β-Glucan synthesis in the cotton fiber: II. Regulation and kinetic properties of β-glucan synthases

The regulation and kinetic properties of cellulose synthase as well as β-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 β-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) β-1,3-glucan synthesis is enhanced up to 31-fold by cellobiose with a K_a of 1.16 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 μM); (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 MM); (f) the optimal pH for cellulose synthase (7.6-8.0) is slightly higher than that for β-1,3-glucan synthase (7.2-7.6); (g) the K_m for UCP-Glc for cotton (Gossypium hirsutum) cellulose synthase is 0.40 mM; (h) the K_m for UDP-Glc for for β-1,3-glucan synthase is 0.43 mM.