(1→3)-β-d-Glucan Synthase from Sugar Beet

Abstract

The mode of inhibition of UDP, one of the products of the reaction catalyzed by (1.fwdarw.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 Ca2+, Mg2+, and cellobiose, exhibited Michaelis-Menten kinetics and had an apparent Km of 260 micromolar for UDP-glucose. Complex effects on the kinetics of the (1.fwdarw.3)-.beta.-D-glucan synthase were observed in the presence of UDP. At high UDP-glucose concentrations, i.e. greater than the apparent Km, UDP behaved as a competitive inhibitor with an apparent Ki 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.fwdarw.3)-.beta.-D-glucan synthase. However, papulacandin B, a potent inhibitor of this fungal enzyme, had no effect on the plant (1.fwdarw.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.