Acetyl-CoA and TCA Cycle

Question 1

What comprises acetyl-CoA?

Answer 1

Coenzyme A, acetyl group via thioester bond

Adenine, Ribose, Phosphate, Two additional Phosphates, Pantothenic acid (B5), Mercaptoethylamine (reactive thiol group)

Question 2

Other than TCA, what can acetyl-CoA do?

Answer 2

Synthesis of fatty acids, cholesterol, and ketone bodies

Question 3

Two major sources of acetyl-CoA?

Answer 3

Oxidative decarboxylation of pyruvate (from glycolysis)

Beta-oxidation of fatty acids

Question 4

Pyruvate dehydrogenase

Answer 4

Link between glycolysis, TCA cycle

Irreversible under phsyiological conditions

Pyruvate decarboxylase (E1)

Dihydrolipoyl transacetylase (E2)

Dihydrolipoyl dehydrogenase (E2)

E3-binding protein (E3BP)

Question 5

Where is pyruvate dehydrogenase located?

Answer 5

Mitochondrial matrix

Question 6

How does pyruvate get to pyruvate dehydrogenase?

Answer 6

Non-selective channel proteins (outer mitochondrial membrane)

MCP1/2 specific carrier proteins (inner mitochondrial membrane)

Question 7

Reaction scheme for pyruvate dehydrogenase

Answer 7

1. E1 forms a hydroxyethyl intermediate with pyruvate and TPP
2. Oxidized to an acetyl group as it is transferred to lipoamide group of E2
3. E2 transfers acetyl group to CoA-SH, forming acetyl-CoA
4. Reduced (-SH) lipoamide is reoxidized by E3
5. FADH2 is reoxidized, by E3, electrons dumped to NAD+

Question 8

Four vitamins needed for pyruvate dehydrogenase

Answer 8

1. Thiamine (B1) for E1
2. Riboflavin (B2) for E3
3. Niacin (B3) for NAD+
4. Pantothentic acid (B5) for CoA-SH

Question 9

Two forms of thiamine deficiency

Answer 9

1. Dietary lack (polished rice)
2. Alcoholism

Treated via parenteral injections, followed by oral doses

Question 10

Beriberi

Answer 10

Pain and parathesia (burning, tingling, prickling sensations)

Wet form: cardiovascular symptoms, congestive heart failure, peripheral edema, cardiomegaly,

Dry form: symmetrical peripheral neuropathy

Question 11

Wernicke encephalopathy

Answer 11

Alcohol patients with thiamine deficiency

Horizontal nystagmus (uncontrollable movements of the eyes), ophthalmoplegia (paralysis of one or more of the eyeballs), cerebellar ataxia (difficulty with motor control), mental impairment

Question 12

Chronic lactic acidosis in alcoholic patients or newborns?

Answer 12

Thiamine deficiency (or a mutation in E1) means that pyruvate cannot be efficiently converted into acetyl-CoA.

Instead, NADH reduces pyrvate to lactate, resulting in chronic lactic acidosis. Spectrum is vast: can be fatal in newborns, majority have neurological difficulties due to brain’s reliance on carbohydrates for ATP, structural abnormalities.

Question 13

Describe regulation of the pyruvate dehydrogenase complex

Answer 13

Product inhibition: Acetyl-CoA and NADH competitively inhibit

Reversible phosphorylation: pyruvate dehydrogenase kinase

Activation: ADP and pyruvate inhibit the kinase, promoting dephosphorylation and thus activation of the complex

Ca²⁺ activates the phosphatase and thus activates the complex

Question 14

Where in the cell do TCA reactions occur?

Answer 14

Mitochondria (some enzymes associated with matrix, others associated with inner membrane)

Question 15

Eight enzymatic steps of the TCA cycle

Answer 15

Citrate synthase
Aconitase
Isocitrate dehydrogenase
A-ketoglutarate dehydrogenase complex
Succinyl-CoA synthetase
Succinate dehydrogenase
Fumarase
Malate dehydrogenase

Question 16

Net equation of the TCA cycle

Answer 16

Acetyl-CoA + 2H₂O + 3NAD⁺ +FAD + GDP + P_i

2 CO₂ + CoA-SH + 3NADH + 3H⁺ + FADH₂ + GTP

Question 17

Oxidation of 1 mol acetyl-CoA yields ?

Answer 17

12 mol ATP

Question 18

What is an anaplerotic reaction and why is it needed?

Answer 18

To replenish the TCA cycle intermediates as they are drawn off for other purposes

Question 19

Describe regulation of the TCA cycle (other than pyruvate dehydrogenase)

Answer 19

Citrate synthase

Availability of oxoloacetate (present below Km)

ATP, NADH, succinyl-CoA all inhibit citrate synthase

Isocitrate dehydrogenase

Allosterically activated by ADP (facilitates isocitrate binding)

Small changes of ADP = large changes in rate

NADH is potent inhibitor

Activation by Ca²⁺

a-ketoglutarate dehydrogenase

product inhibition: succinyl-CoA, NADH

Not controlled by de/phosphorylation

Activation by ADP, Ca²⁺

Question 20

Pyruvate decarboxylase

Answer 20

Found in mitochondrial matrix

Conversion of pyruvate to oxaloacetate

Requires bicarbonate, hydrolysis of ATP for input of energy

Prothestic group from vitamin Biotin

Allosterically activated by ADP

Question 21

Glutamate dehydrogenase

Answer 21

Glutamate converted to a-ketoglutarate

Question 22

What five compounds can be converted into propionyl-CoA via oxidation?

Answer 22

1. Isoleucine
2. Valine
3. Methionine
4. Threonine
5. Odd-chain fatty acids

Propionyl-CoA can be converted into succinyl-CoA