(1-->3)-beta-d-Glucan Synthase from Sugar Beet : II. Product Inhibition by UDP.

The mode of inhibition of UDP, one of the products of the reaction catalyzed by (1-->3)-beta-d-glucan synthase in sugar beet (Beta vulgaris L.) was investigated. In the absence of added UDP, the enzyme, in the presence of Ca(2+), Mg(2+), 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 (1-->3)-beta-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 (1-->3)-beta-d-glucan synthase. However, papulacandin B, a potent inhibitor of this fungal enzyme, had no effect on the plant (1-->3)-beta-d-glucan synthase isolated from sugar beet petioles. The inhibitory effect of UDP was also compared with other known inhibitors of glucan synthases.