The hydrogen-deuterium exchange rates of the slowest exchanging β-sheet NH's in the bovine pancreatic trypsin inhibitor (BPTI) have been determined in free BPTI and in the trypsin-BPTI complex at pH 9-10, 25-40 °C. Rate constants for individual protons have been measured from their assigned resonances in the 1H NMR spectrum. Trypsin binding has a highly localized effect; the Tyr-35 NH exchange rate is slowed by a factor of >103 in the complex, while the other NH's measured are slowed by a factor of 3-15. In free BPTI, under conditions where the exchange rate constants have activation energies in the range 11-32 keal/mol, the NH's of Tyr-21, Phe-22, and Tyr-23 are several orders of magnitude slower than the other β-sheet NH's.
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