Fig. 7.

Evolutionary connection of glyoxylate cycle and purine catabolism. The glyoxylate shunt (green arrows) bypasses two decarboxylation reactions of the Krebs cycle by converting isocitrate into succinate and glyoxylate through the enzyme isocitrate lyase (ICL). Glyoxylate is converted into (S)-malate, a Krebs cycle intermediate, by malate synthase (MS) through condensation with acetyl-CoA. Glyoxylate is also formed as the end product of purine degradation from ureidoglycolate in the reaction catalyzed by ureidoglycolate lyase (UGL). The evolutionary origin of MS and UGL by an ancient gene duplication in eukaryotes (red line) suggests a link between the two metabolic pathways and an anaplerotic role of purine catabolism in early eukaryotes.