glyoxylate cycle

Bacteria and some species of higher plants are able to obtain a net increase in malate or oxaloacetate through expression of enzymes of the glyoxylate cycle or glyoxylate shunt. The enzymes of the TCA cycle and the glyoxylate cycle are physically segragated, with the glyoxylate cycle enzymes of some plants localized in a specialized organelle called the glyoxysome. The two additional enzymes that permit the glyoxylate shunt are isocitrate lyase and malate synthase, which convert isocitrate to succinate or to malate via glyoxylate (reaction).

Glyoxysomes import fatty acids and aspartate, which provide acetyl-CoA to the shunt. The glyoxysome lacks means for reoxidizing NADH, so has none of the dehydrogenases of the TCA cycle. Aspartate transaminase (aspartate aminotransferase) converts aspartate into oxaloacetate, permitting incorporation of acetyl CoA into citrate via citrate synthase. Glyoxysomal aconitase is present, but isocitrate lyase is found in the glyoxysome instead of isocitrate dehydrogenase. The succinate generated by the lyase is then exported back to the mitochondria since there is no glyoxysomal succinate dehydrogenase. (diagram) Mitochondria oxidize succinate to oxaloacetate, and aspartate transminase converts it back to aspartate, maintaining the cycle. Meanwhile, the glyoxysomes incorporate a second acetyl CoA to produce malate, which is exported to the cytoplasm for gluconeogenesis.

Two acetyl CoA are input per glyoxylate cycle with no loss of CO2, making possible net synthesis of a 4-carbon product. (diagram) The glyoxylate cycle bypasses reactions of TCA cycle in which CO2 is released, conserving 4 carbon compounds for biosynthesis. The glyoxylate cycle is also called the glyoxylate bypass or glyoxylate shunt. The anaplerotic glyoxylate pathway is active when growth on 2 carbon compounds requires conservation of 4 carbon TCA intermediates. Two molecules of acetylCoA are taken up per turn of the glyoxylate cycle, and acetylCoA is generated by acetate thiokinase in the reaction:

acetate + CoA + ATP = acetyl-CoA + AMP + Pi

Alternatively, acetylCoA is generated by β oxidation of fatty acids. The glyoxylate cycle is repressed during growth on glucose, and induced by growth on acetate.

The the net reaction is
2 acetyl-CoA + NAD+ → succinate + 2 CoA + NADH + H+

Superpathways: superpathway of glyoxylate bypass and TCA , superpathway of glycolysis, pyruvate dehydrogenase, TCA, and glyoxylate bypass : Variants: TCA cycle , pyruvate oxidation pathway , pyruvate dehydrogenase complex :

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