The β-galactosidase-catalysed hydrolyses of β-d-galactopyranosyl pyridinium salts. Rate-limiting generation of an enzyme-bound galactopyranosyl cation in a process dependent only on aglycone acidity

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Abstract
1. β-d-Galactopyranosyl pyridinium salts are well-behaved substrates for the β-galactosidase of Escherichia coli, catalysis occurring by the interaction of the salt itself with the normal active site of the protein. 2. logkcat. values for seven such salts show a linear relationship (correlation coefficient=−0.997) with the pKa of the parent pyridine. 3. The β-d-galactopyranosyl derivatives of pyridine and 4-bromoisoquinoline exhibit α-deuterium kinetic isotope effects of 1.136±0.040 and 1.187±0.046 on their enzymic hydrolysis, indicating formation of a galactopyranosyl cation in the rate-limiting step. 4. This behaviour of the pyridinium salts contrasts with the behaviour of aryl galactosides and this contrast can be accommodated by the β-galactosidase mechanism of Sinnott & Souchard (1973). 5. The α-deuterium kinetic isotope effect for the hydrolysis of β-d-galactopyranosyl azide is 1.098±0.033; comparison of the kcat. value of the azide with that of a pyridinium salt of the same aglycone pKa enables a maximum factor of 70 to be ascribed to the acceleration of the departure of azide by intracomplex general acid catalysis. 6. The possibility of the rate-limiting process in the glycosidase-catalysed hydrolysis of aryl glycosides being a conformation change is considered for a number of glycosidases where correlations of kcat. with aglycone acidity, reported in the literature, have been unsuccessful.