Hydrogen exchange kinetics of core peptide protons in Streptomyces subtilisin inhibitor.

The hydrogen isotope exchange kinetics of the 10 slowest exchanging resonances in the 1H NMR spectrum of Streptomyces subtilisin inhibitor (SSI) have been determined at pH 7-11 and 30-60 degrees C. These resonances are assigned to peptide amide protons in the beta-sheet core that comprises the extensive protein-protein interface of the tightly bound SSI dimer. The core protons are atypical in that their exchange rates are orders of magnitude slower than those for all other SSI protons. When they do exchange at temperatures greater than 50 degrees C, they do so as a set and with a very high temperature coefficient. The pH dependence of the exchange rate constants is also atypical. Exchange rates are approximately first order in hydroxyl ion dependence at pH less than 8.5 and greater than 9.5 and pH independent between pH 8.5 and 9.5. The pH dependence and temperature dependence of the SSI proton exchange rates are interpreted by the two-process model [Woodward, C. K., & Hilton, B. D. (1980) Biophys. J. 32, 561-575]. The results suggest that in the average solution structure of SSI, an unusual mobility of secondary structural elements at the protein surface is, in a sense, compensated by an unusual rigidity and inaccessibility of the beta-sheet core at the dimer interface.