TOPIC 7 - membranes, lipids and signalling Flashcards
1
Q
How many membranes do gram positive bacteria have?
A
No outer membrane and a thick peptidoglycan layer
2
Q
How many membranes do gram negative bacteria have?
A
Outer and inner membrane and a thinner peptidoglycan layer
3
Q
Where are membranes in eukaryotic cells?
A
- nucleolus
- mitochondria
- lysosomes
- endoplasmic reticulum
- golgi
- vesicles
- chloroplasts (in plants)
4
Q
What are the functions of membranes?
A
- functional barrier
- provide cells with energy
- organise and regulate enzyme activities
- signal transduction
- substrates for biosynthesis and signalling molecules
- protein recruitment platform
5
Q
What are membranes composed of?
A
Proteins and lipids
6
Q
What are the three types of lipids?
A
- glycerophospholipids
- sphingolipids
- sterols
7
Q
How are phospholipids amphiphatic?
A
They have a polar head and non polar tail so can from bilayer structures
8
Q
What group is on the end of a fatty acid chain?
A
A terminal carboxylic acid
9
Q
How does a lipid bilayer form?
A
Lipids spontaneously aggregate to bury their hydrophobic tails in the interior and expose their hydrophilic heads to water
10
Q
How do you name fatty acids?
A
Fatty acids vary in chain length, double bond number, double bond position and hydroxylation
XX:Y n-y
XX = number of carbons in the chain
Y = number of double bonds
n-y = position of first double bonded carbon
11
Q
What are the shapes of saturated and unsaturated lipid tails?
A
Saturated - no double bonds, straight
Unsaturated - one or more double bond, can be straight (trans bond) or have a 30º kink (cis bond)
12
Q
Can a phospholipid have tails of different lengths
A
Yes, it influences how the phospholipids pack against one another
- straight chains= thicker membranes
13
Q
What do glycerophospholipids consist of?
A
- 2 fatty acid tails
- a glycerol backbone
- a head group
14
Q
Are the glycerophospholipid tails usually saturated or unsaturated?
A
sn-1 is usually saturated or monounsaturated
sn-2 is more often monounsaturated or polyunsaturated
15
Q
Which glycerophospholipids have a net negative charge/anionic phospholipids ?
A
PS - Serine
PI - Inositol
PG - Glycerol
CL - Cardiolipin
16
Q
What glycerophospholipids have zero charge/ Zwitterionic phospholipids ?
A
PE - ethanolamine
PC - Choline
17
Q
What glycerophospholipids contain amines that can form H bonds?
A
PS - serine
PE - ethanolamine
18
Q
How many tails does cardiolipin have?
A
4 - makes it bulky and so affects its packing
19
Q
How many PIP species?
A
7 - PI(4,5)P2 is the most abundant
20
Q
What leaflet of the membrane are PIPs found?
A
Inner (cytoplasmic)
21
Q
What do sphingolipids consist of?
A
A sphyngoid base, N-acyl chain and head group
-both tails likely to have no double bonds = saturated
22
Q
What bond is there between the acyl chains and glycerol backbone in glycerophospholipids?
A
Ester
23
Q
What bond is there between the acyl chain and sphingoid base backbone in sphingolipids?
A
Amide linkage - the amide group allows it to form hydrogen bonds and so can interact with cholesterol or polar parts of proteins
24
Q
What is the most common sphingolipid?
A
Sphingomyelin (SM) has a phosphocholine headgroup
25
Are the acyl chains of sphingolipids or glycerophospholipids longer?
Sphingolipid acyl chains tend to be longer and more saturated
26
What are glycolipids?
Glycosphingolipids
| - have different oligosaccharides as head groups (composed of mainly sugars)
27
What leaflet of the membrane are glycolipids found?
Exclusively on the outer leaflet (only make up about 5% of the membrane)
28
What role do glycolipids have?
Interaction of the cell with its surroundings (cell to cell adhesion) and allow membranes to act as recognition sites (sugar site exposed to the outside)
29
How many sugars do ganglioside GM1, GM2 and GM3 have?
GM1 - 4
GM2 - 3
GM3 - 2
-If these lipids found on outer membrane= tend to aggregate= have these suagrs and sugras can interact with outer sugars = clustering
30
What do sterols consist of?
Hydroxyl group and a hydrocarbon tail
31
What is the most common sterol in animals?
Cholesterol
32
What effect does cholesterol have on membranes?
- increases thickness
- increases packing
- increases compressibility
- decreases mobility of lipids and proteins
33
What type of tail does cholesterol interact more tightly with?
Straight saturated tails - more packing due to its shape
34
What affects membrane curvature?
The relative size of the head group and hydrophobic tails of lipids affect the shape of the lipid and the spontaneous curvature of the membrane
35
What are the two types of lipid diversity?
Chemical/structural - defines specific properties of lipids
| Compositional between tissues, organelles and leaflets - affects collective behaviour of lipids
36
Why is lipid asymmetry functionally important?
Change in membrane leaflet composition could act as a signal - eg phosphatidylserine in animal cells translocate to the extracellular monolayer when cell is dying - acts as a signal to neighbouring cells
37
What enzyme aids the movement of PS lipids?
Scramblases
38
What is lipid interdigitation?
When lipids from separate leaflets overlap due to lipid length asymmetry. This couples the two leaflets together and decreases thickness
39
What is rotational movement of lipids?
Spinning of lipids around their axes, does not alter their position but affects interactions with neighbouring molecules
40
What is lateral movement of lipids?
Neighbouring lipids exchange places within a bilayer leaflet
41
What is transverse movement of lipids?
Exchange of lipids between leaflets - can move spontaneously by transverse diffusion or it can be mediated by proteins
42
What are the three phases of lipids when in the lamellar (bilayer) phase?
Lamellar liquid crystalline - membrane more fluid due to loose packing
Solid gel - long unsaturated lipid tails pack more tightly
Liquid-ordered - presence of cholesterol
43
What is the fluid mosaic model?
The bulk of the lipids forms the bilayer providing the solvent for embedded proteins.
The bilayer is fluid – lateral mobility of lipids and of some proteins. It is mosaic in the sense that proteins are scattered across it.
Most proteins are integral and some are peripheral.
This model emphasises the fluidity of the bulk lipids allowing random diffusion
44
What is the concept of membrane domains/rafts?
Suggests that in a eukaryotic plasma membranes there will be different areas 'domains' with different composition of cholesterol and sphingomyelin .
Proteins are either excluded or included in the raft regions
45
What are lipid droplets?
Storage organelles that help to maintain the lipid and energy homeostasis.
Hydrophobic core of neutral lipids enclosed by a phospholipid monlayer.
originate from ER
46
Where do lipid droplets originate from?
The endoplasmic reticulum and are initiated when neutral lipids are produced
47
How do neutral lipids form?
Result from the esterification of a fatty acid to a triacylglycerol or a sterol to a sterol ester
48
Where are neutral lipids normally and what happens if their storage is impaired?
Normally dispersed in the leaflets of ER bilayer, as their concentration increases neutral lipids accumulate (demixing)
If fatty acid storage in lipid droplets is impaired can result in diseases such as type 2 diabetes and fatty liver disease
49
What is a monotopic protein?
an integral membrane protein that inserts into the membrane but does not span it
50
what is a bitopic protein?
integral membrane protein with one helix that spans the bilayer once
type 1 - N-terminal is outside
type 2 - N-terminal is inside
51
what is a polytopic protein?
integral membrane protein with segments spanning the membrane, connected by loops
52
what is a oligomeric protein?
integral membrane protein formed when multiple bitopic proteins oligomerize (join together, but not connected by loops)
53
Give the 5 functions of integral proteins
transport - move molecules across the membrane by changing shape
enzymatic activity - participate in electron transport and metabolism of phospholipids and sterols
signal transduction - transfer information in response to the binding of a ligand or molecule
cell-cell interactions - glycoproteins are recognised by other cells
attachment to cytoskeleton/extracellular matrix - can be non-covalently bound to cytoskeleton, regulates cell shape and stabilises protein position in membrane. also facilitate cell-cell adhesion
54
what is the transmembrane region of integral proteins made up of?
amino acids with hydrophobic side chains
55
how is glucose transported into cells?
glucose transporters (GLUTs) - made up of 12 ⍺-helices
1. glucose binds to membrane outer surface, causing a conformational change
2. at inner surface, glucose is released and protein returns to its original conformation
56
what can cause ion channels to open?
ligand biding, electric potential, pH, temperature, pressure or lipids
57
what do potassium channels do in the
- cardiovascular system?
- epithelial cells?
- regulate heartbeat
| - regulate passage of salt and water
58
how many subunits do potassium channels have?
4
| - each subunit has two transmembrane helices and a pore half helix
59
what do mechanosensitive channels respond to?
mechanical stress (touch, sound and gravity)
60
what are the critical roles of ß barrel proteins?
cell structure and morphology, nutrient acquisition, protection of bacteria against toxic threats
61
what are peripheral membrane proteins?
proteins that bind to the surface of the membrane or to an integral protein
62
how do peripheral proteins bind?
interact with anionic lipid head-groups or with charged groups on another protein
63
what are lipid binding molecules?
positively charged surfaces on a peripheral protein that recognise specific lipids in the membrane - means they bind in specific places
64
what type of membrane proteins are cytoskeleton?
peripheral membrane proteins
- form a scaffold on the cytosol side of membrane
- attach to integral proteins
- in RBC's this is important as cytoskeleton allows cell to undergo stress without fragmentation
65
what is hereditary elliptocytosis?
condition of RBC's which causes disruption of interactions with the cytoskeleton, forming elliptically shaped cells - causes anaemia
66
what is hereditary spherocytosis?
condition of RBC's as a result of loss of cohesion between plasma membrane and cytoskeleton due to defective anchoring - cells become spherocyte
67
define dementia
an umbrella term for the serious deterioration in mental functions, such as memory, language, orientation and judgement
alzheimers is the most common cause
68
what are the clinical features of alzheimers?
- amnesia
- aphasia (language problems)
- agnosia (difficulty recognising and naming objects)
- apraxia (difficulties in complex tasks)
- visuospatial difficulties
- functional impairment
- mood disorders
- psychosis
- personality change
69
what causes alzheimers?
neurons malfunction causing the chemical and electrical signalling to go wrong - the nerve cells die and the connections deteriorate
- post mortems find senile plaques made of amyloid-beta peptide
70
what are amyloid-beta peptides?
short peptides derived from the larger membrane bound amyloid precursor protein (APP), goes onto form senile plaques which are abundant in alzheimer brains
71
how does cholesterol impact alzheimers?
- more amyloid plaques in those dying of heart disease
| - in cholesterol rich areas of the membrane there is more cleavage of APP and so more amyloid-beta peptide is made
72
what is an O-linked glycoprotein?
a carbohydrate attached to the oxygen atom in the side chain of serine or threonine
- often 2-5 sugars
73
what is an N-linked glycoprotein?
carbohydrate attached to the amide nitrogen atom in the side chain of asparagine
- usually large and branched with up to 30-40 sugars
74
what sequence must the amino acids be for an asparagine residue to accept an oligosaccaride?
asn - X - ser
| asn - X - thr
75
why do cells communicate with each other?
- regulate their development and organisation into tissues
- control their growth and division
- co-ordinate their functions
76
give an example of a disease where cell signal is lost/ no longer sent
Type 1 diabetes
- beta cells are damaged
- so no insulin produced
- insulin receptors do not mobilise glucose transporters
77
give an example of a disease where the target cell ignores the signal
type 2 diabetes
- insulin binds to receptors
- but receptors do not send signal to mobilise glucose transporters
78
give an example of a disease where the signal doesn't reach its target
multiple sclerosis
- damage to myelin sheath
- signal from brain doesn't reach extremities
79
give an example of a disease where there is too much signal
brain damage (ie stroke)
- blood vessels blocked so neurone cells die due to lack of O2
- glutamate released which causes neighbouring cells to die
80
give an example of a disease where there are multiple breakdowns
cancer
- hallmarks of cancer
- eg cell proliferation - too much cell signal so too much growth factor produced so too much cell division
81
what is autocrine signalling?
where a cell produces the signal and has the receptors
82
what is endocrine signalling?
signal is released from a cell into the blood stream and travels to a remote target cell
83
what is paracrine signalling?
signal is released to a nearby cell
84
what is juxtacrine signalling?
contact signalling by plasma membrane-bound molecules (eg T helper cells and antigen presenting cells)
85
what is gap junction signalling?
physical connection between cells via channels (eg cardiomyocytes)-allows movement of some cytoplasmic contents between cells
86
give 4 examples of extracellular signalling molecules (1st messengers)
- growth factors
- neurotransmitters
- hormones
- cytokines
87
what is the difference between exocrine, endocrine and paracrine hormones?
exocrine - secreted directly into the blood
endocrine - secreted into ducts first: distant cells
paracrine - diffuse through interstitial tissues to target cells - act on nearby cells
88
what are the four classes of receptor?
```
(cell surface)
- ligand gated ion channel
- G protein coupled receptor
- receptor tyrosine kinase
(intracellular)
- nuclear hormone receptor
```
89
ligand-gated ion channels
- ionotropic receptors
- binding and opening is very fast - involved in fast synaptic transmission
- ligand binding site on extracellular side
- example - nicotinic acetylcholine receptor (increases Na+ and K+ permeability, Na+ in and K+ out - causes membrane depolarisation)
- also GABA, GlyR and 5-HT3R
90
G-protein-coupled receptors
- metabotropic (ie no pore but triggers signalling response) or heptahelical receptors
- couple to an intracellular effector system via a G-protein
- examples - muscarinic ACh receptor, adrenoceptors, angiotensin II receptors
91
what does the renin-angiotensin system control
blood pressure (negative feedback system)
92
outline RAS
- low blood pressure = renin released by kidneys
- renin converts angiotensinogen to (through cleaving) angiotensin I
- ACE 1 (from lungs and kidney) converts angiotensin I to agiotensin II
- agiotensin II has a variety of effects to increase blood pressure
93
what affects does angiotensin II have
- increase nervous activity (contraction of vessels)
- retention of water
- aldosterone secretion (retention of water)
- vasoconstriction
- ADH secretion (reabsorption of water in collecting duct)
94
why is understanding RAS important?
plays an important role in pathogenesis of heart failure (all work by reducing BP)
- can control bp by inhibiting renin release or stop trigger of RAS syestem
- ACE inhibitors
- AT1 receptor antagonists
- aldosterone receptor antagonists
- example of negative feedback
95
kinase-linked receptors
- single transmembrane helix with a large extracellular binding domain and an intracellular catalytic domain
- some receptors are enzymes themselves- catalytic receptors
- some act through cytoplasmic tyrosine kinases
- extracellular c terminus and intercellular n terminal
96
nuclear hormone receptors
- intracellular receptors in the cytosol or nucleus - ligand activated transcription factors
- Regulate gene transcription
- monomeric structure
- hormones diffuse across membrane Interact with cytosolic or nuclear receptors and form hormone-receptor complexes, bind to regions of DNA and affect gene transcription
- examples - steroid and thyroid hormones
97
what are the 5 neurotransmitters?
- acetylcholine (ACh)
- monoamines (noradrenaline, adrenaline, dopamine, histamine, serotonin)
- amino acids (glutamate, aspartate, glycine, GABA)
- peptides (endorphins, substance P, neurokinins, neurotensin)
- lipids (anandamide)
98
outline the lifecycle of a neurotransmitter
1. synthesis
2. storage - synaptic vesicles
3. release
4. receptor activation
5. neurotransmitter inactivation and reuptake
99
what are 5 treatments for depression
1. monoamine reuptake inhibitors (tricyclic antidepressants, selective seretonin reuptake inhibitors, seretonin/noradrenaline reuptake inhibitors)
2. monoamine oxidase inhibitors
3. atypical antidepressants
4. electroconvulsive therapy
5. mood-stabilising drugs
100
how do reuptake inhibitors work?
prevent neurotransmitters going back into presynaptic cleft so they stay in synapse longer and more stimulation of receptors
101
how do autoreceptor blockers work?
inhibit negative feedback, Ca2+ still released into presynaptic cleft so neurotransmitter still released into synapse
102
how do monoamine oxidase inhibitors work?
stop breakdown of serotonin, so more serotonin in vesicles so more released into synapse
103
what type of signalling do gasotransmitters take part in?
paracrine - pass readily across cell membranes
104
outline the enzymatic production of NO
L-arginine converts to NO + L-citrulline aided by eNOS, nNOS and iNOS
105
outline the enzymatic production of CO
haem + O2 = NADPH
| HO-2 and HO-1 remove Fe2+ and bilirubin = CO
106
outline the enzymatic production of H2S
homocysteine with CBS = cystathionine
cystathionine with CSE = L-cysteine
L-cysteine with CSE, CBS, 3MST = H2S
107
where do we get cholesterol from?
25% from diet, 75% synthesised in the liver
108
how soluble is cholesterol?
insoluble in blood plasma so transported with a carrier - lipoprotein
109
give the 4 classifications of lipoproteins from biggest to smallest and their function
Chylomicrons, VLDL - large not v dense, made in intestines - transport dietray fats from intestines to tissues
VLDL - transport lipids made in liver to peripheral tissues
LDL - smaller, denser- main cholesterol carrier, provide cholesterol to peripheral tissues
HDL - transport cholesterol to liver from peripheral tissues "good cholesterol"
110
what is an apolipoprotein?
- single protein strand that runs around the outside of lipoprotein
- allows lipids to be transported to areas they are required in the body
- determine start and end points for cholesterol transport
- LIPID+APOLIPOPROTEIN = LIPOPROTEIN
111
where is cholesterol found in a lipoprotein?
free cholesterol is found in the external monolayer
| cholesterol esters and triacylglycerols are in the hydrophobic particle core
112
what are the 4 major classes of apolipoprotein?
ApoA - present in HDL, mediates efflux from tissues and influx to liver
ApoB - facilitates LDL uptake into tissues
ApoC - activator of lipoprotein lipase
ApoE - stabilises LDL for cellular uptake
113
how is synthesis of apoplipoproteins regulated?
intestine - dietary fat intake= made according to demand
| liver - hormones (insulin, glucagon) and drugs (statins, alcohol)
114
what are the main functions of apolipoproteins?
- regulate key enzymes in lipoprotein metabolism
| - ligands for interaction with lipoprotein receptors, targeting lipoproteins to the correct tissues
115
what are the structural differences between LDL and HDL?
LDL has ApoB apolipoprotiens
HDL has Apo A-I and Apo A-II, so highly resistant to being oxidised = anti inflamatory properties
(LDL oxidation = chief culprit of CV disease in form artherosclerosis)
116
What are the different types of membrane transport?
- Passive transport (simple diffusion, facilitated diffusion)
- active transport (ATP-driven and ion-driven)
117
Describe simple diffusion.
- Type of passive transport
- Can be used by gases, hydrophobic molecules and small polar molecules
- No metabolic energy required
- No specificity
- Rate of diffusion proportional to conc. gradient
118
Describe facilitated diffusion.
- Type of passive transport
- Occurs down the conc. gradient
- No metabolic energy required
- Depends on integral membrane proteins (carriers, permeates, channels, transporters)
- Specific
- Effected by temperature and pH
- Able to be inhibited
119
What are ionophores?
- Example of facilitated diffusion
- Ion carrying proteins
- Carrier ionophores (carry molecule across) or channel-forming ionophores (molecule passes through channel)
120
What are ion channels?
- Example of facilitated diffusion
- Allow rapid and gated passage of ions
- Highly selective- stimulated by a specific stimulus that allows it to open
- Down conc. gradient
- Essential for maintaining osmotic balance, signal transduction and nerve impulses
121
How is glucose transported across the membrane?
- Integral membrane protein GLUT1
- Facilitated diffusion
- GLUT1 has 12 alpha helical transmembrane domains and a central pore where glucose binds
- Transporter undergoes conformational change when glucoses binds
- Important that glucose conc. is higher outside of the cell
122
How is a low conc. of glucose maintained within the cell?
Hexokinase forms G6P from glucose using ATP
123
What are aquaporins?
- Example of facilitated diffusion
- Water channel proteins required for the bulk flow of H2O across membranes
- 6 transmembrane alpha helices come together to form a pore
- Tetramer with four pores through which H2O can pass
- Abundant in erythrocytes and kidney cells
124
Describe ATP-driven active transport.
- Na+, K+, Ca+ and H+ transport is directly coupled to ATP hydrolysis
- There is a high conc. of K+ and low conc. of Na+ inside the cell = Na+/K+ gradient
- Facilitated by Na+/K+ ATPase
- Na+/K+ ATPase is a tetramer with two alpha and two beta subunits. It pumps 3 Na+ ions out and 2K+ ions into the cell which polarises the cell membrane
- ATP hydrolysis induces conformational changes, pumping Na+ and K+ against their conc. gradients
- coupled system: ATP is not hydrolysed unless Na and K+ are transported and visa versa
125
Describe ion-driven active transport.
- Coupled to the movement of an ion (Na+ or H+) down its conc. gradient
- Symports: both molecules move in the same direction, e.g. Na+/glucose transporter
- Antiports: both molecules move in opposite directions, e.g. Na+/Ca2+ exchanger
126
What are Cardiac Glycosides (Cardiotonic Steroids)?
- Used in congestive heart failure
- Naturally occurring plant steroids (e.g. Digitalin, Ouabain, Digoxin, Digitoxin) that inhibit Na+/K+ ATPases
- In healthy people digitalis causes heart failure but in lower doses it strengthens the heart beat of patients with congestive heart failure- used at treatment
- Inhibition of Na+/K+ ATPase leads to increased conc. of Na+ inside the cell as less pumped out and a decreased Na+ gradient across the membrane. This decreases Na+/Ca2+ exchange, so there is a greater conc. of Ca2+ inside the cell. Increased intracellular Ca2+ leads to enhanced strength of cardiac muscle contraction
127
How do intestinal epithelial cells transfer molecules from the intestinal lumen to the blood?
- Movement of glucose from the lumen into the cell is driven by movement of Na+ ions down the conc. gradient (Na+/glucose transporter transports both into cells)
- Glucose diffuses into the blood via GLUT1
- Low conc. of intracellular Na+ is maintained by Na+/K+ ATPase
- As glucose flows through cell, water follows by osmosis due to osmotic gradient
128
How does oral rehydration therapy work?
Uptake of glucose is dependant on Na+ so an oral solution of both glucose and Na+ is given. This increases osmotic pressure in epithelial cells, so H2O follows.
129
How are macromolecules moved across membranes?
Exoctyosis/endocytosis
130
What is exocytosis and what are its different types?
Exocytosis is the excretion/secretion of molecules. It can be constitutive or regulated.
131
What is constitutive exocytosis?
Constitutive exocytosis is used by all cells. Secreted proteins & plasma membrane. It is continuous or regulated.
132
What is regulated exocytosis?
Regulated exocytosis is used only by specialised cells such as neurones and pancreatic cells. It is Ca2+ dependent.
133
What is endocytosis and what are its different types?
Endocytosis is the ingestion/uptake of molecules. It can be phagocytosis, pinocytosis or receptor-mediated endocytosis
134
What is phagocytosis?
- Ingestion of large particles (bacteria/debris)
- Specialist cells - macrophages, neutrophils, dendritic cells
- Bacteria/debris is degraded by enzymes in the lysosomes and the remnants are displayed on the surface of the phagocyte to alert other immune cells to fight the infection
135
What is pinocytosis?
- Cell 'drinking'
- Uptake of fluid
- All cells are able to do this
136
What is receptor-mediated endocytosis?
- Selective (receptor recognition)
- Uptake from extracellular fluid via Cathrin-coated vesicles and pits
- The ligand being taken up must first bind to a specific cell surface receptor
- Receptor-molecule complexes accumulate in a Cathrin-coated pit and are endocytose in a clathrin-coated vesicle
- fuse with endosome and lysosomes = lysosome degradation
- cell membrane recycled by cell
- Way of concentrating specific macromolecules that are present at low conc.
- E.g. cholesterol uptake via LDL receptor
- Receptor-mediated endocytosis may be exploited by viruses to gain entry to cells (eg. rhinovirus and covid-19)
137
what is signal transduction
also known as intracellular signalling
| = converts extracellular signal into cellular response
138
what are the 3 main stages of signal transduction
1- reception
2- signal transduction
3- cellular response
139
what are the 4 Important features of signal transduction pathways
Hierarchy, amplification, specificity, complexity
140
what is hierarchry
things happen in a particular order to transmit signal from outside to inside cell
eg. 1st messenger, receptor, G protein, effector enzyme, 2nd messenger, protein kinase, target protein , cellular response
141
what is amplification
A single molecule of first messenger can induce many downstream signalling molecules and lead to a larger cellular response
142
what are the main points of amplification
1-G-protein activation - is activated by the receptor as long as the receptor remains in an activated state = single G-protein molecule can activate many molecules of effector enzyme
2-Effector enzyme -catalyse reactions without being used up. So a single effector enzyme molecule can catalyse the production of many molecules of second messenger.
3- ) Protein kinase - single protein kinase molecule can catalyse the phosphorylation of many molecules of protein substrate
143
what is the process in glycogen breakdown pathway
```
1- binding of epinephrine to G protein receptor
2- activates G protein
3- g protein activates adenylyl cyclase
4- which turns ATP to cAMP
5- a activates protein kinase A
6- activates phosphorylase kinase
7- activates glycogen phosphorylase
8- turns glycogen to glucose-1-phosphate
```
144
what is specificity
signal transduction pathways are highly specific
| one stimulus can cause different cellular function depending on what receptor and where it binds
145
what hormone/1st messenger activates different processes on different cells
adrenaline
| as it binds to different receptors and activates different signalling pathways
146
3 examples of different cellular functions regulated by Adreneline
- in muscle/liver = glycogen degradation
- in adipose = fatty acid production
- in cardiovascular = increases heart rate and blood pressure
147
what is complexity
Signal transduction pathways are highly COMPLEX
due to number of different types of receptor, G-proteins, effector enzymes, second messengers, protein kinases and downstream substrates
148
what is 'cross talk'
when signalling pathways interact with one another
| allows for fine tuning of cellular responses
149
what are Main components of signal transduction pathways
G proteins, effector enzymes, 2nd messengers, kinases
150
what are G proteins
Guanine nucleotide binding proteins
151
what does GTP do
high energy molecule that activates G proteins
152
what are G proteins also known as
GTPases
153
what do G proteins do to GTP
hydrolyse GTP to GDP
| create lipid anchored membrane proteins to associate them with different parts of membrane via process prenylation
154
what are the 2 major groups of G proteins
- G prtoeins (receptor associated) :
heterotrimeric(3 subunits) (a,b,y subunits)
- small GTPases:
monomeric (one subunit)- Rho and Ras
155
why are G proteins reffered to as molecular switches
- switched on by ligand binding to receptor
| - switched off by intrinsic GTPase activity
156
how are G proteins switched off
removal of phosphate group by GTPase from GTP = GDP
| off always bound to GDP
157
how are G proteins switched on
binding of ligand = confrontational change = allows G protein bind to receptor = GDP exchanged to GTP
on always bound to GTP
158
What is the Gs protein
trimeric G protein
combination of a, B, and gamma subunits
alpha subunit is the one that binds to GDP/GTP
159
step 1 - what happens when ligands bind to Gs protein
confrontational change in receptor
causes the G-protein to release GDP and swap it for GTP = ON
alpha unit separates from beta and gamma subunits
160
step 2- what happens in the activation of effector enzyme and 2nd messnegr
GTP-bound Gs-alpha subunit binds to and activates adenylyl cyclase which catalyses conversion of ATP to the second messenger cyclic AMP
161
step 3 - what happens in hydrolysis of GTP to GDP
The GTPase activity of the Gs-alpha subunit hydrolyses GTP to GDP (with release of inorganic phosphate, Pi)= G-protein “OFF” state
162
Step 4 - what happens in re-association
The GDP-bound alpha subunit then re-associates with the beta and gamma subunits
system resets itself
163
step 5- what happens to the 2nd messenger
loss of second messenger
| Cyclic AMP is broken down to AMP by phosphodiesterases
164
what happens if the receptor is still active?
process will be repeated so the signal continues to be amplified
165
what enzyme and 2nd messenger does Gs effect?
- adenylate cyclase - stimulation
| - increase in cAMP
166
what enzyme and 2nd messenger does Gai effect?
- adenylate cyclase - inhibition
| - decrease in cAMP
167
what enzyme and 2nd messenger does Gaq effect?
- phospholipase C stimulation
| - increase DAG, IP3
168
what 2 G proteins have opposing effects
Gs and Gi on cyclic AMP levels
169
what G protein does cholera toxin target
Gs
170
what G protein does pertussis toxin (whooping cough) target
Gi
171
what is the name of the bacteria causing cholera
Vibrio cholerae bacteria in contaminated water & food
172
what is the treatment for cholera
Oral rehydration therapy
| water, salts, glucose
173
what is the mechanism for the cholera toxin on Gs
- Cholera toxin prevents GTPase activity of Gs =therefore GTP remains bound to Gs and it stays in the “ON” state
- over-stimulation of adenylate cyclase and accumulation of cyclic AMP
- intestinal epithelial cells, elevated cAMP increases loss of Cl- ions through chloride channels
- resultant osmotic gradient= water excreted into the intestinal lumen= diarrhoea and dehydration
174
what bacteria is whooping cough caused by
Bordetella pertussis
175
what is the virulence factor of whooping cough
Pertussis toxin
176
what is the virulence factor of cholera
Virulence factor = Cholera toxin
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mechanism of pertussis toxin on Gi
inhibition of Gi G-protein
- Pertussis toxin prevents GDP/GTP exchange by Gi
- the Gi protein is locked in the “off"
- unable to inhibit adenylate cyclase, resulting in accumulation of cyclic AMP
- physiological effects = increased insulin secretion and increased sensitivity to histamine
178
what are second messengers
• Short-acting intracellular molecules that are rapidly formed or released as a result of receptor activation
179
Five common second messengers
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• Cyclic AMP (cAMP)
• Cyclic GMP (cGMP)
• Diacylglycerol (DAG)
• Inositol 1,4,5-trisphosphate (IP3)
- intracellular calcium (Ca2+i)
```
180
production of cAMP?
cyclic AMP from ATP; a reaction catalysed by adenylate cyclase (also called adenylyl cyclase)
181
production of cGMP?
cyclic GMP from GTP; a reaction catalysed by guanylate cyclase (also called guanylyl cyclase
182
what breaks down cAMP and cGMP
Cyclases and Phosphodiesterases (PDEs)
- some Phosphodiesterases break down cAMP or anther one will specifically breakdown cGMP
- same cyclases can break both down
183
well known PDE inhibitors?
caffeine and Viagra
- Caffine inhibits breakdown cAMP
- Viagra inhibits breakdown of cGMP
184
step 1 of Gq activation?
- ligand binding GDP/GTP exchange,ON, due to confrontational change
- alpha subunit dissociates from beta/gamma subunits
185
step 2 of Gq activation?
activation of effector enzyme and production 2nd messenger
- alpah subunit binds Phospholipase C
- catalyses production of two different second messengers 1,2-diacyclycerol (DAG) and inositol 1,4,5-trisphosphate (IP3)
- release of stored Ca2+ ions into the cytosol due to interaction of ER with IP3
- DAG and stimulates protein kinase C (PKC) that phosphorylates target proteins leading to cellular response
- release of calcium also stimulates responses
186
why is Ca importnant 2nd messenger?
(NB: not “formed” but is “released” into the cytosol)
1-activate various molecules (e.g calcium-dependent kinases) to modulate cellular function
2- trigger opening of calcium channels in the plasma membrane = more Ca floods in
187
how is calcium removed from the cell
taken back up into the ER through a calcium ATPase in the ER membrane, or is pumped or exchanged out of the cell
188
what are protein kinases?
- enzymes that transfer Pi from ATP to a SPECIFIC AA residue: Ser, Thr,Thy on a SPECIFIC protein
- phosphorylation may activate or inhibit protein function
189
what are the 3 main groups of protein kinase
- serine/theronine kinases
- thyrosine kinases
- dual specificity kinases
190
function of serine/theronine kinases
- phosphorlyate Ser and or Thr residues
| - eg. PKA,PKC,PKG
191
function of thyrosine kinases
-phosphorlyte only Tyr residues
192
function of dual specificity kinases
phosphorlyate Ser, Thr, Tyr resides
| eg.MAP kinase kinases- MKKs
193
how is LDL receptor gene expression regulated (endocytosis of LDL)?
intracellular cholesterol concentration, binding of LDL to receptor (binds to apoB or apoE) stimulates endocytosis into hepatic or peripheral cells
- form a coated vesicle
- become uncoated and triskelions return to plasma membrane
- ph decreases and LDL -> amino acids, cholesterol, fatty acids
- receptors recycled to membrane
194
what does high intracellular levels of cholesterol do to LDL uptake and so why is excess LDL bad?
suppresses LDL receptor synthesis - prevents excess cholesterol uptake
as a result excess cholesterol remains in the blood as LDL
195
what does high levels of HDL correlate with?
low incidence of atherosclerosis as is resistant to oxidation through apoAand scavenges excess cholesterol, returns to liver and is excreted as bile
REVERSE CHOLESTEROL TRANSPORT
196
what is normal level of cholesterol in a healthy adult?
175 mg/100ml
197
what are the cardiac, cerebral and peripheral manifestations of atherosclerosis?
cardiac - chest pain, palpitations, hear tattack
cerebral - stroke, cerebral haemorrhage
peripheral - pain, ischaemia , ulceration and gangrene
198
what is the cholesterol synthetic pathway?
1. HMG-CoA
- ---> HMG-CoA reductase
2. mevalonate (key intermediate)
3. IPP
4. FPP
5. Squalene (fatty precursor)
6. cholesterol
199
what do statins do?
prevent cholesterol synthesis in the liver as inhibit HMG-CoA reductase ---> less mevalonate ---> less cholesterol ---> more expression of LDL receptor ---> more uptake of cholesterol from blood
200
what is pleiotropism?
extra unintended affects from a drug - could be good or bad
201
what are statins pleiotropic effects?
- improve endothelial dysfunction
- antioxidant
- inhibition of inflammatory response
- stabilise atherosclerotic plaques
202
what do kinases do
put phosphate groups onto molecules
| - very specific action
203
what do phosphateses do
remove phosphate groups from AA
| -not very specific
204
how can kinases modulate protein function
- Phosphorylation to induce change in protein conformation / function
- § Phosphorylation of a transcription factor that alters gene transcription and hence protein expression levels
205
Protein kinase inhibitors as therapeutic agents in...
```
Cancer
Cardiovascular Disease
HIV/AIDS
Rheumatoid Arthritis
Alzheimer’s Disease
```
206
how can disregulation in kinases be linked to cancer?
in solid tumours changes in protein kinase expression levels and activities + posttranslational modifications can contribute to cancer development
207
what are kinase therapies used for?
- as a cancer drug therapy
- Conventional chemotherapy can be ineffective and harmful- by combining chemotherapeutics and protein kinase inhibitors= more succesfull
208
functions of lipids?
- energy storage - as fat deposits
- major components of cell membranes
- signalling molecules
- required to solubilise fat soluble vitamins
- biosynthetic precursors (eg. steroid hormones from cholesterol)
209
function of lipoproteins?
-means of lipid (trigyceriol and cholesterol) transport in blood
210
LIPID+ APOLIPOPROTEIN=?
LIPOPROTEIN
211
similarities between apoA and apoB structure
- surface monolayer of phospholipids and free cholesterol
| - hydrophobic core of mainly cholesterol esters and some triglycerides
212
How are lipids transported by lipoproteins?
- Triglycerides hydrolysed by lipoprotein lipase to fatty acids= taken up by target tissues for energy production (eg muscle) or stored (adipose tissue).
- this means Chylomicrons shrink, remnants transported back to liver.
- VLDL transport lipids to target tissues- same process as above
213
how to HDL work?
- HDL= opposite function to LDL and can remove cholesterol from the tissues.
- HDL synthesised in the blood and acquire their cholesterol by extracting it from cell membranes and transporting back to the liver.
- The liver is the only organ that can dispose of significant quantities of cholesterol, primarily in the form of bile salts.
214
what is the main function of lipoproteins
- Membrane-bound receptors to enable cholesterol entry to hepatic and peripheral cell
- Lipoprotein receptors=necessary for cholesterol uptake into cells.
215
what happens to VLDL remnants?
-VLDL remnants remain in the blood, become LDL that are then taken up by target cells by the LDL receptor, digested in the lysosome to release the cholesterol.
216
how is LDL taken into cells?
- apoB recognised by receptors in clathrin coated pit
- membranes takes this in by endocytosis
- clathrin released back onto surface
- receptor and cholesterol processed in vesicle then released to endosome
- receptors released from LDL receptor
- receptors recycled back to plasma membrane
- cholesterol processed by lysosome and degraded to fatty acidsf
217
how does reverse cholesterol transport work?
- nascent HDL extracts cholesterol form cell membrane of macrophage and swells
- turns to mature HDL
- recognisable by scavenger cells on liver
- cholesterol in membrane of HDL processed in liver and secreted in bile
218
mutations with LDL receptors are a common cause of?
FAMILIAL HYPERCHOLESTEROLAEMIA= cells lacking LDL receptors= amount LDL in blood increases
219
major consistent of arhteroscelroic plaque?
cholesterol-enriched LDL
220
how do arhterocelrtic plaques form?
1-fatty streak in lumen (at young ages - process takes decades)
2- fibrous plaque forms = reduces area blood flows
3- advanced plaque = complete occlusion - plaque rupture on top of blood clot = MF
221
what drugs control hypercholesterolaemia?
-Statins
Prevent cholesterol synthesis in the liver
-Cholesterol absorption inhibitors
Prevent uptake from the intestine
-Fibrates
Reduce triglycerides and increase HDL, less effective
222
how do statins work?
- inhibit HMG-CoA reductase
- decreases cholesterol synthesis
- increased expression of LDL receptor
- increase uptake cholesterol from blood
223
what are Ras and Rho?
small G proteins only function when prenylated
224
what prenylates Rho and Ras
FPP and GGPP = called isoprenoids
- required for a process known as PRENYLATION (sticking a lipid tail onto intracellular signalling molecules) = so associated with cell membrane and signal cascade
- Ras is farnesylated whilst Rho is geranylgeranylated
225
why is lipid tail G proteins important?
Without lipid tail G proteins floating in cytosol and cant signal
226
what is PCSK9?
* Protease enzyme expressed by liver & intestine
| * Important role in lipid metabolism
227
why is PCSK9 important?
* Promotes intracellular degradation of LDL-R
* Prevents recycling of LDL-R to cell surface
* Reduces LDL-R population on cell membrane
* Link between PCSK9 mutations and coronary heart disease
* Implicated in familial hypercholesterolaemia
* Genetic variation in PCSK9 (gain/loss of function)- beneficial if loss of function? LDL receptors remain in tact?
228
how does PCSK9 work?
- PCSK9 binds LDL and internalised with LDL receptor
- degrades receptor and cannot be recycled to accept LDL at surface
- PCSK9 inhibitor stops this from happening
229
what are neutral lipids a result of?
-the esterification of a fatty acid to triacylglycerol or a sterol (such as cholesterol) to sterol ester
230
what is a sterol ester
- cholesteral with lipid tail attached to one side of cholesteral
- At low concentrations, neutral lipids are dispersed between the leaflets =no deformation
- As their concentration increases, neutral lipids accumulate (de-mixing- membranes start deforming ) = may threaten burst membrane
231
what are the 3 proteins that help lipids move between leaflets?
* P type fliptases – proteins which help lipids move from outside to inside of cells-require energy from ATTP
* ABC fliptase- help proteins move from inner to outer membrane – ATP
* Scrambalses- can move proteins in any directions- no ATP but Ca needed
232
what membrane curvature do cylindrical lipids from?
cylindrical shape so can pack next to each other tightly= flat membrane
233
what membrane curvature do conical shape lipids form?
negative curvature 'U shape'
234
what membrane curvature do inverted conical lipids form?
only one tail= create a positive curvature 'N shape'
235
why CO bad?
- highly toxic
- exposure is common
- chronic exposure leads long term neurological CV disorders
236
why is CO good?
- endogenous signalling molecule
-cardioprotective
neuroprotective
237
what are. gasotransmitters?
gaseous molecules synthesised in the body. They include nitric oxide (NO), carbon monoxide (CO) and hydrogen sulphide (H2S).
238
what other effects do kinase linked enzymes have?
```
•Act by indirectly regulating gene transcription
•Roles in controlling:
cell division
tissue repair
apoptosis
-cellualr effects: cellular poliferation
```
239
what is angiotensin II made of?
* Octapeptide
* Angiotensin II receptors are GPCRs]
* only 8 AA
240
what do hormones regulate?
```
•body energy needs
•protein and nucleic acid metabolism
•mineral and electrolyte metabolism
•synthesis and release of hormones
Activity is regulated through negative or positive feedback mechanisms
```
241
how do cells communicate with one another?
* Remote signalling by secreted molecules
* Contact signalling by plasma membrane-bound molecules (‘Juxtacrine’ signalling)
* Contact signalling via gap junctions- channels that form between two cells and allow molecules to move across
242
which isomoer of glucose is the glucose transporter specific for?
D-isomer of glucose relative to the L-isomer or to other related sugars
243
what ions are directly transported into a cell by ATP hydrolysis?
Na+, K+, Ca2+ and H+ transport is directly coupled to ATP hydrolysis
e.g. Na+/K+ ATPase
§ Coupled system: ATP is not hydrolysed unless ions (eg.Na+ and K+) are transported and vice versa
244
what does the Na+/K+ gradient ensure?
* controls cell volume
* makes nerve and muscle cells electrically excitable
* facilitates ion-driven active transport of amino acids and sugars
245
what is the symport system?
both molecules travel in same direction
246
what do intestinal epithelial cells do?
•Line the lumen of small intestine
•Large surface area for absorption from villi and microvilli
•Absorb nutrients from digested food (sugars, amino acids, lipids, etc.)
•Transfer nutrients into the blood
•Similar cells found in kidney tubules
-cells are polarised - have 2 distinct sides: the apical (brush-border) membrane that faces the lumen of the gut and the basolateral surface that faces the bloodstream
