Properties of beta-glucan synthetase from Saccharomyces cerevisiae

Properties of beta-glucan synthetase from S. cerevisiae were studied. The enzyme exhibited optimal activity at pH 6.7 and 24 C. Km for UDP-glucose was 0.12 mM. 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 beta-glucan synthetase. Acid proteases were neither stimulatory nor destructive. Thus it seems unlikely that beta-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, beta-glucan or the protease inhibitors PMSF, Antipain or Pepstatin A did not protect beta-glucan synthetase from inactivation. Glucan synthetase was not stimulated by addition of cellobiose or beta-glucans. The synthesis of beta-glucans was competitively inhibited by UDP (Ki = 0.45 mM). Glucono-delta-lactone, a known inhibitor of beta-glucosidases was a strong non-competitive inhibitor of beta-glucan synthetase.