Glyoxylate Cycle

1. Glyoxylate Cycle Dr. Sooad Al-Daihan Biochemistry department

2. Overview • An anabolic metabolic pathway occuring in plants, and several microoragnisms , BUT not animals. • Occurs in glyoxysome • Glyoxysomes are not present in all plant tissues at all times. They develop in lipid-rich seeds during germination. • The enzymes common to the TCA cycle and the glyoxylate cycle are isoenzymes, one specific to mitochondria, the other to glyoxysomes. • The glyoxylate cycle allows plants to use acetyl-CoA derived from β-oxidation of fatty acids for carbohydrate synthesis

3. Glyoxylate Cycle • Acetyl CoA condenses with oxaloacetate to form citrate, which is catalyzed by citrate synthase. • Citrate converted to isocitrate,byaconitase. • Isocitrateiscleaved by isocitratelyase forming succinate and glyoxylate • Glyoxylate condenses with 2nd acetyl CoA to yield malate by malate synthase • Malate is then oxidized to oxloacetate which will condense with another molecule of Acetyl CoA to start another turn Or enters the cytosol and oxidized to oxaloacetate (precursor of glucose via gluconeogensis) 1 5 2 4 3

4. Relationship between the glyoxylate and citric acid cycles. • Succinate returns to mitochondria , where it re-enters the TCA cycle and is transformed to oxaloacetate, which can again be exported (via aspartate) to the glyoxysome • Notes: • Each turn of this cycle consumes 2 molecules of Acetyl CoA and produce 1 molecule of succinate • The glyoxylatecycle, shares three steps with the citric acid cycle but bypasses the two decarboxylation steps