Starch Biosynthesis in Plants

Abstract Starch is a major storage compound in plants that is present both in leaves and in storage tissues. Biochemical and molecular biological data show that ADP‐glucose is the glucosyl donor for plant starch synthesis, and its synthesis is catalyzed by ADP‐glucose pyrophosphorylase. Subsequently, starch synthases catalyze the transfer of the glucosyl residue from ADP‐glucose to the oligosaccharide chains of the starch components amylose and amylopectin to form new α‐1,4‐glucosidic residues. After elongation of these α‐1,4‐glucosidic chains, the branching enzyme catalyzes a cleavage of the elongated chain and transfers the cleaved portion of the oligosaccharide chain to either another region in the amylopectin molecule or to a new amylopectin and forms a new α‐1,6‐glucosidic linkage. Amylose synthesis is catalyzed by the granule‐bound starch synthase. Regulation of starch synthesis occurs at the ADP‐glucose pyrophosphorylase step. The enzyme from higher plants, green algae, and cyanobacteria is activated allosterically by 3‐phosphoglycerate and inhibited by inorganic phosphate. Isolation of mutants and control analyses indicate that the allosteric activation and inhibition are of physiological and functional importance in the regulation of starch synthesis. Furthermore, evidence indicates that ADP‐glucose pyrophosphorylases can also be regulated by a redox mechanism. The current knowledge of the enzyme structures and critical amino acids necessary for substrate binding, allosteric effector binding, regulation, and catalysis for the ADP‐glucose pyrophosphorylase is reviewed.