Substrate Activation of beta-(1 --> 3) Glucan Synthetase and Its Effect on the Structure of beta-Glucan Obtained from UDP-d-glucose and Particulate Enzyme of Oat Coleoptiles.

UDP-d-glucose, at a micromolar level in the presence of MgCl(2) and oat (Avena sativa) coleoptile particulate enzyme which contains both beta-(1 --> 3) and beta-(1 --> 4) glucan synthetases, produces glucan with mainly beta-(1 --> 4) glucosyl linkages. An activation of beta-(1 --> 3) glucan synthetase by UDP-d-glucose and a decrease in the formation of beta-(1 --> 3) glucan in the presence of MgCl(2) have been observed. However, at high substrate concentration (>/= 10(-4)m), the activation of beta-(1 --> 3) glucan synthetase is so pronounced that the formation of beta-(1 --> 3) glucosyl linkage predominates in synthesized glucan regardless of the presence of MgCl(2). These observations may explain the striking shift in the composition of glucan of particulate enzyme from a beta-(1 --> 4) to beta-(1 --> 3) glucosyl linkage when UDP-d-glucose concentration is raised from a low concentration (</= 10(-5)m) to a higher concentration (>/= 10(-4)m).Besides UDP-d-glucose, CDP-d-glucose can also serve as substrate for the formation of beta-(1 --> 3) glucan in the presence of beta-(1 --> 3) synthetase.