m-Fluorotyrosine substitution in β-galactosidase; Evidence for the existence of a catalytically active tyrosine

The pH profiles of β-galactosidase, having tyr replaced by m-fluorotyrosine, were compared to those of normal enzyme. The inflection point on the alkaline side was lowered about 1.5 pH units in the fluoro-enzyme, corresponding to the difference in the phenolic pKa values of m-fluorotyrosine and tyr. When glycosidic bond breakage was rate-limiting, the Vm at pH 7.0 was higher for the fluoro-enzyme. When hydrolysis was rate-limiting or when acceptors which made transgalactosylis rate-limiting were used, the Vm was lower for the fluoro-enzyme. This shows that a tyr in β-galactosidase is a general-acid catalyst in the glycosidic bond breaking reaction and a tyr (probably the same one) is a general-base catalyst in the hydrolytic reaction.