Enzymatic Characteristics of Cellobiose Phosphorylase from Ruminococcus albus NE1 and Kinetic Mechanism of Unusual Substrate Inhibition in Reverse Phosphorolysis

Cellobiose phosphorylase (CBP) catalyzes the reversible phosphorolysis of cellobiose to produce alpha-D-glucopyranosyl phosphate (Glc1P) and D-glucose. It is an essential enzyme for the metabolism of cello-oligosaccharides in a ruminal bacterium, Ruminococcus albus. In this study, recombinant R. albus CBP (RaCBP) produced in Escherichia coli was characterized. It showed highest activity at pH 6.2 at 50 degrees C, and was stable in a pH range of 5.5-8.8 and at below 40 degrees C. It phosphorolyzed only cellobiose efficiently, and the reaction proceeded through a random-ordered bi hi mechanism, by which inorganic phosphate and cellobiose bind in random order and D-glucose is released before Glc1P. In the synthetic reaction, RaCBP showed highest activity to D-glucose, followed by 6-deoxy-D-glucose. D-Mannose, 2-deoxy-D-glucose, D-glucosamine, D-xylose, 1,5-anhydro-D-glucitol, and gentiobiose also served as acceptors, although the activities for them were much lower than for D-glucose. D-Glucose acted as a competitive-uncompetitive inhibitor of the reverse synthetic reaction, which bound not only the Glc1P site (competitive) but also the ternary enzyme-Glc1P-D-glucose complex (uncompetitive).