BETA-GLUCAN SYNTHESIS IN THE COTTON FIBER .2. REGULATION AND KINETIC-PROPERTIES OF BETA-GLUCAN SYNTHASES

The regulation and kinetic properties of cellulose synthase as well as beta-1,3-glucan synthase have been studied. The cellulose was detected using acetic/nitric acid insolubility as an indicator of cellulose (this product contained only beta-1,4-linked glucans; K. Okuda, L. Li, K. Kudlicka, S. Kuga, R.M. Brown, Jr. [1993] Plant Physiol 101: 1131-1142). These studies reveal that (a) beta-1,3-glucan synthesis is enhanced up to 31-fold by cellobiose with a K. of 1. 1 6 mm; (b) cellulose synthesis is increased 12-fold by a combination of cellobiose (K(a) = 3.26 mm) and cyclic-3':5'-GMP (K(a) = 100 mum); (c) the common components in the reaction mixture required by both enzymes are cellobiose, calcium, and digitonin; (d) cellulose synthase has an essential requirement for magnesium (K(a) = 0.89 mm); (e) cellulose synthase also requires a low concentration of calcium (K(a) = 90 mum); (f) the optimal pH for cellulose synthase (7.6-8.0) is slightly higher than that for beta-1,3-glucan synthase (7.2-7.6); (g) the K(m) for UGP-Glc for cotton (Gossypium hirsutum) cellulose synthase is 0.40 mm; (h) the K(m) for UDP-Glc for for beta-1,3-glucan synthase is 0.43 mm.