Chapter 16.2:
The Citric Acid Cycle
CHEM 7784
Biochemistry
Professor Bensley
CHAPTER 16
The Citric Acid Cycle
Today’s Objectives:
(To learn and understand the)
– Reactions of the citric acid cycle
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
In this chapter, we learned that:
• 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