TCA Regulation

Question 1

pathways connected to:
carbohydrate metabolism
lipid metabolism
protein metabolism

Answer 1

Carbs: glycolysis, gluconeogenesis, glycogenesis, glycogenolysis, pentose phosphate pathway
Lipids: beta oxidation, FA synthesis, TG synthesis, cholesterol synthesis
Proteins: transamination, urea cycle, AA catabolism, AA synthesis, AA derivative synthesis, nucleic acid synthesis

Question 2

electron flow overview cellular respiration

Answer 2

Stage 1 sources: glycolysis and pyruvate oxidation, beta oxidation and AA catabolism
Stage 2: citric acid cycle (4 enzymatic reactions)
All feed into NADH and FADH2 to enter into electron transport chain

Question 3

fates of pyruvate from glycolysis

Answer 3

1) oxidation for energy in TCA
2) converted to Acetyl-coA and used as starting material for FA and sterol synthesis
3) precursor of AA synthesis

Question 4

sources of acetyl coA for TCA

Answer 4

beta oxidation, ketogenic amino acids, glycolysis (followed by pyruvate oxidation)

Question 5

Pyruvate oxidation overview

Answer 5

Irreversible oxidative decarboxylation reaction (carboxyl group removed)
Enzyme: pyruvate dehydrogenase complex (E1, E2, E3)
Cofactors: CoA-SH, NAD+, TPP, Lipoate, FAD
Produced: NADH (2-5 ATP), CO2, Acetyl-CoA

Question 6

Pyruvate Dehydrogenase Complex

Answer 6

cluster of multiple copies of 3 enzymes:

E1: pyruvate dehydrogenase, bound by TPP
E2: dihydrolipoyl transacetylase, covalently bound lipoyl group
E3: dihydrolipoyl dehydrogenase, cofactors NAD+ and FAD

Question 7

Pyruvate oxidation pathway

Answer 7

1) Pyruvate decarboxylated and attached to TPP, forming acetyl group and CO2 released
2) Reduction of acyl-lipoyllysine with addition of acetyl group
Enzyme: E1 pyruvate dehydrogenase

3) Addition of CoA to acetyl group forming acetyl coA
Enzyme: E2 dihydrolipoyl transacetylase

4) FAD reduced to FADH2 to return lipollysine to oxidized state
5) NAD+ reduced to NADH, regenerating FAD
Enzyme: E3 dihydrolipoyl dehydrogenase

Question 8

TCA intermediates

Answer 8

Acetyl-coA + oxaloacetate
Citrate
Isocitrate
Alpha-ketoglutarate
Succinyl-coA
Succinate
Fumarate

9 total

Question 9

TCA CO2 loss steps

Answer 9

CO2 carbons come from oxaloacetate molecule originally (acetyl-coA molecules don’t contribute to CO2 in first past)

2CO2 lost: Isocitrate –> a-ketoglutarate –> succinyl coA

Question 10

TCA net equation

Answer 10

Acetyl-coA = 2CO2 + 3 NADH + 1 FADH2 + 1 ATP

Question 11

Energy production steps in TCA

Answer 11

NADH produced:
Isocitrate –> a-ketoglutarate
a-ketoglutarate –> succinyl-coA
malate –> oxaloacetate

FADH2 produced: succinate –> fumarate

GTP (ATP) produced: succinyl-coA –> succinate

Question 12

Anaplerotic reactions from TCA

Answer 12

Oxaloacetate replenishing reactions:
1) pyruvate + HCO3 + ATP –> oxaloacetate,
Enzyme: pyruvate carboxylase
Tissues: liver, kidney

2) phosphoenolpyruvate + CO2 + GDP –> oxaloacetate + GTP,
Enzyme: phosphoenolpyruvate carboxykinase
Tissues: heart and skeletal muscle

3) Phosphoenolpyruvate + HCO3 –> oxaloacetate,
Enzyme: phosphoenolpyruvate carboxylase
Tissues: in higher plants, yeast and bacteria

Malate replenishing reaction:
1) Pyruvate + HCO3 + NAD(P)H –> malate + NAD(P)+
Enzyme: malic enzyme
Tissues: everywhere in eukaryotes and bacteria

Question 13

Citrate can be used to synthesize

Answer 13

fatty acids and sterols

Question 14

a-ketoglutarate can be used to synthesize

Answer 14

glutamate
Which can then be used to make: proline, arginine and glutamine, and purine nucleotides

Question 15

Succinyl-coA can be used to synthesize

Answer 15

Along with glycine can synthesis porphyrins/heme

Question 16

Oxaloacetate can be used to synthesize

Answer 16

aspartate, asparagine and from them, pyrimidine nucleotides

Question 17

Phosphoenolpyruvate (glycolysis) can be used to synthesize

Answer 17

Either glucose or
Serine, glycine, cysteine (via 3-phosphoglyceratet)
Phenylalanine, tyrosine, tryptophan (via phosphoenolpyruvate + erythrose-4-phosphate)

Question 18

Which enzymes are regulated in the TCA?

Answer 18

Enzymes of irreversible reactions: pyruvate dehydrogenase complex, citrate synthase, isocitrate dehydrogenase, a-ketoglutarate dehydrogenase complex

Question 19

Pyruvate dehydrogenase complex inhibition

Answer 19

phosphorylation of the complex inactivates it

Question 20

Succinyl-coA as an inhibitor of TCA

Answer 20

inhibitor of citrate synthase and a-ketoglutarate dehydrogenase complex
regulates a-ketoglutarate levels for AA metabolism

Question 21

3 levels of regulation of TCA

Answer 21

1) transport of pyruvate into mitochondria
2) conversion of pyruvate to acetyl coA
3) entry into CAC

Products inhibit the cycle

Question 22

Activators/inhibitors of pyruvate dehydrogenase complex

Answer 22

Activators: AMP, CoA, NAD+, Ca2+

Inhibitors: ATP, acety-coA, NADH, fatty acids

Question 23

Activators/inhibitors of citrate synthase

Answer 23

Activators: ADP

Inhibitors: citrate, succinyl-coA, NADH, ATP

Question 24

Activators/inhibitors of isocitrate dehydrogenase

Answer 24

Activators: Ca2+, ADP

Inhibitors: ATP

Question 25

Activators/inhibitors of a-ketoglutarate dehydrogenase complex

Answer 25

Activators: Ca2+

Inhibitors: succinyl-coA, NADH