The Tricarboxylic Acid Cycle

A time-lapse photograph of a ferris wheel at night. Aerobic cells use a metabolic wheel—the tricarboxylic acid cycle—to generate energy by acetyl-CoA oxidation. (Ferns Wheel, DelMar Fair © Corbis/Richard Cummins)

The glycolytic pathway converts glucose to pyruvate and produces two molecules of ATP per glucose—only a small fraction of the potential energy available from glucose. Under anaerobic conditions, pyruvate is reduced to lactate in animals and to ethanol in yeast, and much of the potential energy of the glucose molecule remains untapped. In the presence of oxygen, however, a much more interesting and thermodynamically complete story unfolds. Under aerobic conditions, NADH is oxidized in the electron transport chain, rather than becoming oxidized through reduction of pyruvate to lactate or acetaldehyde to ethanol, for example. Further, pyruvate is converted to acetyl-coenzyme A and oxidized to CO2 in the tricarboxylic acid (TCA) cycle (also called the citric acid

Thus times do shift, each thing his turn does hold;

New things succeed, as former things grow old.

Robert Herrick (Hesperides [1648], "Ceremonies for Christmas Eve")

Outline

20.1 • Hans Krebs and the Discovery of the

TCA Cycle

20.3 • The Bridging Step: Oxidative

Decarboxylation of Pyruvate

20.4 • Entry into the Cycle: The Citrate

Synthase Reaction

20.5 • The Isomerization of Citrate by

Aconitase

20.6 • Isocitrate Dehydrogenase—The First

Oxidation in the Cycle

20.7 • a-Ketoglutarate Dehydrogenase—A

Second Decarboxylation

20.8 • Succinyl-CoA Synthetase—A Substrate

Level Phosphorylation

20.9 • Succinate Dehydrogenase—An

Oxidation Involving FAD

20.10 • Fumarase Catalyzes Trans-Hydration of

Fumarate

20.11 • Malate Dehydrogenase—Completing the Cycle

20.13 • The TCA Cycle Provides Intermediates for Biosynthetic Pathways

20.14 • The Anaplerotic, or "Filling Up,"

Reactions

20.15 • Regulation of the TCA Cycle

20.16 • The Glyoxylate Cycle of Plants and

Bacteria

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