Solution dynamics of the trp repressor: a study of amide proton exchange by T1 relaxation.

The amide proton exchange rates of Escherichia coli trp repressor have been measured through their effects on the longitudinal relaxation rates of the amide protons. Three types of exchange regimes have been observed: (1) slow exchange (on a minute/hour time-scale), measurable by isotope exchange, but not by relaxation techniques in the core of the molecule; (2) relatively rapid exchange, with the rates on a T1 relaxation time-scale (seconds) in the DNA-binding region and (3) very fast exchange at the N and C termini. The results have been analyzed in terms of the two-site exchange model originally proposed by Linderstrøm-Lang, and of a three-site extension of the model. The values of the intrinsic exchange rates calculated using the two-state model agree with the values expected from the studies of Englander and co-workers for the very fast case of the chain terminals, but disagree with the literature values by two orders of magnitude in the intermediate case found in the DNA-binding region. The implication of these findings is that the "open" state of the two-state model in the DNA-binding region is not completely open and has an intrinsic exchange rate different from that of a random coil peptide. Alternatively, if the literature values of the intrinsic exchange rates are assumed to apply to the open states in all parts of the repressor molecule, two "closed" helical states have to be postulated, in slow exchange with each other, with only one of them in rapid exchange with the open state and hence with the solvent. Kinetically, the two models are indistinguishable.