Amide proton exchange in proteins by EX1 kinetics: studies of the basic pancreatic trypsin inhibitor at variable p2H and temperature.

With the use of one-dimensional 1H nuclear magnetic resonance, two-dimensional correlated spectroscopy, and two-dimensional nuclear Overhauser enhancement spectroscopy, the exchange mechanisms for numerous individual amide protons in the basic pancreatic trypsin inhibitor (BPTI) were investigated over a wide range of p2H and temperature. Correlated exchange under an EX1 regime was observed only for the most slowly exchanging protons in the central hydrogen bonds of the antiparallel beta-sheet and only over a narrow range of temperature and p2H, i.e., above ca. 55 degrees C and between p2H 7 and 9, where the opening rates of the structure fluctuations which promote the exchange of these protons are of the order 0.1 min-1. At p2H below 7, the exchange of this most stable group of protons is uncorrelated and is governed by an EX2 mechanism. At p2H above 9, the exchange is also uncorrelated and occurs via either EX2 or EX1 processes promoted by strictly local structure fluctuations. For all other backbone amide protons in BPTI, the exchange was found to be uncorrelated and by an EX2 mechanism under all conditions of p2H and temperature where quantitative measurements could be obtained with the methods used, i.e., for kex approximately less than 5 min-1. From these observations with BPTI it can be concluded that the amide proton exchange in globular proteins is quite generally via EX2 processes, with rare exceptions for measurements with extremely stable protons at high temperature and basic p2H. This emphasizes the need for further development of suitable concepts for the structural interpretation of EX2 amide proton exchange [Wagner, G. (1983) Q. Rev. Biophys. 16, 1-57; Wagner, G., Stassinopoulou, C. I., & Wüthrich, K. (1984) Eur. J. Biochem. 145, 431-436] and for more detailed investigations of the intrinsic exchange rates for solvent-exposed amide protons in the "open" states of a protein [Roder, H., Wagner, G., & Wüthrich, K. (1985) Biochemistry (following paper in this issue)].