Starch Synthesizing Reactions and Paths: in vitro and in vivo Studies

Abstract

This review is written for plant biologists, biochemists and biotechnologists. It presents recent results and views on the metabolism of transitory and reserve starch. We discuss several topics related to starch biosynthesis and focus on the elongation of α-glucan chains as mediated by the various starch synthases and by the plastidial phosphorylase. We briefly discuss the two main glucosyl donors, i.e. ADPglucose or glucose 1-phosphate, that are used by the two types of glucosyl transferases. In the next section, we present a novel theoretical approach to analyse reiterating reactions mediated by a single enzyme. This type of reactions occurs frequently in carbohydrate metabolism of pro- and eukaryotic cells. We then give a schematic presentation of the structure of starch synthases from Arabidopsis thaliana and discuss the action of the different isoforms. The two types of phosphorylases that are common in plants are described and their in vivo actions are presented. We describe functional properties of heterogeneous protein complexes and explain why they are needed in starch metabolism. Finally, we discuss phenotypical analyses of starch-related mutants and explain some of the difficulties inherent to the attempt to deduce biochemical paths from phenotypical features.