SN1 hydrolyses of glycosyl pyridinium salts, and quantification of the main source of catalytic power of E. coli(lacZ)-β-galactosidase
- 1 January 1977
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in Journal of the Chemical Society, Perkin Transactions 2
- No. 9,p. 1191-1198
- https://doi.org/10.1039/p29770001191
Abstract
The hydrolysis of the 3-chloro-1-(β-D-galactopyranosyl) pyridinium ion is independent of pH between pH 2 and 8; this hydrolysis and those of another four pyridine-substituted cations exhibit positive entropies of activation. Rates at 25 °C are between 10–10 and 10–12.5 times the kcat values for the β-galactosidase-catalysed hydrolyses of the same compounds; for the 3-chloropyridinium salt ΔH‡ is lowered by 21 kcal mol–1 and ΔS‡ by 25 cal mol–1 K–1. The α-deuterium kinetic isotope effect for the spontaneous hydrolysis of the β-D-galactopyranosylpyridinium ion is the same as that for its enzymic hydrolysis, indicating that both processes involve a galactosyl cation. The whole of the catalytic effect of the enzyme towards these substrates must therefore arise from non-covalent interactions with parts of the substrate other than the bond being cleaved. Arguments are presented that such interactions have comparable importance in the hydrolyses of oxygen glycosides.Keywords
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