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

Abstract

UDP-d-glucose, at a micromolar level in the presence of MgCl2 and oat (Avena sativa) coleoptile particulate enzyme which contains both β-(1 → 3) and β-(1 → 4) glucan synthetases, produces glucan with mainly β-(1 → 4) glucosyl linkages. An activation of β-(1 → 3) glucan synthetase by UDP-d-glucose and a decrease in the formation of β-(1 → 3) glucan in the presence of MgCl2 have been observed. However, at high substrate concentration (≥ 10−4m), the activation of β-(1 → 3) glucan synthetase is so pronounced that the formation of β-(1 → 3) glucosyl linkage predominates in synthesized glucan regardless of the presence of MgCl2. These observations may explain the striking shift in the composition of glucan of particulate enzyme from a β-(1 → 4) to β-(1 → 3) glucosyl linkage when UDP-d-glucose concentration is raised from a low concentration (≤ 10−5m) to a higher concentration (≥ 10−4m).