Evidence for intermediate formation in the mechanism of potato starch -hosphorylase from exchange of the ester and phosphoryl oxygens of alpha-D-glucopyranose 1-phosphate.
We have examined under a variety of conditions the ability of potato starch phosphorylase to cause exchange of the ester and phosphoryl oxygens of alpha-D-glucopyranose 1-phosphate (Glc-1-P). In the presence of phosphorylase and strach, under conditions where 40-50% of the glc-1-P is consumed in starch elongation, little if any exchange occurs that cannot be accounted for by accompanying starch phosphorolysis. Nor are the oxygens scrambled in the same or longer times by enzyme only when no release of inorganic phosphate occurs. But when D-maltotriose is used as a primer, or during primerless synthesis, and in the presence of phosphorylase and alpha- or beta-cyclodextrin, a large degree of scrambling does occur. We conclude that under these latter conditions a glucosyl cation or covalent glycosyl-enzyme intermediate in formed. If this same intermediate is formed in the absence of starch or its analogue, then the phosphate counterion is not free to rotate; if the intermediate is formed with starch, then again one must assume that the rotation of phosphate ion is hindered, or that formation of the intermediate is rate determining.
D000469:Alkaline Phosphatase; D055598:Chemical Phenomena; D002621:Chemistry; D005958:Glucosephosphates / Q000378:metabolism*; D006868:Hydrolysis; D007700:Kinetics; D010103:Oxygen Isotopes; D006005:Phosphorylases / Q000378:metabolism*; D010944:Plants / Q000201:enzymology*; D013213:Starch
D005958:Glucosephosphates; D010103:Oxygen Isotopes; D013213:Starch; D006005:Phosphorylases; D000469:Alkaline Phosphatase