Biochemistry - The Chemical Reactions of Living Cells, Volume 2 - D. Metzler 1980

Organization of Metabolism: Catabolic Pathways
Oxidation Pathways Associated with the Tricarboxylic Acid Cycle
Dicarboxylic Acid Cycle

Some Bacteria can grow on a medium containing either glycolate, Glycine, or oxalate as the sole carbon source. All of these compounds are oxidized to glyoxylate [Eq. (9-10)]. Glyoxylate, in turn, is oxidized to CO₂ and Water, providing the bacteria with energy and serving as a precursor for biosynthetic processes. Energy is generated via the dicarboxylic acid cycle (Fig. 9-5), which catalyzes the Complete oxidation of glyoxylate to CO₂. The removal of four hydrogen atoms yields two molecules of NADH, which can be oxidized via the Respiratory Chain to release energy [27]. Notably, glyoxylate acts as the primary substrate in the dicarboxylic acid cycle, whereas acetyl-CoA (which serves as the principal substrate in The Tricarboxylic Acid Cycle) Functions here as a regenerating substrate.

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The "logic" of the dicarboxylic acid cycle is quite straightforward. Acetyl-CoA contains a potentially free carboxyl group. Following the Condensation of acetyl-CoA with glyoxylate and The oxidation of the resulting hydroxyl group, this carboxyl group occupies the ß-position relative to the carbonyl group in the oxaloacetate molecule, whereas the carboxyl group derived from glyoxylate remains in the a-position. Subsequent ß-Cleavage and oxidative a-cleavage release both carboxyl groups as carbon dioxide, thereby regenerating a molecule of the substrate. The cycle is both simple and efficient. Like the tricarboxylic acid cycle, it requires thiamine pyrophosphate, without which a-cleavage would be impossible. Comparing the tricarboxylic acid cycle (Fig. 9-2) with the simpler dicarboxylic acid cycle, we see that in the former case, the initial condensation product, citrate, contains a hydroxyl group at a tertiary carbon atom. Such a hydroxyl group cannot be directly oxidized to a carbonyl group, a step necessary for subsequent chain cleavage. This necessitates the action of aconitase, which shifts the —OH group to the adjacent carbon atom. Both cycles involve the oxidation of a hydroxy acid to a keto acid, followed by ß-cleavage and oxidative a-cleavage. However, the tricarboxylic acid cycle requires additional oxidation steps to convert succinate into oxaloacetate—this is because the tricarboxylic acid cycle utilizes a more reduced substrate than the dicarboxylic acid cycle.