The citric acid cycle

The pyruvate dehydrogenase complex

Highly ordered clusters of enzymes and cofactors that oxidized pyruvate in the mitochondrial matrix to acetyl-CoA and CO₂

Conversion of puryvate to Acetyl CoA

• pyruvate dehydrogenase complex (mitocondria)

• aerobic complex

• carried out by most all cells

• oxidative decarboxylation

Parts of the mitochondrion

Inner membrane

Outer membrane

Intermembrane space

Cristae

Matrix: like the cytosol of the mitocondria, where the CAC and pyruvate DH takes place

Pyruvate dehydrogenase reaction

Pyruvate + CoA + NAD+ → Acetyl-Coa + CO₂ + NADH

• Irreiverislbe

• pyruvate is getting oxidized

• NAD+ is getting reduced

Pyruvate dehydrogenease complex channels its intermediates through 5 reactions

1. pyruavte is decarboxylacted; CO₂ is released

2. Hydroxethyl group is transferred to lipoic acid

3. An Acetyl group is transferred to CoA

4. FAD is reduced to re-oxidize the lipoic acid

5. FADH₂ is reoxidized to FAD by reducing NAD+ to form NADH

Step 1 of pyruvate dehydrogenase complex

Cofacter = TPP

Pyruvate loses a CO₂ group and becomes a hydroxyethyl-TPP derivative.

Step 2 of pyruvate dehydrogenase complex

Cofactor = Lipoamide

Transfer of hydroxyethyl from TPP to lipoic acid

Step 3 of pyruvate dehydrogenase complex

Cofactor = Coenzyme A
Transfer of acetyl group from lipoamide to CoA, forming acetyl-CoA.

Produces a reduced lipoamide group as the hydrozyethyl group was oxidized to the acetyl group

Step 4 of pyruvate dehydrogenase complex

Cofactor: FAD

FAD is reduced to FADH₂; lipoic acid is reoxidized

Step 5 of pyruvate dehydrogenase complex

Cofactor = NAD⁺
FADH₂ is reoxidzed to FAD and NAD+ is reduced to FADH

5 cofactors if purvate Dehydrogenase complex

1. TPP

2. Lipoamide

3. CoA

4. FAD

5. NAD+

3 prouducts of purvate dehydrogenase complex

1. CO₂

2. Acetyl-CoA

3. NADH

Citric acid cycle

Oxidation of acetyl groups to CO₂ in the citric acid cycle

• nearly universal pathway

• Generares NADH, QH₂ and one GTP

• Mitochondrial matrix

Where to the steps of the CAC occur

7 out of 8 steps occur in the mitochondrial matrix

1 out of 8 steps occurs in the inner mitochodnrial membrane

• succinate drhydrogenase

3 sources of Acetyl-COA (the start)

1. Pyruvate from glycolysis

2. Fatty acid oxidation

3. Amino acid catabolism

Reaction 1 of the CAC

Citrate synthase adds an acetyl group (2c) to oxaloactate (4C)

• regulated by builtup of citrate

• irreversible

Reaction 2 of the CAC

Aconitase isomerized citrate to isocitrate by removing and adding a water molecule.

Step 3 of the CAC

Isocitrate dehydrogenase releases the first CO₂

Isocitrate is oxidized to alpha-ketoglutarate (5C) and generates NADH + CO₂.

• irreverisble

• regulated

Step 4 of the CAC

Alpha-ketoglutarate dehydrogenase releases the second CO₂and converts alpha-ketoglutarate into succinyl-CoA (4C), generating NADH and another CO₂.

• similar to pyruvate DH compelx (uses the same cofactors)

• Irrerivisble

Step 5 of the CAC

Succinyl-CoA synthetase catalyzes substrate level phosphorlaytion

Turns Succinyl CoA into Succinate

• generates 1 GTP (mammals) or 1 ATP (plants/bacteria)

Step 6 of the CAC

Succinate drhydrogenase generates ubiquinol

Turns succinate into fumarate while reducing FAD to FADH2. This step is important for the electron transport chain.

Step 7 of the CAC

Fumerase atalyzes a hydration reaction

Fumerate → Maltate

Step 8 of the CAC

Maltate dehydrogenase regenerates oxaloacetate

Maltate → Oxaloactate while reducing NAD+ to NADH. Completees the cycle

Products in 1 turn of the CAC

• 2 CO₂

• 1 ATP?GTP

• 3 NADH

• 1 FADH₂ / QH₂

the citric acid cycle is an energy generating cycle

1 NADH = 2.5 ATP

1 QH₂ = 1.5 AP

End up with 32 ATP from the CAC

Regulation in the CAC

Steps 1, 3 and 4

Turn off:

• High NADH

• High ATP

• Plenty of energy

Turn on:

• High NAD+

• High ADP

• Low energy

CAC regulated through three exergonic steps

• citrate synthase

• Isocitrate dehydrogenase compelx

• A ketoglutarate dehydrogenase complex

Fluxes are affected by the concentrations of substrate and products

• end products ATP and NADH inhibit

• NAD + and ADP activate

Citric acid cycle intermedaites

Citrate: fatty acids, steroids

A ketoglutarate: glutamate (turns into AA or DNA backbone)

Succinyl CoA: Porphyins, heme

Oxaloacetate: Turns into AA and glucose through gluconeogenesis.

Intermediates that are diverted to other pathways can be replenished

What replenshises CAC intermediates

Anaplerotic reactions = chemical reactions that replenish intermediates

When intermediates are shunted from the CAC to othrer plathways they are repleninshed

Mutation in CAC enzymes

Mutations in succinate dehydrogenase and fumarase cause tumors

Many glial cell tumors have mutant NADPH-dependent isocitrate dehydrogenase

• lose ability to convert isocitrate to a-ketoglytarate

• gain ability to convert a ketoglutarate to 2-hydroxygluatarate