University of Wisconsin

Glucose-1,6-bisphosphate as substrate for enzymatic phosphorylation of starch

— Pan, D

The presence of phosphorylated polyglan in corn starch as well as in other cereals has been known for several decades, but the precise biological mechanism responsible for phosphorylating starch is not fully understood. Although the levels of starch phosphate appear low, starch-bound phosphates constitute a major part of the total phosphate pool in cereals. The level of phosphates attached to starch affects the viscosity of gelatinzed starch. This is significant because of the diversified uses of starch for foods and for industrial purposes. The phosphate group of starch granula had been thought to attach to the C6 of the glucose moiety. However, the linking phosphate group to C1 or C3 of glucose residue has not been ruled out. We report here that an enzyme found in developing maize endosperm can phosphorylate starch with glucose-1,6-bisphoshate as a substrate by the reaction mechanism shown in Figure 1.

The phosphorylation of starch can be demonstrated by incubation of soluble polyglan such as phytoglycogen with the enzyme preparation and 14C or 32P labeled glucose-1,6-bisphosphate as a substrate. The phosphorylated phytoglycogen is identified by measuring the incorporation of radioactivity into phytoglycogen through ethanol precipitation versus control experiments in the absence of the enzyme preparation.

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