biostudies-otherUnknown143(3)Sinnott MLThe Biochemical journal751-62https://www.ebi.ac.uk/biostudies/studies/S-EPMC11684441. beta-d-Galactopyranosyl pyridinium salts are well-behaved substrates for the beta-galactosidase of Escherichia coli, catalysis occurring by the interaction of the salt itself with the normal active site of the protein. 2. logk(cat.) values for seven such salts show a linear relationship (correlation coefficient=-0.997) with the pK(a) of the parent pyridine. 3. The beta-d-galactopyranosyl derivatives of pyridine and 4-bromoisoquinoline exhibit alpha-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 beta-galactosidase mechanism of Sinnott & Souchard (1973). 5. The alpha-deuterium kinetic isotope effect for the hydrolysis of beta-d-galactopyranosyl azide is 1.098+/-0.033; comparison of the k(cat.) value of the azide with that of a pyridinium salt of the same aglycone pK(a) 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 k(cat.) with aglycone acidity, reported in the literature, have been unsuccessful.biostudies-otherEurope PMCSinnott MLWithers SGfalseThe beta-galactosidase-catalysed hydrolyses of beta-d-galactopyranosyl pyridium salts. Rate-limiting generation of an enzyme-bound galactopyranosyl cation in a process dependent only on aglycone acidity.1. beta-d-Galactopyranosyl pyridinium salts are well-behaved substrates for the beta-galactosidase of Escherichia coli, catalysis occurring by the interaction of the salt itself with the normal active site of the protein. 2. logk(cat.) values for seven such salts show a linear relationship (correlation coefficient=-0.997) with the pK(a) of the parent pyridine. 3. The beta-d-galactopyranosyl derivatives of pyridine and 4-bromoisoquinoline exhibit alpha-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 beta-galactosidase mechanism of Sinnott & Souchard (1973). 5. The alpha-deuterium kinetic isotope effect for the hydrolysis of beta-d-galactopyranosyl azide is 1.098+/-0.033; comparison of the k(cat.) value of the azide with that of a pyridinium salt of the same aglycone pK(a) 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 k(cat.) with aglycone acidity, reported in the literature, have been unsuccessful.1974-01-01T00:00:00Z1974 Dec2019-08-04T08:06:15Z2019-08-04T08:06:15ZS-EPMC116844410.1042/bj1430751