ADPglucose pyrophosphorylase: basic science and applications in biotechnology

Abstract

The enzymatic reactions of bacterial glycogen and plant starch synthesis are similar and some of the properties of the biosynthetic enzymes are compared. Regulation occurs at the synthesis of ADPglucose and in almost all cases, ADPglucose pyrophosphorylase, is allosterically activated about 10- to over 40-fold by glycolytic intermediates and inhibited by AMP, ADP or Pi. The activator specificity of the ADPglucose pyrophosphorylase varies with respect to the source of enzyme and can be correlated to the major assimilation pathway occurring in the organism. For example, ADPglucose pyrophosphorylases from plants and other oxygenic photosynthetic organisms are activated by 3-phosphoglycerate. Organisms using glycolysis for carbon assimilation have ADPglucose pyrophosphorylases with fructose-1,6-bis-phosphate as the major activator. Chemical modification and site-directed mutagenesis studies that have determined the activator binding sites for some enzymes are described. The structural genes of Escherichia coli ADPglucose pyrophosphorylase allosteric mutants which no longer require activator for activity have been isolated. Transformation of plant systems with an allosteric bacterial mutant gene (but not with the wild-type gene) increases their starch content. Transformed potato tubers can have 25-60% more starch than the normal tuber indicating the importance of allosteric regulation of ADPglucose synthesis. The increase of a normal plant product by transformation of the plant with a gene encoding the rate-limiting enzyme in starch synthesis is an important biotechnological advance and suggests the possibilities of changing starch composition (extent of branching and chain sizes) via transformation with the starch synthase and branching enzyme genes.