Hydrolyses of alpha- and beta-cellobiosyl fluorides by cellobiohydrolases of Trichoderma reesei

Cellobiohydrolase II hydrolyses alpha- and beta-D-cellobiosyl fluorides to alpha-cellobiose at comparable rates, according to Michaelis-Menten kinetics. The stereochemistry, absence of transfer products and strict hyperbolic kinetics of the hydrolysis of alpha-cellobiosyl fluoride suggest that the mechanism for the alpha-fluoride may be the enzymic counterpart of the SNi reaction observed in the trifluoroethanolysis of alpha-glucopyranosyl fluoride [Sinnott and Jencks (1980) J. Am. Chem. Soc. 102, 2026-2032]. The absolute factors by which this enzyme accelerates fluoride ion release are small and greater for the alpha-fluoride than for the beta, suggesting that its biological function may not be just glycoside hydrolysis. Cellobiohydrolase I hydrolyses only beta-cellobiosyl fluoride, which is, however, an approx. 1-3% contaminant in alpha-cellobiosyl fluoride as prepared and purified by conventional methods. Instrumental assays for the various components of the cellulase complex are discussed.