anaplerotic reactions

Anaplerotic reactions form intermediates for the Krebs, TCA, citric acid cycle.

Two major types of anaplerotic reactions have been observed:
1. Anaplerotic carbon dioxide fixation such as the pyruvate carboxylase reaction.
2. Glyoxylate cycle used by acetogens, microorganisms that can grow on acetate as a sole carbon source in this modified TCA cycle. The enzymes isocitrate lyase and malate synthase, which convert isocitrate into succinate and malate (via glyoxylate) into the glyoxylate cycle.

Four reactions are classed as anaplerotic, although the production of oxaloacetate from pyruvate is probably the most important physiologically. The anaplerotic reactions are:

1. carboxylation of pyruvate to oxalocetate (malate can be formed similarly, though thermodynamics favour the reaction malate to pyruvate)

pyruvate + CO2 + H2O + ATP →pyruvate carboxylase→ oxaloacetate + ADP + Pi + 2H+

2. transamination of aspartate to oxaloacetate by aspartate aminotransferase, which allows incorporation of acetyl CoA into citrate via citrate synthase in glyoxysomes.

3. hydration of glutamate to α-ketoglutarate

glutamate + NAD+ + H2O →glutamate-dehydrogenase→ NH4+ + α-ketoglutarate + NADH + H+

4. β-oxidation of fatty acids to succinyl-CoA

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