Microsecond to minute dynamics revealed by EX1-type hydrogen exchange at nearly every backbone hydrogen bond in a native protein

A previous comprehensive analysis of the pH dependence of native-state amid e hydrogen (NH) exchange in turkey ovomucoid third domain (OMTKY3) yielded apparent opening and closing rate constants (k(op) and k(cl)) at 14 NH grou ps involved in global conformational changes. This analysis has been extend ed to 18 additional slowly exchanging NH groups. Quench-flow experiments we re performed to monitor NH exchange in native OMTKY3 from neutral to very a lkaline pH (similar to 12) conditions. Above pH 10 the mechanism of exchang e switched from one governed by a rapid equilibrium preceding the chemistry of exchange (i.e. EX2 exchange), to one where exchange was limited by the rate of opening (i.e. EX1 exchange). Kinetics of solvent exposure are now k nown for nearly all backbone NH groups in native OMTKY3, yielding rate cons tants that span five orders of magnitude, 0.004 to 200 s(-1) (C) 2000 Acade mic Press.