Starch granules bound enzyme

Isozymes of plant starch synthases (2, 16-20) and branching enzymes (2, 16, 21-26) have been characterized. Presumably, they play distinct roles in amylopectin and amylose synthesis. Also, plant and Chlamydomonas reinhardtii granule-bound starch synthases (27-33) are responsible for synthesis of amylose. Mutants defective in this enzyme are known as waxy mutants and contain starch granules that have little or no amylose. 

The activity of starch-UDPglucosyl transferase in potato tubers is high, and fractionation of the subcellular particles in sucrose-citrate media has given a starch-granule fraction showing 400-2000 times the activity of the above bean preparation. The presence of an enzyme which is active only when bound to starch granules (where the structural relationship between enzyme and substrate is similar to that in plant cells) is suggestive of an in vivo function. 

Proteins, particularly granule-bound starch synthesis enzymes entrapped in dried starch granules, are also involved in the determination of the starch-pasting characteristics, even when they are present at less than 0.2% in dried starch. Figure... 

Genetically modiHed (anti-GBSS) potato. Potato starch consists of two components amylose (20%) and amylopectin (80%). Granule-bound starch synthase (GBSS) is the major enzyme involved in the formation of amylose. 

UDP-Glc essentially vanished. Either the activity observed with UDP-Glc was not solubilized, suggesting that a different enzyme catalyzed the UDP-Glc activity, or became inactive during the amylase procedure. Alternatively, the ability of the starch-bound starch synthase to utilize UDP-Glc is dependent on the close association of the enzyme with the starch granule. In other words, the conformation of the GBSS is altered in the presence of starch to allow UDP-Glc to be catalytically active. 

A C. reinhardtii mutant lacking D-enzyme activity has been characterized and has been shown to have significantly lower levels of starch. Other enzymes involved in starch metabolism such as ADP-Glc PPase, granule-bound and soluble starch synthase, BE, phosphorylase, a-glucosidase, amylases, and debranching enzyme activities were not affected. The starch content in the mutant was about 6-13% of wild type and there was an excessive accumulation of maltooligosaccharides up to a polymer size of 16 glucose units. 

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