TCA (Citric acid/Krebs) Cycle

Where does it take place?

Mitochondrial Matrix

Primary function

Oxidize Acetyl-CoA and generate redox energy under aerobic conditions, but does not use O2

How is acetyl-CoA sourced in the matrix?

Pyruvic acid (from carbohydrate metabolism), amino acids, fatty acids (from beta oxidation)

If TCA cycle is the metabolic engine, what is the fuel and what is the exhaust? What about the work done?

Fuel is acetyl coa. CO2 ×2 is the exhast. 1 GTP, 1 FADH2, and 3NADH are the work done

How does pyruvate from glycolysis turn into acetyl coa?

PDH, this is the bridge between glycolysis and citrate cycle. PDH and TCA cycle are found in the matrix (soup) of mitochondria

PDH consists of how many enzymes and requires which coenzymes?

Enzymes: pyruvate dehydrogenase, lipoate transacetylase, lipoate dehydrogenase. Coenzyme: thiamine pyrophosphate, a-lipoic acid, coenzyme A, FAD/FADH2, NAD+/NADH + H+

Overall reaction of PDH

Pyruvate + CoA + NAD+ → Acetyl coa + CO2 + NADH + H+

mechanisms of PDH complex

1. decarboxylation of pyruvate and attachment of acetyl group onto TPP 2. transfer of acetyl group and electron to lipoate 3. transesterification: transfer of acetyl group onto CoA 4. Oxidation of lipoamide by FAD → to FADH2 5. Oxidation of FADH2 by NAD+ → NADH + H+

What inhibits PDH? Why?

Arsenic. It irreversibly blocks catalytic activity of lipoamide-containing enzymes. The oxidized lipoamide cannot be regenerated, suicidal inhibition.

How is PDH regulated? 2 ways

1. feedback inhibition by the end products (acetyl coa, NADH and H+) 2. covalent modifications (kinase activated by NADH and acetyl CoA, turns PDH INACTIVE. Inhibited by pyruvate and ADP) PDH phosphatase turns PDH ON, activated by Mg and Ca.

Net reaction of citrate cycle

1 acetyl co-a + 3NAD+ + 1FAD + 1GDP + 1 Pi + 2H2O → 2 CO2 + 3NADH + 1GTP + 1CoA + 1FADH2

Reaction 1

Acetyl-CoA + Oxaloacetate + H2O —citrate synthase—> CoA + Citrate

Reaction 2

Citrate ←aconitase→ isocitrate

Reaction 3

Isocitrate + NAD+ —isocitrate dehydrogenase—> a-ketoglutarate + NADH + CO2 + H+

Reaction 4

a-ketoglutarate + CoA + NAD+ —a-ketoglutarate dehydrogenase—> succinyl-coa + NADH + CO2 + H+

Reaction 5

Succinyl-CoA + GDP + Pi ← succinyl-coA synthetase→ succinate + GTP + CoA

Reaction 6

Succinate + FAD ←succinate dehydrogenase→ Fumarate + FADH2

Reaction 7

Fumarate + H2O ← Fumarase→ Malate

Reaction 8

Malate + NAD+ ←malate dehydrogenase→ Oxaloacetate + NADH + H+

total ATP per TCA cycle

10 ATP, from 3NADH, 1FADH2 and 1GTP. PDH also yields 1NADH, putting 1 pyruvate = 12.5 ATP (unofficial)

Which reactions in the TCA cycle are exergonic and irreversible? These are also the main regulatory enzymes

1. citrate synthase 3. isocitrate dehydrogenase 4. a-ketoglutarate dehydrogenase

Pyruvate Carboxylase

Converts pyruvate to oxaloacetate; acetyl coa activates the enzyme, requires biotin as coenzyme

Phosphoenolpyruvate carboxylase

Found mainly in plants, yeast and bacteria; replenishes oxaloacetate

malic enzymes

found in proks and euks, replenishes malate

Redox potential difference DE*’

DE*’= (E*’ e acceptors) - (E*’ e donor)

DG*’= -nFDE*’

E*’ = standard redox potential in V

n= number of electrons transferred

F = faraday constant

Reduction potential: nernst equation

E = DE*’ + RT/nF ln(e acceptor/e donor)