Glyoxylate Cycle Dr Sooad AlDaihan Biochemistry department Overview

Glyoxylate Cycle Dr. Sooad Al-Daihan Biochemistry department

Overview ü An anabolic metabolic pathway occuring in plants, and several microoragnisms , BUT not animals. ü Occurs in glyoxysome ü Glyoxysomes are not present in all plant tissues at all times. They develop in lipid-rich seeds during germination. ü 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-Co. A ü derived from β-oxidation of fatty acids for carbohydrate synthesis

Glyoxylate Cycle Acetyl Co. A condenses with oxaloacetate to form citrate, which is catalyzed by citrate synthase. Citrate converted to isocitrate, by aconitase. Isocitrateis cleaved by isocitrate lyase forming succinate and glyoxylate Glyoxylate condenses with 2 nd acetyl Co. A to yield malate by malate synthase Malate is then oxidized to oxloacetate which will condense with another molecule of Acetyl Co. A to start another turn Or enters the cytosol and oxidized to oxaloacetate (precursor of glucose . 1 1. 2 5 . 3 2 . 4 4. 5 3

Relationship between the glyoxylate and citric acid cycles. üSuccinate 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: üEach turn of this cycle consumes 2 molecules of Acetyl Co. A and produce 1 molecule of succinate üThe glyoxylate cycle, shares three steps with the citric acid cycle but bypasses the two decarboxylation steps