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SURVEY OF BIOCHEMISTRYCitric Acid Cycle

2

PRS

• Which substrates in the TCA cycle are coupled to the production of CO2?

1. Isocitrate and alpha-ketoglutarate2. Citrate and alpha-ketoglutarate3. Oxaloacetate and citrate4. Succinate and malate

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PRS Answer

Isocitrate

and

Alpha-ketoglutarate

4

PRS

• In the net TCA cycle rxn, what are the products?

1. 3 NADH + FAD + GDP + 2CO2 + CoA

2. 3 NADH + FAD + GTP + 2CO2 + CoA

3. 3 NAD+ + FADH2 + GTP + 2CO2 + Acetyl-CoA

4. 3 NADH + FADH2 + GTP + 2CO2 + CoA

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PRS - Answer

• In the net TCA cycle rxn, what are the products?

• 3 NADH + FAD + GDP + 2CO2 + CoA

• 3 NADH + FAD + GTP + 2CO2 + CoA

• 3 NAD+ + FADH2 + GTP + 2CO2 + Acetyl-CoA

• 3 NADH + FADH2 + GTP + 2CO2 + CoA

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PRS

In what form does carbon enter the TCA cycle?

1.Pyruvate2.Glucose3.CO2

4.Acetyl CoA

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PRS

• Which enzyme catalyzes the reversible isomerization of citrate and isocitrate?

1. Citrate synthase2. Aconitase3. Triose Phosphate Isomerase4. Isocitrate Dehydrogenase

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TCA Cycle Overview

Pyruvate Acetyl CoA

Oxaloacetate Citrate

Isocitrate

Alpha-Ketoglutarate

Succinyl-CoASuccinate

Fumarate

L-Malate3 NAD+ + FAD + GDP + Pi + Acetyl-CoA

3NADH + FADH2 + GTP + 2 CO2

CO2

NAD+ NADH

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Pyruvate Dehydrogenase

• Multienzyme complex: E1, E2, E3

• Oxidative decarboxylation• Requires 5 cofactors

– CoA– NAD+

– FAD– Lipoic Acid– Thiamine Pyrophosphate

Pyruvate Acetyl CoA

CO2

NAD+ NADH

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Pyruvate DehydrogenaseE1 Cleaves off CO2

+

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Pyruvate DehydrogenaseE2 Catalyzes Acetyl-CoA Formation

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Pyruvate DehydrogenaseE2 Catalyzes Acetyl-CoA Formation

Acetyl-CoA is formed, but the oxidized form of the lipoic acid cofactor must beregenerated.

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Pyruvate DehydrogenaseE3 Regenerates the Disulfide

Notice the redox chemistry here:

The lipoic acid group of E2 has been regenerated.

Now the oxidized form of E3 needs to be regenerated. How does this happen?

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Pyruvate DehydrogenaseE3 Produces NADH

Now the oxidized form of E3 needs to be regenerated. How does this happen?

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Citrate Synthase

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Aconitase

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Isocitrate Dehydrogenase

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Alpha-Ketoglutarate Dehydrogenase

Notice the similarities between this reaction and that of pyruvate dehydrogenase.

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Alpha-Ketoglutarate Dehydrogenase

Notice the similarities between this reaction and that of pyruvate dehydrogenase.

Both Pyruvate and alpha-ketoglutarate are alpha-keto acids.

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Alpha-Ketoglutarate Dehydrogenase

Notice the similarities between this reaction and that of the pyruvate dehydrogenase complex.

Both rxns produce CO2 and NADH and H+.Both rxns use CoA as a cofactor.

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Succinyl-CoA Synthetase

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Succinate Dehydrogenase

Rxn is strongly inhibited by malonate

By what mode of inhibitionwould you expect malonate

to work?

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Fumarase

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Malate Dehydrogenase

+

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Malate Dehydrogenase

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Regulation of the TCA CycleExcess Products

1. Prohibit entry by buildingexcess Acetyl-CoA and NADH

2. Deactivate pyruvate dehydrogenase

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Regulation of the TCA CycleInactivate Pyruvate Dehydrogenase

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Controlling Enzymes in the TCA Cycle

Citrate Synthase

Isocitrate Dehydrogenase

Alpha-Ketoglutarate Dehydrogenase

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Coming Up

• Wed: Cover Ch. 18

• Thur: Exam #3 – Chapter 15: Glycolysis– Chapter 16: Glycogen Metabolism– Chapter 17: TCA Cycle– Chapter 18: Electron Transport and

Oxidative Phosphorylation