Enzymic Mechanism of Starch Synthesis in Plants

An important development in the field of polysaccharide biochemistry has been the discovery of starch synthetase in beans, potatoes and corn seedlings by Leloir and coworkers•>. The enzyme was found to be closely bound to the starch granules and to catalyze the transfer of glucose moiety from UDP-glucose into polysaccharides containing linkage of the a-D-(H4) type. Subsequently, the granule-bound starch synthetase was observed to transfer the glucose residues into starch being 10 times as fast from ADP-glucose as from UDP-glucoseu. The author et al. have also found the similar enzyme in some plants, including rice grains>.•> and soybean leaves» as well as a soluble starch synthetase in glutinous rice grains'>, where the starch granule-bound enzyme is not detectable. At present it may be said that the accumulation of _many data has established a basic enzymic mechanism of the starch synthesis. It is not, however, still known about a relationship between the physical structure of starch granules and its biosynthetic process and about the enzymic mechanism for the formation of a-D-(1,6) bonds (branching points) in starch molecules. I have recently found that the granule-bound starch synthetase of various plant origins requires K · for its maximum activityn.o and that sucrose synthetase has a regulatory property in the reaction of sucrose cleavage0 ·m. This paper deals with these experiments. Mechanism of glucose transfer from sucrose into starch