Water catalysis of peptide hydrogen isotope exchange.

The temperature dependence of the hydrogen-tritium and deuterium-hydrogen exchange reactions in poly(DL-alanine) has been reexamined. The results indicate a significant contribution to the observed exchange rates from the water-catalyzed reaction at pD values near pDmin. The activation enthalpy for water-catalyzed deuterium-hydrogen exchange in poly(DL-alanine) is found to be 21 kcal mol-1. As a result, the contribution to the observed exchange rate from the water-catalyzed reaction increases with increasing temperature which in turn leads to broad, shallow pD minima and the appearance of apparent reaction orders with respect to [D+] and [OD-] that are substantially less than first order over an extended range of pD values. The importance of water catalysis in protein hydrogen exchange is demonstrated by a reanalysis of data for the exchange of single protons in bovine pancreatic trypsin inhibitor [Hilton, B.D., & Woodward, C. K. (1979) Biochemistry 18, 5834; Richarz, R., Sehr, P., Wagner, G., & Wüthrich, K. (1979) J. Mol. Biol. 130, 19]. The pD dependence of these protons can be explained in terms of an increased contribution from water catalysis.