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Metabolism and Enzymology

(13)--D-Glucan Synthase from Sugar Beet ¹

II. Product Inhibition by UDP

Department of Botany, University of California, Davis, California 95616

The mode of inhibition of UDP, one of the products of the reaction catalyzed by (13)--D-glucan synthase in sugar beet (Beta vulgaris L.) was investigated. In the absence of added UDP, the enzyme, in the presence of Ca²⁺, Mg²⁺, and cellobiose, exhibited Michaelis-Menten kinetics and had an apparent K_m of 260 micromolar for UDP-glucose. Complex effects on the kinetics of the (13)--D-glucan synthase were observed in the presence of UDP. At high UDP-glucose concentrations, i.e. greater than the apparent K_m, UDP behaved as a competitive inhibitor with an apparent K_i of 80 micromolar. However, at low UDP-glucose concentrations, reciprocal plots of enzyme activity versus substrate concentration deviated sharply from linearity. This unusual effect of UDP is similar to that reported for fungal (13)--D-glucan synthase. However, papulacandin B, a potent inhibitor of this fungal enzyme, had no effect on the plant (13)--D-glucan synthase isolated from sugar beet petioles. The inhibitory effect of UDP was also compared with other known inhibitors of glucan synthases.