulin.
membrane trafficking isn't 100% perfect; if mis-trafficking occurs
e.g. from the ER to the cell surface, the released preproinsulin/proinsulin isn't biologically active
can package into specialised 'secretory vesicles' at very high concentrations and regulate them easily
[compartmentalisation leads to specialisation and hence greater efficiency]
**ability to package insulin into specialised secretory vesicles at high concentrations is extremely important.
the basic idea that you can compartmentalise to create highly efficient systems is the key basis on which eukaryotic life is built
>based on membrane compartmentalisation and membrane trafficking
77
what is exocytosis?
Exocytosis (/ˌɛksoʊsaɪˈtoʊsɪs/) is a form of active transport and bulk transport in which a cell transports molecules (e.g., neurotransmitters and proteins) out of the cell (exo- + cytosis). As an active transport mechanism, exocytosis requires the use of energy to transport material.
e.g. release of hormones from vesicles by - regulated exocytosis
regulated exocytosis; a common mechanism in many endocrine, neuroendocrine and entero-endocrine cells.
78
explain insulin release.
> insulin is released in response to elevation in blood sugar- the main physiological trigger
> regulated exocytosis
> in response to this trigger, the insulin vesicles have to dock and fuse with the plasma membrane and release their contents
79
what triggers insulin release ?
1. glucose in the blood enters the beta cell via glucose transport protein called GLUT2
2. when glucose in cell, metabolised TO glucose-6-phosphate, enters glycolysis enters glycolysis. glycolysis increases and cells increase their ATP/ADP ratio. there's a marked increase in ATP content in these cells
3. the beta cell, like every cell in our body, has an ATP sensitive K channel in the cell surface
4. when it binds ATP, it closes, so K can no longer cross the membrane
5. this has the effect of depolarising the membrane -> the beta cell membrane is polarised
6. the depolarisation opens a voltage sensitive Ca channel
7. the [Ca] is usually 2 maybe 3 orders of magnitude greater extracellular than intracellular. as soon as you open the channel, you get movement of Ca from outside to inside the cell
8. Ca ions trigger the fusion of secretory vesicles with the cell membrane
80
explain [glucose]ex is accurately sensed
.> vital to this mechanism is the ability of the cell to sense the extracellular concentration of glucose and adjust intracellular metabolism in direct proportion
> linear
> a key facet of this mechanism is the low affinity glucose transporter expressed in the β-cell plasma membrane - GLUT2
- expressed mainly in liver and beta cells
>vital to this mechanism is the ability of the cell to sense the extracellular concentration of glucose and adjust intracellular metabolism in direct proportion
>a key facet of this mechanism is the low affinity glucose transporter expressed in the β-cell plasma membrane (GLUT2)
81
what are facilitative diffusion transporters?
Facilitated diffusion is the diffusion of solutes through transport proteins in the plasma membrane.
Channel proteins, gated channel proteins, and carrier proteins are three types of transport proteins that are involved in facilitated diffusion.
*glucose transporters are facilitative diffusion transporters
82
what is the alternating conformation model for membrane transport?
They are proposed to use an alternating access model of transport [1], the principle of which is that the protein switches conformations to present the substrate binding site to alternate sides of the membrane without ever fully opening a channel from one side to the other.
83
describe the glucose transporter
alternating conformation model for membrane transport
v-shaped protein with outward facing binding site, which can switch to an inward facing binding site- alternating conformation.
glucose binds to outward facing, protein switches to inward facing binding site, glucose enters interior of cell.
they have channel across the membrane with a 'plug' at the bottom- this plug is a voltage sensor. if you change the potential difference across the plasma membrane, then you either close/open the plug. and if you open the plug, then the ions can flow.
84
transporters vs channels
transporters and channels work differently.
transporters mediate movement of (usually) one solute molecule at a time. Hence, Glut's have a turnover number of ~17000-20000/min
channels 'open' and allow flow of many ions quickly. this may be as high as 10^6 ions/second
structures are different to reflect this. there are 12 different GLUTs (glucose transporters) in human genome. even more channels...
85
discuss glucose metabolism in β-cells
in most cells (ATP) is relatively invariant; this is not the case in β-cells.
glucose metabolism in β-cells is inefficient.
as a result, β-cells response to changes in extracellular glucose concentrations by rapidly changing [ATP]/[ADP] ratio
glucose phosphorylation in β-cells uses the low affinity (high Km) enzyme; glucokinase.
this enzyme is not saturated as physiological (glucose), so phosphorylation rate directly proportional to intracellular (glucose).
glycolytic rate therefore depends on glucose entry/[blood glucose].
86
what enzyme does glucose phosphorylation in β-cells use?
low affinity (high Km) enzyme glucokinase
87
is glucose metabolism in β-cells efficient of inefficient?
inefficient
*as a result β-cells respond to changes in extracellular glucose concentrations by rapidly changing [ATP]/[ADP] ratio
88
what does glycolytic rate depend on?
glucose entry/[blood glucose]
glycolytic rate= a measure of the maximum rate of conversion of glucose to pyruvate or lactate that can be achieved acutely by a cell.
89
what is glycolytic rate?
Glycolytic capacity is a measure of the maximum rate of conversion of glucose to pyruvate or lactate that can be achieved acutely by a cell. Since glycolytic ATP synthesis is obligatorily linked to glycolytic carbon flux, glycolytic capacity is also a measure of the maximum capacity of glycolysis to generate ATP.
Glycolysis is a central metabolic pathway that is used by all cells for the oxidation of glucose to generate energy in the form of ATP (Adenosine triphosphate) and intermediates for use in other metabolic pathways.
Glycolysis is the process by which glucose is broken down within the cytoplasm of a cell to form pyruvate. Under aerobic conditions, pyruvate can diffuse into mitochondria, where it enters the citric acid cycle and generates reducing equivalents in the form of NADH and FADH2.
90
what is micharlis menten equation?
v = (Vmax [S])/Km + [S]
Km= [substrate] that produces Vmax/2
Km an expression of affinity:
high Km=low affinity ... low Km=high affinity
91
what is low vs high Km?
low = high affinity
high= low affinity
Km= [substrate] that produces Vmax/2
92
are channels open or closed in beta-cells?
although widely expressed in other cells these channels tend to be permanently closed as a consequence of high ATP.
In beta-cells, they are often open as ATP levels are far lower.
In beta cells, channels are open, allowing movement of K+ across membrane. Potassium ions are moving out of the cell and the cell membrane is polarised, maintaining a negative membrane potential.
beta cell membrane is polarised- when ATP channel closes, cell depolarises.
*change in polarity of membrane transmitted to entire surface because its electrical; depolarisation wave felt across entire cell surface in a fraction of a second
93
describe the opening an closing of glucose channels across the membrane
in beta cells, channels are open, allowing movement of K+ across the membrane. Potassium ions are moving out of the cell and the cell membrane is polarised, maintaining at a negative membrane potential.
As [ATP] rise the channels close, and so the PM (plasma membrane) depolarises. (the change in polarity of membrane transmitted to entire cell surface because it's electrical; depolarisation wave felt across entire cell surface in a fraction of a second)
When ATP levels rise, sensitive K+ channels close, this results in a change in potential difference across the plasma membrane.
The inside becomes more positive and the voltage-gated Ca2+ channels sense this and open.
This depolarisation opens voltage-gated calcium channels, allowing a rapid influx of Ca2+.
The rapid rise in intracellular calcium triggers the fusion of insulin-containing secretory vesicles with the plasma membrane.
*this all occurs in fractions of a second
an electrical signal is really quickly transduced over the entire cell
EXQUISITELY SENSITIVE to the amount of glucose in the blood
amplification at various points (particularly at the electrical signal)
multiple control points (various other hormonal and neuronal systems can regulate this)
94
what are the two glucose sensors?
GLUT2
Glucokinase
95
what does compartmentalisation mean in terms of insulin?
- insulin only folds in the ER
- insulin is only correctly processed in secretory vesicles/granules
- high concentrations by Zn-mediated 'crystalline' array (Zn high in secretory granules; nowhere else)
- the secretory granules are 'lined up' ready to be released
- Ca channels are nearby, generating a rapid influx of Ca at exactly the right place
96
what shapes and controls the insulin exocytotic response?
several modifying ion channels and receptors.
beta cells, alpha cells and delta cells in cluster:
- interact with each other
- signal to each other and integrate their behaviour/metabolism
1000s of islets in the pancreas signal to each other
- they have neuronal and hormonal connections
- release insulin in waves
clusters release insulin at different times
integrate output
97
what are the key targets of insulin?
pancreas releases insulin from beta cells in the islets of langerhans.
insulin targets muscle and liver for glucose uptake and lipolysis (fat) to make fatty acids.
98
what is lipolysis?
Lipolysis is the metabolic process through which triacylglycerols (TAGs) break down via hydrolysis into their constituent molecules: glycerol and free fatty acids (FFAs). Fat storage in the body is through adipose TAGs and is utilized for heat, energy, and insulation.
Insulin plays a crucial role in regulating glucose and lipid metabolisms. Insulin promotes lipid synthesis and storage, reduces plasma FFAs, and inhibits the catabolism of lipids and FFA oxidation. Insulin is the most important hormone that inhibits lipolysis.
99
how does insulin signal?
insulin is a hormone that once released from the pancreas circulates in the blood stream.
it signals liver, muscle and fat to change their metabolic profiles.
binds to a specific receptor expressed only on the surface of those tissues.
specific hormones/signals bind to specific receptors in specific tissues to drive specific effects.
100
where are insulin receptors found on tissues?
expressed on the SURFACE of tissues
101
what type of polypeptide is the insulin receptor and what bond holds it together?
an α2β2 polypeptide
held together by disulphide bonds
102
what part of the insulin receptor does insulin bind to and what does this cause?
to the α2 subunits (specific)
>causing a conformational change in the alpha subunit
> transmits a signal to the β2 subunits
> activates an intrinsic tyrosine kinase within the cytosolic domain
(the signal has crossed the membrane)
*binding is specific- only insulin can bind the receptor
103
what function to kinase enzymes have?
adds phosphate
104
what part of an amino acids is phosphate group added to?
the -OH (hydroxyl) group
105
what is autophosphorylation?
Autophosphorylation is a biochemical process in which a phosphate group is added to protein kinase by the action of the protein kinase itself. During the course of autophosphorylation the phosphate group is supplied by a unique biochemical compound known as adenosine triphosphate (ATP).
106
what is amplification in relation to insulin signalling?
a single molecule of insulin activates a kinase that can phosphorylate many target molecules.
insulin binds receptor, inducing conformational change in alpha subunits.
transmitted through beta subunits.
cytosolic domain becomes activated tyrosine kinase.
107
what are the two main classes of kinase enzymes?
those that phosphorylate TYROSINE residues
those that phosphorylate SERINE/THREONINE residues
108
how can changes to the enzyme induced by phosphorylation be reversed?
by virtue of the couples kinase/phosphatase system
*kinases catalyse transferring of a phosphate group from ATP to proteins, and phosphatases remove the phosphate
109
what steps are involved in insulin signalling?
tyrosine kinase receptor phosphorylates specific Tyr residues within the receptor
(auto-phosphorylation)
these p-Tyr residues recruit signalling molecules (IRS-1 shown here) to the receptor which are then themselves phosphorylated.
>> forms a signalling complex which will selectively recruit molecules.
auto-phosphorylated insulin receptor recruits proteins to the phosphorylated tyrosine residues.
>one such protein is insulin receptor substrate 1 (IRS1)
- signal amplification
- receptor phosphorylates multiple targets
- these recruit multiple effectors
- signal strength is amplified
110
how do activated insulin receptors assemble signalling complexes?
via phosphorylated-tyrosine and SH2 domain-containing proteins.
> the auto-phosphorylation of specific tyrosine residues function to recruit SH2 (Src homology 2) domain containing proteins to the receptor
> these are then activates
> signal propagated
> insulin receptors selectively recruit a specific subset of SH2-domain containing proteins
111
what does SH2 domain mean and do?
Src homology 2
Their function is to bind tyrosine-phosphorylated sequences in specific protein targets. Binding of an SH2 domain to its cognate tyrosine-phosphorylated target links receptor activation to downstream signalling, both to the nucleus to regulate gene expression and throughout the cytoplasm of the cell.
All SH2 domains bind P-Tyr (however they are distinct)
all SH2 domains bind phosphotyrosine
all SH2 domains have a 'pocket' into which the P-Tyr residue fits
not all SH2 domains bind all P-Tyr residues; there is specificity dictated by the surrounding sequence in the target (receptor) polypeptide.
(two pronged plug concept)
112
explain the two pronged plug concept of SH2 domain
pocket contains +ve residues at the bottom to interact with -ve phosphate
if tyrosine isn't phosphorylated; affinity of tyrosine for pocket is gone
second binding pocket; e.g. hydrophobic pocket which recognises isoleucine (confers specificity)
113
which part of the SH2 domain recognises phosphotyrosine and which part recognises surrounding residues?
containing pocket recognises phosphotyrosine
specificity pocket recognises surrounding residues
114
what part of the polypeptide chain does the specificity pocket of SH2 domain recognise?
recognises surrounding residues.
recognises the hydrophobic side chain of isoleucine.
*placement of the pocket is important- this specifically recognises an isoleucine three amino acids down from a phosphotyrosine
115
what is isoleucine ?
Isoleucine is an essential amino acid. Isoleucine is the oxygen-carrying pigment inside of red blood cells and helps to make haemoglobin. It is also helpful in controlling blood sugar, boosting energy, and improving endurance
116
how is specificity achieved in assembling a signalling complex?
specific ligand binds to specific receptor
> insulin is an example
autophosphorylation of specific tyrosine residues
recruits SH2 domain-containing proteins to the receptor
different receptors recruit different SH2 domain-containing proteins
different SH2 domains recognise P-Tyr (phosphorylated tyrosine) in different sequence contexts
117
what is the name of activated ser/thr kinase?
Akt
118
what are the core concepts which lead to a signal amplification?
>receptors specific for individual receptor ligand.
>signal transduced across the membrane- in this case by activation of an intrinsic tyrosine kinase activity (common in many different hormone receptors)
>the receptor phosphorylates itself (autophosphorylation)- the key 'on' switch
>this amplifies the signal, as the kinase can phosphorylate multiple copies of the target
>specific targets are recruited by binding to the phospho-tyrosine residues (there is specificity here, different receptors recruit different effectors)
>signal complex created- efficient
>these then activate down-stream pathways, many often in a cascade
>causes marked signal amplification
119
what is Akt?
a key enzyme which is activated by insulin
> active ser/thr kinase
120
what does insulin stimulate which is impaired in type-2 diabetes?
- insulin stimulates glucose uptake into fat and muscle cells (requires ATP)
- the key action of insulin to increase glucose transport into muscle and fat cells
- impaired in type-2 diabetes (resistance)
121
what is required for glucose uptake into fat and muscle cells?
ATP
122
what specialised form of glucose transporter is used for glucose uptake into fat and muscle cells?
GLUT4
123
what triggers GLUT4 to move to cell surface in response to insulin binding its receptor?
Akt
specific glucose transporter (GLUT4)
moves to the cell surface in response to insulin binding its receptor
this is triggered by activation of Akt
**another example of regulated exocytosis
124
what is an example of regulated exocytosis?
insulin stimulates glucose uptake into fat and muscle cells
- specific glucose transporter (GLUT4)
- moved to the cell surface in response to insulin binding its receptor
- this is triggered by activation of Akt
125
what is GLUT4 packages into?
secretory vesicles.
(in the absence of insulin GLUT4 is inside the cells in specialised vesicles - GSVs
*GLUT4 is the only GLUT isoform that exhibits this trafficking
126
what are the specialised vesicles which store GLIT4?
GSVs
>GLUT4 stored internally in the membranes of GSVs
127
when is glucose transport low?
glucose transport is low in the absence of insulin
128
what happens to GSVs on addition of insulin?
vesicles dock and fuse with the plasma membrane and GLUT4 is inserted into plasma membrane
129
what is the process of vesicles in release of insulin in beta cells and in fat and muscle cells?
- same idea as insulin secretion, vesicles are triggered to move to the cell surface where they dock and fuse
- in the beta-cell, this releases insulin (soluble content)
- in fat and muscle cells, this delivers GLUT4 to the cell surface and increases glucose entry into the cells
- this delivery of GLUT4 to the cell surface required Akt
130
what enzyme is required for vesicles to deliver GLUT4 to the cell surface?
Akt
131
what are effects of insulin and how quickly do these take place?
inhibits lipolysis (IRS/Akt pathway)
>very fast
increases glucose transport (GLUT4)
> 2-10 min
increases glycogen synthesis
> 10-20 min
cell specific effect (promotes lipid droplet formation in fat cells)
>??
activates gene expression
> hours
132
how does insulin promote glycogen synthesis in liver and muscle?
it turns on glycogen synthase and turns off glycogen phosphorylase.
it does this by switching on protein phosphatase-1, which dephosphorylates both proteins.
133
what does insulin promote in fat cells?
promotes lipid droplet formation
134
effects of active Aks on fat cells and liver
active insulin receptor leads to activation of Akt.
Akt phosphorylates target proteins- different subsets in different cell types.
different Akt effectors expressed in different cells.
fat cells;
activates GLUT4 trafficking to the cell surface
lipid droplet formation
liver;
phosphorylates and ACTIVATES protein phosphotase1 - promotes glycogen storage
135
what activates Akt?
active insulin receptor
136
what effect does Akt have on liver cells?
phosphorylates and activates protein phosphatase 1
> promotes glycogen storage
136
how does insulin modulate gene expression?
insulin signals through IRS1 (insulin receptor substrate 1) to a variety of transcription factors that control gene expression
insulin activates the LxR transcription factor in liver and fat
in far, this drives increased expression of SREBP1 transcription factor
SREBP1 controls fatty acid synthase (acetylcoA carboxylase, lipid metabolism genes turned ON)
LxR -> SREBP1 -> storage of fat ON
137
what transcription factor control fatty acid synthase?
SREBP1
138
what two things can transcription factors be?
activators or repressors
activator; helps general transcription factors and RNA polymerase assemble
repressor; blocks general transcription factors and RNA polymerase
139
discuss transcription factors
- can turn gene expression on/off
- are specific for certain genes
> dependent on sequence recognised by transcription factor
> acetyl CoA carboxylase; fatty acid synthase have the same sequence
- many targets for signalling pathways
- activated by phosphorylation and subsequent movement into the nucleus
> phosphorylation can do different things to a protein
> change activity; change its stability; change its location
140
what is a signalling cascade?
A biochemical cascade, also known as a signalling cascade or signalling pathway, is a series of chemical reactions that occur within a biological cell when initiated by a stimulus
141
what cells sense changes in glucose?
beta cells
The beta cells of the pancreatic islets of Langerhans respond to changes in glucose concentration by varying the rate of insulin synthesis and secretion. Beta cells sense glucose concentration by the levels of the products of glucose catabolism.
142
how is pedigree analyses used to determine whether a phenotype is genetic in origin?
By analysing a pedigree, we can determine genotypes, identify phenotypes, and predict how a trait will be passed on in the future. The information from a pedigree makes it possible to determine how certain alleles are inherited: whether they are dominant, recessive, autosomal, or sex-linked
By mapping out family relationships and trait inheritance patterns over several generations, pedigree analysis helps reveal if a phenotype is likely due to genetic factors or if it might be influenced by environmental factors
143
how can gene mapping strategies identify affected chromosomal domains?
Each chromosome has a distinct banding pattern, and each band is numbered to help identify a particular region of a chromosome. This method of mapping a gene to a particular band of the chromosome is called cytogenetic mapping.
[human gene mapping with STRs; short tandem repeats]
144
how can we use a 'candidate gene' approach to link a gene to a phenotype?
a 'candidate gene' is a gene that (when mutant) might plausibly cause the mutant phenotype we observe
the two STRs (short tandem repeats) found to be co-inherited with the 'inappropriate aggression' phenotype identify an X-chromosome region Xp11.23-Xp11.30
145
roughly how many genes are contained in the X chromosome?
~ 1000 genes
146
what are STRs?
Short tandem repeats (STRs) occur when a short sequence of DNA is repeated many times in a row – for example, a triplet repeat such as CAG. These occur throughout the genome, often with little or no consequence.
different STRs map to different positions on the human X-chromosome
147
how is the human gene mapped with STRs?
look for STRs (short tandem repeats) that are co-inherited with the 'inappropriate aggression' phenotype.
by 1990 a set of STRs had been identified that covered the entire human genome
the nearest STRs will be inherited at about 100% with the phenotype
the STRs further away will be inherited less frequently down to 50%
148
what is a candidate gene?
a gene that (when mutant) might plausibly cause the mutant phenotype we observe
149
what is the human genome project?
The Human Genome Project is an international research project whose primary mission is to decipher the chemical sequence of the complete human genetic material (i.e., the entire genome), identify all 50,000 to 100,000 genes contained within the genome, and provide research tools to analyse all this genetic information.
150
what are two enzymes which are involved in calming the fight/flight response?
monoamine oxidase-A (MOXA)
monoamine oxidase-B (MOXB)
151
what is the function of MOXA and MOXB enzymes?
to metabolise excess neurotransmitters, which 'calms' the fight/flight response
152
discuss the genetics and biochemistry of monoamine oxidase genes.
genetics;
two human monoamine oxidase genes in Xp11.23-Xp11.30
they have identical intron/exon structure of 15 exons
they lie next to each other in a head to tail orientation
biochemistry;
relative activity of MOXA c85% and MOXB c15%
at the amino acid level MOXA and MOXB are 70% identical
metabolise serotonin, dopamine, and noradrenaline
153
what do the monoamine oxidase genes metabolise?
serotonin, dopamine and noradrenaline
154
is there a genetic component to aggressive behaviour?
yes in relation to the monoamine oxidase A deficiency
155
explain the concept of 'personalised medicine'?
Personalised medicine is an emerging practice of medicine that uses an individual's genetic profile to guide decisions made in regard to the prevention, diagnosis, and treatment of disease.
156
how has DNA sequencing technology advanced in the last 20 years?
Recent advancements have focused on faster and more accurate sequencing, reduced costs, and improved data analysis. These advancements hold great promise for unlocking new insights into genomics and improving our understanding of diseases and personalized healthcare.
157
how are most phenotypes complex?
they involve multiple genes and the environment
158
how does GWAS link genes to complex phenotypes?
genome-wide association studies
159
how does personalised medicine uses genetic data to improve healthcare outcomes?
The study of how a person's genes affect their drug response, pharmacogenomics (also called pharmacogenetics), is crucial in personalized medicine as it enables the tailoring of drug selection and dose to a patient's genetics
160
what is heritability?
a measure of how well differences in people's genes account for differences in their traits.
traits can include characteristics such as height, eye colour, and intelligence, as well as disorders like schizophrenia and autism spectrum disorder.
161
what are SNPs?
single nucleotide polymorphisms
162
what are the main source of genetic variation?
SNPs; single nucleotide polymorphisms
163
what percentage of the human genome does variation arise from?
0.1%
99.9% of the human genome is conserved between people, all variation arises from the other 0.1%
164
what is a haplotype?
a set of DNA variants along a single chromosome that tend to be inherited together
>idea that even if you have variation in a non-coding region, these can still be inherited over multiple generations so we can detect them in a population
165
discuss herceptin and breast cancer
over-expression of the HER2 growth factor (cell division) receptor.
cell receives too much signal and divides inappropriately (breast cancer)
bock HER2 receptor with 'neutral' ligand (herceptin)
166
identify and explain the latest clinical guidelines for exercise in the prevention and management of diabetes
physical activity
lower body weight
167
168
what is PKC?
Protein kinase C (PKC) form a key family of enzymes involved in signalling pathways that specifically phosphorylates substrates at serine/threonine residues. Phosphorylation by PKC is important in regulating a variety of cellular events such as cell proliferation and the regulation of gene expression.
169
what are ROS?
reactive oxygen species
170
discuss PKC and oxidative stress
as blood glucose increases, diacylglycerol (DAG) synthesis increases.
this increase in DAG leads to activation of PKC pathway.
> this pathway mediates different cellular signals which increase the production of pro-inflammatory cytokines, leading to microvascular damage
> this can cause damage at the kidney (nephropathy) and nerves (neuropathy)
oxidative stress leads to an increased production of reactive oxygen species (ROS)
> they cause endothelial dysfunction, along with contributing to insulin resistance and damaging pancreatic β-cells
171
what is the word for damage to kidney and damage to nerves?
kidney damage; nephropathy
nerve damage; neuropathy
172
why is the Mediterranean diet benificial?
helps reduce inflammation and oxidative stress
173
what are GI foods?
glycaemic foods
low GI (less than 55) – examples include soy products, beans, fruit, milk, pasta, grainy bread, porridge (oats) and lentils. medium GI (55 to 70) – examples include orange juice, honey, basmati rice and wholemeal bread. high GI (greater than 70) – examples include potatoes, white bread and short-grain rice.
174
GWAS analyses are likely to shed light on genetic factors contributing to which condition?
prostate cancer.
genome-wide association studies (GWAS) are designed to identify common genetic variants that contribute to complex, polygenic diseases with multifactorial inheritance patterns (such as prostate cancer).
conditions like prostate cancer are influenced by multiple genes and environmental factors making GWAS an effective tool to explore associated genetic risk factors.
175
give examples of monogenic disorders?
monogenic; caused by mutations in a single gene
cystic fibrosis
Huntington disease
Tay Sachs disease
sickle-cell anaemia
176
the inheritance of type-2 diabetes cannot be explained by the segregation of a single gene. What is the correct term for this type of trait?
complex trait.
type-2 diabetes is considered a complex trait because it is influenced by multiple genetic and environmental factors rather than the segregation of a single gene.
complex traits typically involve the combined effects of several genes, each contributing a small effect to the overall risk, along with lifestyle and environmental factors.
177
what are the different types of traits?
complex trait; influenced by multiple genetic and environmental factors rather than the segregation of a single gene.
qualitative trait; refers to traits that are often controlled by a single gene and have distinct categories (e.g. blood type)
omnigenic trait; suggests a theory where almost all genes may contribute in some wat to complex traits, but it is not the specific term used to describe polygenic conditions like type-2 diabetes
sporadic trait; describes a trait that appears randomly and is not strongly inherited
discrete trait; refers to traits that have distinct, separate categories (e.g. presence/absence of a trait) rather than a continuous distribution
178
GLUT4 is expressed primarily in which tissues?
adipose
skeletal muscle
179
describe an example of metabolic control
in cases where the direction of a metabolic pathway has to be reversed, the pathway is controlled at an irreversible step
180
what are some statements that are true for exercise for Type-2 diabetes?
> aerobic exercise can have a metabolic impact for up to 72hrs
> compliance with prescribed exercise is poor
> increased intensity exercise will shift substrate utilisation towards carbs
> exercise will promote the movement of GLUT4s to the plasma membrane
181
phosphorylation is a form of post-translational modification. what effect does it have on an enzyme?
it results in a conformational change in the protein
181
during insulin biosynthesis, insulin goes through several stages of processing and packaging. give some statements relating to this.
> proinsulin is co-translationally inserted into the ER through the Se61 complex
> disulphide bonds do not form effectively in the cytosol because it is a reducing environment; they typically form in the ER
> different enzymes are involved in processing preproinsulin to proinsulin and then proinsulin to insulin
> high levels of zinc in secretory vesicles help to package and crystallise mature insulin storage and secretion
182
is proinsulin biologically active?
proinsulin is not biologically active. it is an inactive precursor to insulin that requires further processing to become the mature, biologically active form of insulin. during biosynthesis, proinsulin undergoes enzymatic cleavage to remove the C-peptide, resulting in the formation of active insulin composed of the A and B chains connected by disulphide bonds.
183
what is the preferred substate for ATP at rest or during low levels of activity?
free fatty acids.
during rest or low activity levels, free fatty acids are the preferred substrate for ATP generation. In these states, the body's energy demand is relatively low, so it conserves glucose for tissues that depend on it, like the brain and red blood cells.
fatty acids are metabolised in the mitochondria via β-oxidation to produce ATP, providing a steady and sustained energy source that's efficient during low-intensity activity/rest.
**glucose is more often reserves for higher-intensity activities and immediate energy needs
184
what enzyme convers glucose-6 P to glucose-1 P?
phosphoglucomutase
PGM catalyses the reversible reaction that shifts the phosphate group from the 6-position to the 1-position on glucose.
185
to exert its effect on cells, insulin binds specifically to the insulin receptor. what type of receptor is the insulin receptor?
tyrosine kinase receptor.
when insulin binds to this receptor on the cell surface, it activates the receptors intrinsic tyrosine kinase activity, which triggers autophosphorylation and initiates a signalling cascade.
this cascade facilitates glucose uptake and other metabolic effect within the cell.
186
why is glucose a good source of energy?
glucose is an excellent energy source because it yields high ATP, is metabolically efficient, readily available, and crucial for brain function.
187
outline in a few sentences the metabolic fate of glucose under aerobic and anaerobic conditions
Under aerobic conditions (with oxygen), glucose is metabolized through glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation. First, glucose is broken down in glycolysis to form pyruvate, which enters the mitochondria. In the citric acid cycle, pyruvate is further broken down, and high-energy electrons are transferred to the electron transport chain, where they produce a large amount of ATP, yielding up to 36-38 ATP molecules per glucose molecule.
Under anaerobic conditions (without oxygen), glucose is still broken down through glycolysis, but pyruvate cannot enter the citric acid cycle. Instead, it is converted into lactate (in animals) or ethanol (in yeast) to regenerate NAD+, allowing glycolysis to continue. This process, called fermentation, produces only 2 ATP molecules per glucose, providing much less energy than aerobic respiration.
188
During what stage of cellular respiration is the most ATP synthesised?
The most ATP is synthesized during the electron transport chain (ETC) and oxidative phosphorylation stage of cellular respiration. This stage occurs in the inner mitochondrial membrane, where high-energy electrons from NADH and FADH₂ (produced during earlier stages like glycolysis and the citric acid cycle) are passed along protein complexes in the ETC.
As electrons move through the chain, they create a proton gradient across the membrane. This gradient drives ATP synthase, an enzyme that produces ATP by combining ADP with inorganic phosphate. The majority of ATP, about 28-34 molecules per glucose molecule, is generated during this stage.
189
Outline briefly the role of the pancreas, liver and muscle in the regulation of blood glucose levels.
The pancreas, liver, and muscle each play essential roles in regulating blood glucose levels:
Pancreas: The pancreas monitors blood glucose and releases the hormones insulin and glucagon to regulate it. When blood glucose is high, insulin is released to help cells absorb glucose, lowering blood levels. When glucose is low, glucagon is released to signal the liver to release stored glucose.
Liver: The liver stores glucose as glycogen and releases it back into the bloodstream when needed. In response to glucagon, the liver breaks down glycogen (glycogenolysis) or even synthesizes new glucose from non-carbohydrate sources (gluconeogenesis) to maintain blood glucose levels during fasting or low glucose intake.
Muscle: Muscle cells store glucose as glycogen for energy during physical activity. Although muscles don’t release glucose into the blood directly, they take up glucose in response to insulin, helping to reduce blood glucose levels after a meal.
Together, these organs coordinate to keep blood glucose levels within a narrow, healthy range.
190
Describe in series of bullet points what happens in these three tissues which blood
glucose levels are (a) elevated and (b) low, such as during an overnight fast.
(a)
Elevated (e.g., after a meal)
Pancreas:
Detects the increase in blood glucose.
Releases insulin into the bloodstream to lower glucose levels.
Liver:
Insulin signals the liver to take up glucose.
Glucose is converted to glycogen (glycogenesis) for storage.
If glycogen stores are full, excess glucose may be converted into fatty acids and stored as fat.
Muscle:
Insulin signals muscle cells to absorb glucose from the blood.
Glucose is stored as glycogen within the muscle for future energy needs.
Muscle cells also use glucose immediately for energy, especially if there is physical activity.
(b)
Low (e.g., during an overnight fast)
Pancreas:
Detects the decrease in blood glucose.
Releases glucagon into the bloodstream to raise glucose levels.
Liver:
Glucagon signals the liver to break down glycogen into glucose (glycogenolysis).
If glycogen stores are depleted, the liver produces glucose through gluconeogenesis (creating glucose from non-carbohydrate sources like amino acids).
Glucose is released into the bloodstream to maintain blood sugar levels.
Muscle:
Muscle glycogen stores are used locally for energy during activity but are not released into the bloodstream.
Muscles don’t directly contribute to raising blood glucose levels but conserve energy by using stored glycogen if needed.
During prolonged fasting or exercise, muscles may rely on fatty acids for energy to conserve glucose.
191
List three ways in which an enzyme might be regulated.
Enzymes can be regulated through various mechanisms to control their activity. Here are three common ways:
Allosteric Regulation:
Allosteric regulators (activators or inhibitors) bind to a site on the enzyme other than the active site, causing a conformational change. This can either increase or decrease the enzyme's activity.
Covalent Modification:
Enzymes can be modified by the addition or removal of chemical groups, such as phosphorylation (addition of a phosphate group). This modification often changes the enzyme's shape and activity and is a common way to quickly activate or deactivate enzymes.
Feedback Inhibition:
In metabolic pathways, the end product can act as an inhibitor of an enzyme earlier in the pathway. This prevents the overproduction of the end product by slowing down the pathway when sufficient product is available.
Each of these mechanisms allows cells to finely tune enzyme activity in response to changing needs and conditions.
192
Define post-translational modification and give one example which is widely used to
control protein function.
Post-translational modification (PTM) refers to the chemical modification of a protein after it has been synthesized (translated) in the ribosome. These modifications can alter a protein’s function, stability, localization, or interaction with other molecules and are essential for regulating various cellular processes.
Example: Phosphorylation
Phosphorylation is a widely used post-translational modification where a phosphate group is added to specific amino acids, typically serine, threonine, or tyrosine residues.
This modification is often catalyzed by enzymes called kinases and can be reversed by phosphatases.
Phosphorylation can activate or deactivate enzymes, change protein structure, or create binding sites for other molecules, playing a key role in signal transduction and regulation of cellular activities.
193
Draw the reaction catalysed by a serine-specific kinase and a tyrosine-specific kinase.
