beta]-Glucan Synthesis in the Cotton Fiber (II. 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 Ka of 1.16 mM; (b) cellulose synthesis is increased 12-fold by a combination of cellobiose (Ka = 3.26 mM) and cyclic-3prime]:5prime]-GMP (Ka = 100 mu]M); (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 (Ka = 0.89 mM); (e) cellulose synthase also requires a low concentration of calcium (Ka = 90 mu]M); (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 Km for UGP-Glc for cotton (Gossypium hirsutum) cellulose synthase is 0.40 mM; (h) the Km for UDP-Glc for for beta]-1,3-glucan synthase is 0.43 mM.