Human Biochemistry, Volume 1 - Murray R. 1993

Bioenergetics and Carbohydrate and Lipid Metabolism
The Citric Acid Cycle: Catabolism of Acetyl-CoA
Catabolic Role of the Citric Acid Cycle

The cycle begins with the reaction between an acetyl-CoA molecule and a four-carbon dicarboxylic acid, oxaloacetate, yielding a six-carbon tricarboxylic acid known as citric acid. This is followed by a series of reactions that release two CO2 molecules and regenerate oxaloacetate (Fig. 17.1). Since The amount of oxaloacetate required to process A large number of acetyl groups into CO2 is quite small, oxaloacetate can be considered to play a catalytic role.

The Citric Acid Cycle serves as the mechanism that captures the major portion of the Free energy released during The oxidation of CARBOHYDRATES, Lipids, and Proteins. During the oxidation of acetyl-CoA, the action of specific dehydrogenases generates reducing equivalents in the form of hydrogen or electrons. These equivalents enter the Respiratory Chain, where Oxidative Phosphorylation takes place, resulting in ATP synthesis (Fig. 17.2; see also Chapter 13).

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Fig. 17.2. The Citric Acid cycle is the primary catabolic pathway for acetyl-CoA in aerobic organisms. Acetyl-CoA, the breakdown product of carbohydrates, proteins, and lipids, enters the cycle alongside H2O and is oxidized to CO2, supplying reducing equivalents (2H). The subsequent oxidation of 2H in the respiratory chain is coupled with the phosphorylation of ADP. Per single turn of the cycle, 11 bonds are formed via oxidative phosphorylation, and one bond is formed at the substrate level during The conversion of succinyl-CoA to succinate. — respiratory chain, FP — flavoprotein, Cyt — cytochrome, — high-energy phosphate.

The Enzymes of the citric acid cycle are localized in the mitochondrial matrix, where they exist either in a free state or attached to the inner surface of The inner mitochondrial membrane. The latter arrangement facilitates The transfer of reducing equivalents to the respiratory chain enzymes embedded in the inner mitochondrial membrane.



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