Metabolism, synthesis and enzyme-linked receptors Flashcards
1
Q
What happens to excess carbohydrates?
A
Excess is stored either as glycogen in the liver and muscle, or as fat in adipose tissue, for mobilisation when food is scarce or there’s a sudden increase in demand
2
Q
What is hypoglycaemia and the symptoms?
A
Deficiency of glucose in the blood stream (below 4mmol/L) (below 70mg/dL)
Symptoms include muscle weakness, loss of coordination, sweating and hypoglycaemic coma and death
3
Q
What is hyperglycaemia and its symptoms?
A
An excess of glucose in the blood stream (above 7mmol/L) (above 126mg/dL)
Symptoms are increased thirst, short concentration span, blurred vision, frequent urination, fatigue and weight loss
4
Q
What does hyperglycaemia involve on a histological level?
A
The non-enzymatic modification of proteins (cataracts or lipoproteins important in atherosclerosis)
5
Q
How does glycogen synthesis occur?
A
Glycogenin reacts with UDP-glucose and catalyses the addition of the first glucose molecule. This then acts as a substrate for glycogen synthase
6
Q
Why can’t glucose be stored?
A
It is osmotically active
7
Q
What makes glycogen a better energy storage source than fat?
A
Fat cannot be mobilised as readily as glycogen, nor can it be used as an energy source in the absence of oxygen
8
Q
What are the 2 main activities of the debranching enzymes responsible for glycogen breakdown?
A
Transferase activity moves the last glucose residues to the non-reducing end of an existing chain and glucosidase removes the 1-6 link, releasing glucose
9
Q
What 4 enzymes are required for glycogen breakdown?
A
Phosphorylase breaks the alpha 1-4 links
Transferase
Debranching enzyme alpha-1-6
Phosphoglucomutase converts G1P to G6P
10
Q
What enzyme is required for conversion of G6P to glucose?
A
G6-phosphatase
11
Q
What happens to glucose-6-phosphate formed in the liver?
A
It’s dephosphorylated and secreted into the blood to maintain the 5mmol/L blood sugar
12
Q
What allows glycogenolysis to be switched on very rapidly?
A
Many phosphorylases are bound to each glycogen particle
13
Q
Phosphorylase is an allosteric enzyme. What does this mean?
A
It has sites away from its active site that control its activity by inducing shape changes in the protein
14
Q
What converts inactive glycogen phosphorylase b to active glycogen phosphorylase a?
A
A special enzyme called phosphorylase b kinase, which transfers a phosphate from an ATP to one serine residue on each phosphorylase subunit
15
Q
What stimulates glycogenolysis in the liver and muscle?
A
Glucagon stimulates glycogenoolysis in the liver, while adrenaline stimulates it in muscle
16
Q
How is glycogen phosphorylase controlled in muscle tissue?
A
Glycogen phosphorylase b can be activated by 5’-AMP without being phosphorylated. 5’-AMP is a molecule formed when ATP stores are depleted. ATP binds to the same allosteric site as 5’-AMP and blocks activation, hence 5’-AMP only causing activation when ATP stores are depleted. Glucose-6-phosphate also blocks 5’-AMP activation
17
Q
How is glycogen phosphorylase controlled in the liver?
A
Glucose inhibits activated glycogen phosphorylase a
18
Q
What activates glycogen phosphorylase b in muscle tissue?
A
Ca2+ ions activate it to mediate glycogenolysis during muscle contraction
19
Q
In the liver, phosphorylase kinase is under dual regulation via what 2 different receptor types?
A
Its regulated through elevation of cAMP and activation of PKA, and via Ca2+ through the alpha adrenergic/ IP3 pathway
20
Q
What activates glycogen synthase?
A
ATP and G6P, as well as dephospho rylation by protein phosphatase-1
21
Q
What deactivates glycogen synthase?
A
Phosphorylation by PKA
22
Q
What activates glycogen phosphorylase?
A
Phosphorylation by phosphorylase b kinase
23
Q
What deactivates glycogen phosphorylase?
A
ATP and G6P, and dephospho rylation by protein phosphatase-1
24
Q
What’s the bodily daily requirement for glucose?
A
160g
25
How does the gluconeogenic pathway work?
The gluconeogenic pathway converts pyruvate to glucose. This takes place mostly in the liver and a little in the kidney. Kidney productions rise to 40% during starvation
26
What are the 3 most important substrates to gluconeogenesis?
The amino acid (alanine), lactate and glycerol
27
What process is gluconeogenesis usually accompanied by?
Ketogenesis
28
What can pyruvate and a number of amino acids be converted to for the first step of gluconeogenesis?
Oxaloacetic acid
29
What enzyme converts pyruvate to oxaloacetic acid?
Pyruvate carboxylase
30
What enzyme converts oxaloacetic acid to phosphoenol pyruvate?
Phosphoenol pyruvate carboxykinase
31
How does compartmentalisation affect the gluconeogenesis process?
Pyruvate must by uptaken into mitochondria before it's converted to oxaloacetate. Oxaloacetate must be converted to malate to exit the mitochondria before its resynthesises in the cytosol.
32
What inhibits pyruvate kinase?
The enzyme responsible for conversion of phosphoenol pyruvate to pyruvate is inhibited by glucagon
33
Where do fats come from?
Fats are either obtained from the diet or made de novo from carbohydrates
34
Give 4 roles of fats
Membranes
Uptake of lipid soluble vitamins
Being precursors of steroid hormones
Energy storage
35
How much energy is there in 1g of fat?
37kJ of energy
36
Name 3 important molecules to consider when discussing lipid metabolism
Cholesterol
Fatty acids
Triglycerides
37
What tissue uses fat as its preferred energy source?
Cardiac muscle tissue
38
What's the most common source of metabolic building blocks?
Dietary carbohydrates
39
Where are most fats synthesised?
In the liver
40
What are fatty acids?
Chains of methyl groups with a terminal carboxyl group. Any double bonds are in cis formation
41
What molecules does fatty acid synthesis require?
Acetyl CoA
ATP
NADPH
42
Where does NADPH come from?
A significant proportion of NADPH comes from the pentose phosphate pathway- a pathway active in hepatocytes when there's excess glucose intake
43
What does fatty acid synthesis involve as a process?
The sequential addition of 2 carbon units derived from acetyl CoA
44
How is acetyl CoA transferred to the cytosol from mitochondria?
Acetyl CoA reacts with oxaloacetate to form citrate, which moves across the bilayer membrane before being broken down back into oxaloacetate and acetyl CoA
45
What's the key first step in fatty synthesis?
Acetyl-CoA carboxylase takes acetyl CoA (2C), ATP and carbonate ions (HCO3-) and forms malonyl-CoA (C4), ADP and Pi
46
Via what feedback is this first step of fatty acid synthesis regulated?
A positive feedback mechanism
47
What activates the first step of fatty acid synthesis?
Citrate
48
What vitamin does the first step of fatty acid synthesis require?
Vitamin biotin
49
What is the second step of fatty acid synthesis?
Malonyl-CoA then reacts with ACP (acyl carrier protein) to activate malonyl CoA and prepare it for subsequent reactions. Another acetyl CoA binds to ACP, then malonyl-ACP and acetyl-ACP undergo a condensation reaction to form acetoactyl-ACP (C4)
50
What happens to acetoactyl-ACP in fatty acid synthesis?
Acetoactyl ACP undergoes reduction, dehydration and further reduction to form butyryl-ACP. Butyryl-ACP then reacts with another malonyl-ACP in a condensation reaction. The elongation process can continue until a 60 carbon chain is formed (palmitic acid)
51
What is fatty acid synthase?
A multi-functional complex of enzymes that exists as a dimer, oriented head to tail
52
Where is cholesterol synthesised?
Cholesterol is mostly synthesised in the ER
53
How does cholesterol synthesis begin?
Cholesterol synthesis starts with the activation of acetate, acetyl-CoA.
54
What is a major regulatory step in cholesterol synthesis?
The conversion of 3-hydroxy-3-methylglutaryl CoA (HMGCoA) to mevalonate.
55
How does cholesterol regulate its own synthesis?
Cholesterol inhibits HMGCoA reductase- the enzyme involved in its own synthesis. It's therefore difficult to reduce circulating cholesterol by diet alone, as endogenous synthesis will increase
56
What are the 3 steps of fatty acid degradation to release stored energy and where do they happen?
Mobilisation in the adipocytes.
Activation in the liver cytosol.
Degradation in the liver mitochondria.
57
What stimulates mobilisation of fatty acids?
Mobilisation is stimulated by hormones such as glucagon and adrenaline
58
How do glucagon and adrenaline act to mobilise fatty acids?
They stimulate a 7TM receptor protein to synthesise cAMP, which will then activate PKA, which phsophorylates triacylglycerol lipase, activating the enzyme. The active enzyme starts to break down triacylglycerol to diacylglycerol. This is then broken down by other lipase to form glycerol and free fatty acids
59
What happens to the glycerol formed in fatty acid mobilisation?
Glycerol is phosphorylated to glycerol-3-phosphate, which is then oxidised to dihydroacetone phosphate and isomerism to Glyceraldehyde-3-phosphate
60
What happens to most of the Glyceraldehyde-3-phosphate derived from glycerol in the liver?
Very little glyceraldehyde-3-phosphate goes towards glycolysis. Most goes to gluconeogenesis
61
What happens to fatty acids once they're mobilised?
They are transported to the liver and activated by acyl-CoA synthase in the cytoplasm. Acyl-CoA that's produced is transported across the inner mitochondrial membrane bound to carnitine, an alcohol
62
What can carnitine deficiency cause and why?
Carnitine deficiency can cause muscle weakness or even death, as carnitine is essential to fatty acid degradation and therefore energy release
63
What inhibits transport of the activated fatty acid chain into the IMM?
Malonyl CoA
64
How is Acyl CoA transported across the IMM?
Acyl CoA reacts with carnitine, catalysed by carnitine acyltransferase I, to form acyl carnitine. Acyl carnitine is transported via a translocase into the matrix. Acyl CoA is resynthesised via carnitine acyltransferase II
65
How does fatty acid oxidation occur?
Acyl CoA is degraded by sequential removal of 2 carbon units. As a result. FADH2, NADH and acetyl CoA are produced
66
What is the name of fatty acid oxidation?
ß-oxidation
67
How many ATPs are yielded from complete oxidation of palmitate?
106 molecules
68
What happens to most of the acetyl CoA produced from fatty acid degradation?
Most is converted to ketone bodies, and not much is used in the TCA cycle. Ketone bodies are a way in which the body can transport acetyl CoA for metabolism by non-hepatic tissue
69
How does ketogenesis occur?
Acetyl-CoA is converted to acetoacetyl-CoA, which is then converted to HMG-CoA. HMG-CoA is converted to acetoacetate, which can be reduced to 3-ß-hydroxybutyrate or non-enzymatically to acetone. The products of ketogenesis can be smelt on the breath of diabetics
70
What are ketone bodies for?
They are a major energy source for cardiac muscle and the renal cortex. During starvation, up to 75% of the brain's energy is derived from acetoacetate
71
Where are triglycerides broken down?
Adipocytes
72
How does insulin regulate fat metabolism?
Insulin increases glycolysis in the liver and fatty acid synthesis. This leads to increased triglycerides in adipose tissue and decrease in ß-oxidation
73
How do glucagon and adrenaline regulate fat metabolism?
They increase triglyceride mobilisation
74
What are the possible fates of amino acids?
Amino acids can be used or broken down, but cannot be stored
75
What are amino acids broken down into?
alpha-keto acid and ammonia
76
Why does nitrogen have to be safely removed in amino acid metabolism?
It's toxic
77
What happens to the nitrogen from amino acid metabolism in mammals?
Nitrogen is converted to the non-toxic neutral compound urea and excreted in urine
78
What are the 3 steps of conversion of amino acid nitrogen to urea?
Transamination
Formation of ammonia
Formation of urea
79
How much protein is broken down to form 1g of urea?
3g of protein
| due to the fact urea is 48% nitrogen by weight and protein is 16% nitrogen
80
What is the transamination process in the conversion of amino acid nitrogen to urea?
Amino acid A and alpha-keto acid B combine to form alpha-keto acid A and amino acid B via an enzyme called aminotransferase
81
Name 3 well-known alpha-keto acids
Alpha-ketoglutarate, oxaloacetate and pyruate
82
What are the 2 most important transaminase enzymes?
Alanine transaminase (ALT) and aspartate transaminase (AST)
83
What does ALT do?
ALT forms pyruvate and glutamate from alanine and alpha-ketoglutarate
84
What does AST do?
AST forms oxaloacetate and glutamate from aspartate and alpha-ketoglutarate
85
What can alpha-ketoglutarate, pyruvate and oxaloacetate be oxidised and converted to?
Glucose
86
What are high AST and ALT levels in the blood indicative of?
Liver damage
87
What happens to the glutamate formed from alanine and a-ketoglutarate?
Glutamate reacts with oxaloacetate to form a-ketoglutarate and aspartate, an intermediate in urea formation
88
What does ALT require the presence of?
Pyridoxal phosphate derived from vitamin B6
89
Which 2 amino acids don't undergo transamination?
Threonine and lysine
90
How can glutamate form ammonia directly?
Via the action of glutamate dehydrogenase in the mitochondrial matrix via oxidative deamination. This reaction is fully reversible and can use either NAD or NADP
91
What does free ammonia generated in tissue combine with to give glutamine?
Glutamate
92
What is glutamine the main transporter of?
Nitrogen
93
Where are enzymes for the urea cycle present?
In the liver, but not in muscle tissue
94
Where intracellularly does the urea cycle take place?
The mitochondria and cytoplasm
95
Name 3 substrates involved in the urea cycle
Bicarbonate, aspartate and ammonium ions
96
What's the first step in the generation of urea?
The reaction of CO2 from bicarbonate, and ammonium ions, to form carbamoyl phosphate. Carbamoyl phosphate then reacts with ornithine to form citrulline in the mitochondria.
97
What happens to citrulline formed in the mitochondria in urea formation?
Citrulline combines with aspartate to give arginino-succinate. The aspartate comes from the transamination reactions of glutamate with oxaloacetate
98
Where is arginino-succinate formed?
In cytosol
99
What is arginino-succinate broken down to and how?
Arginine-succinate is broken down into arginine, which then forms ornithine and urea via arginase, and fumarate, which goes on to form malate and reform oxaloacetate
100
In prolonged exercise or starvation, what are used for energy in muscle?
Branched amino acids (leucine, isoleucine and valine)
101
How is pyruvate converted to alanine in muscle tissue?
Branched chain amino acids are broken down to form NH4+ and carbon skeletons. NH4+ then ammonites pyruvate to form alanine
102
What happens to alanine formed in muscle tissue?
Alanine is transported to the liver via the blood
103
What happens to alanine transported to the liver from muscle tissue?
Alanine forms glutamate and ultimately urea, as well as reforming pyruvate via transamination
104
What's the use of pyruvate reformed in the liver?
Pyruvate is converted to glucose via gluconeogenesis. Glucose is then carried in the blood to muscle tissue, where glycogenesis can occur
105
What are the 2 uses of NH4+ produced from breakdown of branched chain amino acids in muscle tissue?
Amination of pyruvate to alanine to be sent to the liver. Formation of glutamine from glutamate, for glutamine to be transported to the liver for metabolism
106
What are the 2 categories of carbon skeletons from amino acid metabolism?
Ketogenic or glucogenic
107
What do ketogenic carbon skeletons go on to form?
Acetoacetyl CoA
108
What do glycogenic carbon skeletons fo on to form?
Glucose (via the TCA cycle and gluconeogenesis for non-hepatic tissue use)
109
What is positive nitrogen balance associated with?
Periods of growth, pregnancy, hypothyroidism and tissue repair
110
What is negative nitrogen balance associated with?
Negative nitrogen balance is associated with burns, trauma, fevers, hyperthyroidism, wasting diseases, and periods of fasting
111
How many transmembrane domains do enzyme-linked receptors tend to have?
1
112
What does activation of enzyme-linked receptors lead to?
Activation of receptor kinases and activation of multiple signalling pathways
113
Name 3 types of enzyme-linked receptors
Tyrosine kinase receptors
JAK/STAT receptors
Serine threonine receptor kinases
114
What are 3 substrates to tyrosine kinase receptors?
Insulin
Epidermal growth factor (EGF)
Platelet-derived growth factor (PDGF)
115
Name 2 substrates to JAK/STAT receptors?
Growth hormone and interferons
116
Name the substrate to serine threonine receptor kinases
TGFß
117
What are enzyme-linked receptors involved in the regulation of?
Cell growth, division, differentiation, survival and migration
118
What happens when an agonist binds to an enzyme-linked receptor?
The receptor often dimerises with a similar receptor, as conformational changes bring the receptors together to interact
119
What is one of the interactions that takes place between the 2 enzyme-linked receptors that dimerise?
The activation of tyrosine kinase enzymes within each receptors
120
What does activation of tyrosine kinase enzymes within each receptor of the dimer lead to?
Autophosphorylation, where one receptor phosphorylates its neighbour on specific tyrosine residues
121
What is inappropriate activation of tyrosine kinase enzymes within receptors associated with?
Diseases, particularly cancer.
122
What is Ras?
A commonly activated protein whose mutations often cause a number of types of cancer. Mutated Ras is found in around 30% of human tumours.
123
What does tyrosine kinase do in a sequence of amino acids?
Tyrosine kinase recognises a specific tyrosine presented in a specific way, so only 1 tyrosine in the chain is phosphorylated. This causes a conformational change which presents a structure recognised by other protein, so other proteins can be recruited
124
What happens to phosphor-tyrosine and surrounding amino acids?
They're recognised by SH2 domains of other proteins, allowing them to bind and undergo activation
125
What are 2 commonly activated protein in tyrosine kinase receptor activity?
Ras and Grb-2
126
What happens to Grb-2?
Grb-2 recognises the receptor through an SH2 domain, which activates Grb-2 and allows another protein (Ras GEF) to bind to it at SH3 domains. This allows activation of Ras via phosphorylation, which leads to some form of cellular activation
127
What are PI-3 kinases?
A set of kinases which are activated by phosphorylating inositol phosphates within the plasma membrane
128
What does activation of PI-3 kinase by phosphorylation of inositol phosphates within the plasma membrane allow?
Activation of PI 3-kinases allows other molecules to bind to the phosphorylated inositol phosphate. One of these is also a kinase, which activates a chain event that results in a physiological response
129
What type of protein is Ras?
A small GTP-binding protein (small G protein) related to G alpha subunits of cAMP.
130
When is Ras activated?
It's inactive when GDP is bound, but activated when this is replaced for GTP, as helped by GEF (guanine exchange factor)
131
What does activated Ras associate with and activate?
Raf
132
How long does removal of GTP from Ras usually take?
Around 30 minutes
133
What aids removal of GTP from Ras?
GTPase activating protein
134
Where do insulin receptor act?
The liver and muscle tissue
135
What do insulin receptors consist of?
2 alpha and 2 beta subunits links by disulphide bridges. There's tyrosine kinase activity within the intracellular portion of the protein
136
What happens upon activation of insulin receptors?
Autophosphorylation of the 2 ß subunits, which leads to activation of a family of small protein substrates called IRS (insulin receptor substrates)
137
How does insulin activate the PI 3-kinase system?
Through IRS protein, which is unique to the insulin family
138
How are IRS' recruited in PI 3-kinase system activation?
IRS is recruited through the activation of SH2 domain via recognition of a particular phosphorylated tyrosine
139
What does activation of PI 3-kinase lead to activation of?
PKB (protein kinase B), which is responsible for activating glycogen synthesis and protein synthesis, and in some cells, the expression of more glucose receptors into cell membranes
140
What's the significance of phosphates being activated as a result of receptor activation?
This means the signalling process sets events in motion that lead to signal termination, as dephospho rylation causes inactivation
141
Besides phosphatase activation, what other means of receptor kinase inactivation?
Receptor internalisation after stimulation.
142
What is PTEN?
One of the phosphatases responsible for the inactivation of the PI 3-kinase pathway
143
What can mutations in key regulatory mechanisms lead to? Give examples of mutations
Cancer
Loss of GTPase activity, preventing inactivation, loss of phosphatase activity or over expression of the receptor in some breast cancers
144
What is a strategy against over expression of receptors?
Prevention of EGF binding to excess receptors. This prevention is controlled by Herceptin, a monoclonal antibody that targets a member of the EGF receptor family and prevents activation
145
What are the 2 domains of receptor tyrosine kinases?
A ligand-binding domain and a protein tyrosine kinase domain
146
What activates the JAK/STAT signalling pathway?
Growth hormone
147
What is JAK?
Janus kinase
148
How many GH molecules bind to a growth hormone receptor?
Just 1
149
What does binding of GH to a GHR lead to?
A conformational change that allows a second growth hormone receptor to interact with it in a dimerisation
150
GHRs don't contain a tyrosine kinase within their structure, but they are intimately associated with what?
JAK (janus kinase), which is what gives GHRs their enzyme-linked activity
151
What is JAK responsible for in GHRs?
The autophosphorylation of itself and the receptor
152
What does activation of GHRs via autophosphorylation produce?
Recognition domains that are recognised by a STAT (signal transduction activator of transcription) (a transcription factor).
153
What happens to STATs when they recognise a recognition domain?
STATs dock on phosphotyrosines and are phosphorylated by JAKs. Activation causes STATs to dissociated from the receptor and then dimerise via SH2 domains
154
What happens to STAT dimers?
They migrate to the nucleus before binding to specific promoters to stimulate the expression of specific genes called gene regulatory proteins.
155
What activates Smad-dependent signalling pathways?
TGF-ß
156
How can binding of TGF-ß affect a cell?
It depends on the cell. In some circumstances, it'll stimulate growth, whereas in others it can inhibit growth
157
How are TGF-ß receptors different to tyrosine kinase receptors?
They have a serine/threonine domain rather than tyrosine kinase, which phosphorylates serines and threonines
158
What does binding of TGF-ß lead to?
Receptor dimerisation and phosphorylation
159
Rather than interacting with Ras or STAT, what do the TGF-ß receptor dimers react with?
Smad 2 and 3, which then associate with Smad 4
160
What are Smads?
The main signal transducers for TGF-ß receptors, which are critically important to regulation of cell development and growth
161
What happens once Smad 4 associates with Smads 2 and 3?
They migrate to the nucleus, where they bind to gene regulatory protein for transcription and gene expression
