Krebs Cycle

Once acetyl CoA has been formed, the acetic acid subunit (two carbons long) combines with oxaloacetic acid (four carbons long) to form a molecule of citric acid (six carbons long). Coen-zyme A acts only as a transporter of acetic acid from one enzyme to another (similar to the transport of hydrogen by NAD). The formation of citric acid begins a cyclic metabolic pathway known as the citric acid cycle, or TCA cycle (for tricarboxylic acid; citric acid has three carboxylic acid groups). Most commonly, however, this cyclic pathway is called the Krebs cycle, after its principal discoverer, Sir Hans Krebs. A simplified illustration of this pathway is shown in figure 5.7.

Glycolysis

3 Pyruvic acid

3 Pyruvic acid

Simplified Glycolysis

MITOCHONDRION

2 Acetyl CoA

MITOCHONDRION

Oxaloacetic acid

2 Acetyl CoA

Krebs cycle

a-Ketoglutaric acid

Krebs cycle

■ Figure 5.7 A simplified diagram of the Krebs cycle. This diagram shows how the original four-carbon-long oxaloacetic acid is regenerated at the end of the cyclic pathway. Only the numbers of carbon atoms in the Krebs cycle intermediates are shown; the numbers of hydrogens and oxygens are not accounted for in this simplified scheme.

C6 Citric acid

Fox: Human Physiology, 5. Cell Respiration and Text © The McGraw-Hill

Eighth Edition Metabolism Companies, 2003

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