Chapter 16.2: The Citric Acid Cycle

CHEM 7784Biochemistry

Professor Bensley

CHAPTER 16 The Citric Acid Cycle

– Reactions of the citric acid cycle

Today’s Objectives: (To learn and understand the)

Respiration: Stage 1

Generates some: ATP, NADH, FADH2

Respiration: Stage 2

Generates more NADH, FADH2 and

one GTP

The Citrate Synthase Reaction (Step #1)

• The only cycle reaction with C-C bond formation

• Essentially irreversible process

Isomerization of Citrate by Aconitase (Step #2)

The Isocitrate Dehydrogenase Reaction (Step #3)

Oxidation of the alcohol to ketone involves the transfer of a hydride from the C-H of the alcohol to the nicotinamide cofactor – 3 step mechanism

Oxidation of -ketoglutarate (Step #4)

Substrate-Level Phosphorylation (Step #5)

Succinate Dehydrogenase (Step #6)

Hydration of

Fumarate to Malate (Step #7)

Oxidation of Malate to Oxaloacetate (Step #8)

Products from One Turn of the Cycle

Net Effect of the Citric Acid Cycle

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

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

• Carbons of acetyl groups in acetyl-CoA are oxidized to CO2

• Electrons from this process reduce NAD+ and FAD

• One GTP is formed per cycle, this can be converted to ATP

• Intermediates in the cycle are not depleted

Direct and Indirect ATP Yield

Role of the Citric Acid Cycle in Anabolism

Chapter 16: Summary

• Citric acid cycle is an important catabolic process: it makes GTP, and reduced cofactors that could yield ATP

• Citric acid cycle plays important anabolic roles in the cell

• A large multi-subunit enzyme, pyruvate dehydrogenase complex, converts pyruvate into acetyl-CoA

• Several cofactors are involved in reactions that harness the energy from pyruvate

• The rules of organic chemistry help to rationalize reactions in the citric acid cycle

In this chapter, we learned that: