GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

12. METABOLIC ACTIVITY OF AEROBIC HETEROTROPHS

12.1. METABOLISM OF C₂ COMPOUNDS

12.1.2. Glyoxylate and oxalate as substrates. Glycerate pathway

The conversion pathway of the most oxidized C₂ compound, oxalate (HOOC-COOH), into glyoxylate (O=CH-COOH) requires preliminary reduction of the substrate. The source of reducing equivalents is The oxidation of a portion of the oxalate to CO₂ following its decarboxylation (Fig. 12.3). Thus, oxaloacetate is converted into oxalyl-CoA by the enzyme oxalyl-CoA synthetase, which is then decarboxylated to formyl-CoA (catalyzed by oxalyl-CoA decarboxylase). Formyl-CoA is transformed into formate, which is oxidized to CO₂ by formate dehydrogenase. The reducing equivalents generated in the formate dehydrogenase reaction are utilized for the reduction of oxalate to glyoxylate. Specifically, oxalyl-CoA is reduced to glyoxylate by the action of glyoxylate dehydrogenase.

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Fig. 12.3. METABOLISM of oxalate and glyoxylate

When glyoxylate (or its precursors, such as glycolate (HOOC-CH₂OH) or uric acid) serves as the carbon source, the Enzymes of the glycerate pathway are induced.

Two molecules of glyoxylate are converted into tartronate semialdehyde by glyoxylate carboligase, with the release of CO₂. This semialdehyde is reduced to glycerate by tartronate semialdehyde reductase, and glycerate is subsequently phosphorylated to 3-phosphoglycerate by glycerate kinase. From 3-phosphoglycerate, acetyl-CoA is formed via standard pathways, which then enters The Tricarboxylic Acid Cycle and undergoes oxidation.

The supply of intermediates required for anabolic metabolism is provided by malate synthase. It catalyzes the Condensation of a glyoxylate molecule with acetyl-CoA to yield malate. Carbohydrate synthesis is achieved through the reactions of Gluconeogenesis.