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MEH session 2 Flashcards

1
Q

Does pyruvate enter the Kreb’s cycle directly?

A

No.

Pyruvate is converted to acetyl coA before entering stage 3 of catabolism

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2
Q

What does pyruvate dehydrogenase do?

A

Converts pyruvate to acetyl coA

This is an irreversible reaction because the loss of CO2 from pyruvate is irreversible

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3
Q

What deficiency is pyruvate dehydrogenase sensitive to?

A

Vitamin B1 deficiency

This provides FAD, thiamine pyrophosphate and lipoid acid

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4
Q

What is pyruvate dehydrogenase inhibited by?

A 
High energy substrates:
Acetyl coA
NADH 
ATP 
Citrate (an intermediate in the krebs cycle)

These substrates phosphorylate the enzyme.
This allosterically inhibits the enzyme

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5
Q

What is pyruvate dehydrogenase activated by?

A 
Low energy substrates and substrates:
Pyruvate 
Coenzyme A 
NAD+ 
ADP
Insulin

These dephosphorylate pyruvate dehydrogenase and allosterically activate the enzyme

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6
Q

What happens when there is a pyruvate dehydrogenase deficiency?

A

Pyruvate is diverted to lactate production via lactate dehydrogenase for the regeneration of NAD+
Leads to lactic acidosis

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7
Q

Where is pyruvate dehydogenase?

A

In the mitochondrial matrix

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8
Q

What feeds into the TCA cycle?

A

2 acetyl coA
2 oxaloacetate
6 NAD+
2 FAD

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9
Q

What are the products of the TCA cycle for one glucose molecule?

A 
6 NADH
2 FADH2
2 GTP 
4 CO2
2 oxoloacetate
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10
Q

Which enzymes regulate the TCA cycle?

A

Enzymes catalysing irreversible steps where CO2 is removed.
Isocitrate dehydrogenase
Stimulated by high [ADP]
Inhibited by high [NADP] [ATP]

Alpha ketoglutarate dehydrogenase
Inhibited by high [NADH] [ATP] [succinyl-coA]

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11
Q

What are the catabolic functions of the TCA cycle?

A
  • –>Oxidise C atoms to CO2

- –>H+ and e- are removed from acetate and are transferred to NAD+ and FAD

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12
Q

What are the anabolic functions of the TCA cycle?

A

C5 and C4 intermediates used for the synthesis of non-essential amino acids.
C4 intermediates used for the synthesis of haem and glucose.
C6 intermediate used for the synthesis of fatty acids.

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13
Q

What would cause the TCA cycle to rapidly stop?

A

Absence of oxygen

NADH and FADH2 are not oxidised

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14
Q

What is the energy yield of ATP from one glucose molecule in aerobic respiration?

A

32 molecules ATP/glucose

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15
Q

Explain the processes of electron transport and ATP synthesis and how they are coupled.

A

Electron transport - electrons in NADH and FADH2 are transferred through a series of carrier molecules to oxygen with the step-wise release of free energy

ATP synthesis - the free energy released in electron transport is used to drive ATP synthesis from ADP and Pi via proton translocating complexes

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16
Q

Describe how ATP is synthesised by oxidative phosphorylation.

A
  • electrons are transferred through series of carrier molecules to oxygen with release of energy
  • Proton translocation get complexes use the free energy from electrons being transferred to them to move protons from inside the mitochondria to the intermembranal space
  • the membrane itself is impermeable to protons so the proton concentration of protons in the intermembranal space increases creating a proton gradient (p.m.f) both electrical and chemical
  • protons can only re-enter the mitochondrial matrix via the ATP synthase complex, driving the synthesis of ATP from ADP and Pi
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17
Q

Does NADH or FADH2 result in the production of more ATP by oxidative phosphorylation?

A

NADH

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18
Q

What regulates the electron transport chain?

A

[ATP] in the cell

When [ATP] is high or [ADP] is low, there is no substrate for ATP synthase

Inward flow of H+ stops

Concentration of H+ in the intermitochondrial membrane space increases

Prevents further H+ from pumping. In the absence of proton pumping, electron transport stops.

[NADH] builds up and inhibits earlier pathways

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19
Q

How can the electron transport chain be inhibited by substances such as cyanide and carbon monoxide?

A

Some carriers contain heme groups to which these substances strongly bind

Inhibitors block electron transport and prevents acceptance of electrons by oxygen

Under these conditions NADH and FADH2 cannot be oxidised by electron transport and there is no energy to drive the pumping of protons so ATP is not synthesised

Irreversible cell damage rapidly occurs

(These substances also bind to haemoglobin and prevent oxygen transport)

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20
Q

What do uncoupling proteins do to the electron transport chain?

A

Increase permeability of membrane to H+
H+ enters mitochondria without driving ATP synthase
This dissipates p.m.f
no phosphorylation of ADP
No inhibition of electron transport so it continues and energy is dissipated as heat

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21
Q

What is dinitrophenol?

A

Often used unwisely as a slimming agent

Acts as an uncoupler for the electron transport chain

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22
Q

How is brown adipose tissue adapted to non-shivering thermogenesis?

A

In response to cold, noradrenaline is released from the sympathetic nervous system. This

-stimulates lipase
Releases fatty acids from triacylglycerol
Beta-oxidation of the fatty acids produced NADH and FADH2

-activates UCP1
Uncoupled electron transport from ATP synthesis
Increases per

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23
Q

What is the difference between oxidative phosphorylation and substrate level phosphorylation?

A

Oxidative phosphorylation:

  • requires membrane associated complexes (inner mitochondrial membrane) whereas substrate level phosphorylation requires soluble enzymes (cytoplasmic and mitochondrial matrix)
  • energy coupling occurs indirectly through generation and subsequent utilisation of a proton gradient (p.m.f) whereas in substrate level phosphorylation energy coupling occurs directly through formation of a high energy of hydrolysis bond (phosphoryl-group transfer)
  • cannot occur in absence of oxygen whereas substrate level phosphorylation can occur to a limited extent in the absence of oxygen
  • major process for ATP synthesis that requires large amounts of energy whereas substrate level phosphorylation is a minor process for ATP synthesis in cells that require large amounts energy
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24
Q

Why do lipids release more energy when oxidised compared to carbohydrates?

A

They are more reduced than carbohydrates

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25
What are the three main classes of lipids?
Fatty acid derivatives Hydroxyl-methyl glutamic acid derivatives (C6 compound) Vitamins
26
Give some examples of fatty acid derivatives.
Fatty acids- fuel molecules Triacylglycerols Phospholipids Eicosanoids
27
Give some examples of hydroxy-methyl glutamic acid derivatives.
Ketone bodies Cholesterol Cholesterol esters Bile acids and salts
28
In an obese person, in which molecule is most of their excess energy stored?
Triacylglycerol
29
What are the major dietary lipids?
Butter Ghee Margarine Vegetable oils
30
Describe the structure of triacylglycerols.
Glycerol backbone | 3 fatty acids
31
What is the process of synthesising triacylglycerol called? | What is the process of breaking down triacylglycerol called?
``` Synthesis = esterification Breakdown = lipolysis ```
32
What kind of fatty acids are essential components of the diet?
Certain unpolyunsaturated fatty acids | Eg. Arachidonic acid
33
Where is the major site of lipid synthesis?
Liver and some in adipose tissue
34
How are triglycerides digested in the GI tract?
Digested to glycerol and fatty acids in small intestine by pancreatic lipases Fatty acids are converted to chylomicrons in lacteals and are absorbed in the lymph and then into the bloodstream Glycerol is absorbed into the blood stream
35
How are triacylglycerols hydrolysed in stress situations (aerobic exercise, starvation, lactation) from adipose tissue?
Adipose tissue triacylglycerols are hydrolysed by the enzyme hormone-sensitive lipase to release fatty acids and glycerol that diffuse from the tissue. Storage/mobilisation is under hormonal control.
36
Which hormones stimulate lipolysis?
``` Glucagon Adrenaline Cortisol Growth hormone Thyroxine ```
37
Which hormones inhibit lipolysis?
Insulin
38
How are metabolites of lipids carried to tissues where they are required?
Fatty acids are carried to tissues via the blood stream bound non-covalently to albumin. Glycerol is transported in the blood to the liver where it may be oxidised, converted to glucose or used in the synthesis of triacylglycerol
39
When are fatty acids used as a fuel?
Low extracellular glucose concentration results in fatty acid release as an alternative fuel as less glycerol 1-phosphate is produced and triglycerides cannot be synthesised
40
Where and how is dietary glycerol metabolised?
Enters bloodstream and transported to liver Glycerol kinase phosphorylates it to glycerol-1-phosphate. This can either: - enter glycolysis by converting to dihydroxyacetone phosphate - be used in triacylglycerol synthesis
41
Which tissues use fatty acids as a source of energy?
Liver Heart muscle Skeletal muscle Other tissues with mitochondria
42
Why can't the cells of the CNS and red blood cells metabolise fatty acids?
Red blood cells- do not possess mitochondria CNS- fatty acids cannot cross blood-brain barrier
43
How are fatty acids metabolised ans where does it occur?
1. Fatty acyl coA synthase catalyses the formation of fatty acyl coA which activates the fatty acid. 2. Fatty acid is transported into the mitochondria via the carnitine shuttle 3. Beta oxidation of fatty acid
44
What inhibits fatty acid transport into the mitochondria and why is this important?
Inhibited by malonyl CoA- an intermediate in the synthesis of fatty acids - important because it prevents fatty acids newly synthesised in the cytoplasm from being immediately transported into the mitochondria and oxidised.
45
How do patients with a defective mitochondrial fatty aid transport system present?
Patients suffer from poor exercise tolerance | Have unusually large amounts of triacylglycerols in cells where they shouldn't be present eg. Muscle as lipid droplets
46
What occurs in beta-oxidation of fatty acids?
Repeated removal of a C2 unit until only two carbons remain, this forms acetyl coA units ---> stage 3 of catabolism Oxidation of the fatty acid producing NADH and FADH2 ---> stage 4 of catabolism
47
Where does beta-oxidation of fatty acids occur?
Mitochondria
48
What is required for the beta-oxidation of fatty acids?
NAD+ and FAD | Oxygen - required for stage 4 of catabolism to re-oxidise the NADH and FADH2
49
Where and how is dietary glycerol metabolised?
In the liver, it is converted to glycerol 1-phosphate by glycerol kinase. Glycerol 1-phosphate either: - triacylglycerol synthesis - converted to dihydroxyacetone phosphate and enters glycolysis
50
What are the catabolic and anabolic functions of acetyl coA?
Catabolic - enters stage 3 of catabolism (krebs cycle) Anabolic - fatty acids: triacylglycerols, phospholipids Hydroxymethyl glutaric acid (HMG): Ketone bodies, cholesterol ---> steroid hormones
51
Describe the structure of acetyl coA.
CH3CO group linked to coenzyme A Linked via S-atom, this has a high energy of hydrolysis Therefore, the acetyl group is activated CoA contains vitamin B5- panthenoic acid
52
Which ketone bodies are produced in the body?
Acetoacetate Acetone Beta-hydroxybutyrate
53
Where are ketone bodies produced?
Acetoacetate and beta hydroxybutyrate are synthesised in the mitochondria in the liver. Acetylene is formed from the spontaneous decarboxylation of acetoacetate The kidney can produce ketone bodies under extreme conditions but at a much lower level than the liver.
54
What is the normal plasma ketone body concentration?
<1 mM
55
When does the concentration of ketone bodies in the circulation increase?
Starvation (2-10mmol/L) - physiological ketosis Untreated type 1 diabetes (>10mmol/L) - pathological ketosis
56
How are ketone bodies produced in starvation and in type 1 diabetes
Reduced blood [glucose] Fatty acids released from adipose stores due to stimulated lipolysis Plasma insulin/glucagon ratio is low, this activates lyase and inhibits reductase Ketone body formation is stimulated
57
Which enzyme do statin drugs inhibit?
HMG -CoA reductase This stimulates the conversion of HMG-coA to (mevalonate to) cholesterol
58
How does your body know that it should synthesise ketone bodies from acetyl coA rather than enter it into the TCA cycle during early starvation?
Blood [glucose] is low Less glycerol for triglycerol synthesis---> less glycerol 1-phosphate---> lipolysis stimulates Lipolysis involved beta oxidation of fatty acids. This reduces NAD to form NADH. NADH inhibits isocitrate dehydrogenase and alpha ketoglutarate dehydrogenase in the TCA cycle so the TCA cycle is inhibited Instead, acetyl coA is diverted to enter the pathway for ketone body formation
59
Which enzymes are involved in ketone production and how are they controlled?
They are synthesised by the actions of lease and reductase enzymes that are reciprocally controlled by the insulin/glucagon ratio. When the ratio falls, lyase is activated and reductase is inhibited ---> ketone body formation activated When the ratio increases, lyase is inhibited and reductase is activated ---> cholesterol synthesis
60
What two factors influence ketone production?
Blood glucose concentration - concentration of NADH | Hormones - insulin/glucagon ratio
61
Which tissues can use ketone bodies as a fuel?
All tissues containing mitochondria including the CNS
62
How are ketone bodies metabolised?
They are converted to acetyl coA and this is oxidised via stage 3 of metabolism
63
Why is ketone body formation important during starvation?
Provides an alternative fuel for tissues such as muscle so that low circulating glucose can be used as fuel by the brain and by tissues that have an absolute requirement for glucose
64
What happens during prolonged starvation?
Blood [glucose] remains low Triglyceride stores are not replaced. Ketone bodies cannot be synthesised Protein in muscle in broken down to amino acids and are transported in the blood to the liver Amino acids are converted into pyruvate and then glucose by gluconeogenesis in the liver
65
Why can ketones be excreted in the urine unlike fatty acids?
They are water soluble molecules which allows high plasma concentrations and excretion in urine. This occurs when ketone body concentration is above renal threshold.
66
What is ketoacidosis?
Acetoacetate and beta-hydroxybutyrate are relatively strong organic acids and when present in high concentrations in the plasma, they may cause acidosis
67
Which enzyme links glycolysis to the TCA cycle by catalysing the formation of acetyl coA?
Pyruvate dehydrogenase catalyses the formation of acetyl coA from pyruvate. This reaction is irreversible.
68
An increase in the cellular concentration of which compound would inhibit the enzyme pyruvate dehydrogenase?
Acetyl coA
69
In which cellular compartment does the citric acid cycle occur?
Mitochondria
70
Which molecule combines with acetyl coA to form citrate in the first step of the citric acid cycle?
Oxaloacetate
71
During the oxidation of glucose to carbon dioxide and water, which stage in metabolism produces the most energy in the form of ATP?
``` The electron transport chain. By far the most ATP is produced here. One molecule of glucose would yield 10 NADH and 2 FADH2 from glycolysis, pyruvate dehydrogenase and the citric acid cycle. Each NADH ---> 2.5 ATP by ETC Each FADH2 ---> 1.5 ATP by ETC ```
72
Explain the function of the two lipid bilayers of the mitochondria in the ETC?
The presence of two mitochondrial membranes allows the maintenance of a proton gradient necessary for oxidative phosphorylation.
73
8) A 22 year old female was admitted to hospital after being found unconcious. Her body temperature was 42 oC. Upon further investigation it was found that she had taken 6 slimming pills that morning which she had purchased from the internet. These slimming pills contained dinitrophenol (DNP), an uncoupler of oxidative phosphorylation. What would be the effect of dinitrophenol on oxidative phosphorylation?
DNP causes operation of the electron transport chain without ATP production. It allows protons to leak across the inner mitochondrial membrane and thus bypass ATP synthase. Therefore, instead of being converted to ATP, the energy from the ETC is released as heat explaining her high body temperature.
74
What effect does the poison cyanide have on the electron transport chain?
Cyanide prevents the flow of electrons through the electron transport chain. It inhibits the enzyme cytochrome c oxidase in complex IV. This is unable to facilitate the acceptance of electrons by oxygen so the ETC chain stops and the proton motive force driving ATP synthesis fails.
75
Describe how oxidative phosphorylation.
Energy from the transfer of electron between carriers is used to create a proton gradient to synthesise ATP.
76
Are lipids more reduced or oxidised than carbohydrates?
More reduced- they have a higher energy content so release more energy when oxidised.
77
What is the typical daily energy requirement of a 70 kg man?
11,000 kJ
78
What is alpha-linolenic acid?
It is an 18 carbon chain with 3 double bonds Found in vegetable oils Often referred to as omega-3
79
With respect to fatty acid catabolism, which enzyme facilitates the activation of fatty acids by linking coenzyme-A and where is this enzyme found?
Fatty acyl coA synthase found in the cytosol
80
Activated fatty acids are unable to cross the mitochondrial membrane yet Beta oxidation occurs in mitochondria. Name the mechanism by which activated fatty acids enter mitochondria.
Carnitine shuttle. The acyl group on coA is transferred to carnitine by carnitine acyltransferase I located on the outer mitochondrial membrane. Acyl carnitine is then shuttled by a carnitine-acylcarnitinine translocase. Acyl carnitine is converted back to acyl coA by carnitine acyltransferase II located on the inner mitochondrial membrane. The liberated carnitine then return back to the cytosol for further use.
81
What substance inhibits the carnitine shuttle?
Malonyl-coA This is an intermediate in fatty synthesis so ensures that newly synthesised fatty acids are not transported into the mitochondria and used for beta-oxidation.
82
During beta oxidation of fatty acids, how many carbons are removed per cycle?
2 carbon units are cleaved from the fatty acid per cycle resulting in release of acetyl coA which can then enter the citric acid cycle
83
After two additional reactions, the glycerol released upon the hydrolysis of triacylglycerol can enter which pathway?
Glycolysis After phosphorylation to glycerol phosphate and the conversion to dihydroxyacetone phosphate, it can enter glycolysis. It can also act as a precursor for gluconeogenesis by these same reactions.
84
Which enzyme converts glycerol to glycerol phosphate?
Glycerol kinase

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