Properties of ?-glucan synthetase from Saccharomyces cerevisiae

Abstract

Properties of β-glucan synthetase from S. cerevisiae were studied. The enzyme exhibited optimal activity at pH 6.7 and 24 C. Km for UDP-glucose was 0.12mm. Addition of Mg⁺⁺ or Mn⁺⁺ stimulated its activity by 60% and 21% respectively. High concentrations of EDTA and hydroxyquinoline were inhibitory. Glucan synthetase was fully active in cell-free extracts. Small concentrations of trypsin or subtilopeptidase A from Bacillus subtilis, caused only a slight increase in glucosyl transferase activity, but larger concentrations destroyed β-glucan synthetase. Acid proteases were neither stimulatory nordestructive. Thus it seemsunlikelythat β-glucan synthetase exists in a zymogen form. Glucan synthetase was unstable. It was inactivated more rapidly at 28 C than at 0 C. The presence of substrate, β-glucan or the protease inhibitors PMSF, Antipain or Pepstatin A did not protect β-glucan synthetase from inactivation. Glucan synthetase was not stimulated by addition of cellobiose or β-glucans. The synthesis of β-glucans was competitively inhibited by UDP (K_i=0.45mm). Glucono-δ-lactone, a known inhibitor of β-glucosidases was a strong non-competitive inhibitor of β-glucan synthetase.