Evidence for intermediate formation in the mechanism of potato starch phosphorylase from exchange of the ester and phosphoryl oxygens of α-D-glucopyranose 1-phosphate

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Abstract
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) was examined. In the presence of phosphorylase and starch, 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 Pi 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. Under these latter conditions a glucosyl cation or covalent glucosyl-enzyme intermediate is apparently 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.

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