Proton nuclear magnetic resonance evidence for the absence of a stable hydrogen bond between the active site aspartate and histidine residues of native subtilisins and for its presence in thiolsubtilisins.

Abstract

The very low field proton nuclear magnetic resonance (1H NMR) found in aqueous solutions of serine proteases and their zymogens is characteristic of the hydrogen bond between the imidazolium and aspartate groups of the catalytic triad: Ser-His-Asp [Robillard, G., & Shulman, R. G. (1972) J. Mol. Biol. 71, 507--511]. According to 1H correlation NMR spectroscopic studies performed in 80/20 (v/v) H2O/2H2O, no such resonance is found in native subtilisins (even at -2 degrees C and pH 6.0), but it is present in thiolsubtilisins and in the phenylboronic acid derivatives of the serine enzymes. The resonance was not visible in the mercuric or carboxamidomethyl derivatives of the thiol enzymes or in the phenylboronic acid--serine enzyme complex if the serine enzyme was first acylated with phenylmethanesulfonyl fluoride. The histidine at the catalytic site of thiolsubtilisin carries a positive charge between pH 5.6 and 8.4, in accord with previous data in favor of a mercaptide--imidazolium ion pair at the catalytic site. The charge distribution (- + -) at the active site of thiolsubtilisin and in the phenylboronic acid derivatives of the serine enzymes resembles that in the tetrahedral transition state formed between a serine enzyme and its substrate. Therefore, the stable hydrogen bond (found in the thiol enzyme and in the phenylboronic acid derivative of the serine enzyme) should be more important during catalysis than in the substrate-free enzyme.