Kinetic beta-deuterium isotope effects suggest a covalent mechanism for the protein folding enzyme peptidylprolyl cis/trans-isomerase.

The cis/trans interconversion of Glt-Ala-Ala-Pro-Phe-4-nitroanilide and Glt-Ala-Gly-Pro-Phe-4-nitroanilide was studied both enzymatically and nonenzymatically by measuring kinetic beta-deuterium isotope effects. The hydrogen atom at the alpha-carbon atom of the Xaa residue within the Xaa-Pro moiety was substituted by deuterium. In the nonenzymatic case the transition state of rotation is reflected by kH/kD greater than 1. When catalysed by 17 kDa PPIase the same bond rotation is characterized by kH/kD less than 1. This suggests a covalent mechanism of catalysis which involves an approximately tetravalent carbon of the prolyl imidic bond for the transition state of reaction.