ABSTRACT
In chloroplasts, both biosynthesis and degradation of starch are strictly regulated but the mechanisms involved are still incompletely understood. Recent studies revealed two novel and regulatory relevant aspects in the biochemistry of starch: the phosphorylation of starch and the starch-related metabolism of cytosolic heteroglycans. Starch phosphorylation occurs by a sequential action of two plastidial enzymes, the glucan, water dikinase (GWD; EC 2.7.9.4) and the phosphoglucan, water dikinase (PWD; EC 2.7.9.5). Both enzymes utilize ATP as dual phosphate donor and transfer the terminal phosphate group to water whereas the beta-phosphate is used for esterification of glucosyl moieties. The metabolism of starch-derived degradation products is closely linked to recently discovered cytosolic heteroglycans that possess, as prominent constituents, arabinose, galactose, glucose and fucose. The pattern of glycosidic linkages is highly complex comprising more than 25 different bonds. During the dark period the size distribution or the amount of the cytosolic heteroglycans increases depending on the plant species. As revealed by in vitro 14C labeling assays, the heteroglycans act as both glucosyl acceptors and donors for two cytosolic glucosyl transferases, the phosphorylase (EC 2.4.1.1) and the transglucosidase (EC 2.4.1.25) and, at least in part, both enzymes utilize the same glucosyl acceptor and donor sites. In mutants of Arabidopsis thaliana L. that are deficient in the cytosolic transglucosidase both the structure and (bio)chemical properties of the heteroglycans are altered.
