SOLUTION DYNAMICS OF THE TRP REPRESSOR - A STUDY OF AMIDE PROTON-EXCHANGE BY T-1 RELAXATION

The amide proton exchange rates of Escherichia coil trp repressor have been measured through their effects on the longitudinal relaxation ra tes of the amide protons. Three types of exchange regimes have been ob served: (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 T-1 rela xation time-scale (seconds) in the DNA-bindingg region and (3) very fa st exchange at the N and C termini. The results have been analyzed in terms of the two-site exchange model originally proposed by Linderstro m-Lang, and of a three-site extension of the model. The values of the intrinsic exchange rates calculated using the two-state modal agree wi th the values expected from the studies of Englander and co-workers fo r the very fast case of the chain terminals, but disagree with the lit erature values by two orders of magnitude in the intermediate case fou nd in the DNA-binding region. The implication of these findings is tha t the ''open'' state of the two-state model in the DNA-binding region is not completely open and has an intrinsic exchange rate different fr om that of a random coil peptide. Alternatively, if the literature val ues of the intrinsic exchange rates are assumed to apply to the open s tates in all parts of the repressor molecule, two ''closed'' helical s tates have to be postulated, in slow exchange with each other, with on ly one of them in rapid exchange with the open state and hence with th e solvent. Kinetically, the two models are indistinguishable.