β‐Secondary deuterium isotope effect and solvent isotope effects in catalysis by subtilisin BPN′

Subtilisin BPN′ catalyzes the hydrolysis in protium and deuterium oxides of p-NO2C6H4OCOCL2NHCO2CH2C6H5(L = H, D) in the pH(D) range 5·0–8·5 (H2O) and 5·4–9·0 (D2O), according to simple Michaelis–Menten kinetics. The parameter kcat/Km exhibits pH(D) inflection points of 7·17 ± 0·05 (H2O) and 7·69 ± 0·08 (D2O), and kcat shows 6·88 ± 0·05 (H2O) and 7·50 ± 0·07 (D2O). The ‘normal’ ΔpK values of 0·5–0·6 indicate no unusual effects of D2O on enzyme properties. The solvent isotope effects (H2O/D2O) on the limiting values of the rate constants at high pH(D) are 1·13 ± 0·07 for kcat/Km and 1·29 ± 0·05 for Kcat. These small effects indicate no more than minor contributions of general acid–base catalysis for rate-limiting events for either kcat/Km or kcat. The β-deuterium secondary isotope effects (2H/2D) are roughly estimated by extrapolation as 0·95 ± 0·01 for kcat/Km, corresponding to substantial tetrahedral character in the transition state, and 1·03 ± 0·03 for kcat, consistent with no tetrahedral character. Models consistent with these results have as rate-limiting events for kcat/Km nucleophilic attack by active-site imidazole and for kcat, among other possibilities, the release of carboxylate product from the imidazolium form of the enzyme.