The pH dependence of hydrogen-deuterium exchange in trp repressor: the exchange rate of amide protons in proteins reflects tertiary interactions, not only secondary structure.

The pH dependence of amide proton exchange rates have been measured for trp-repressor. One class of protons exchanges too fast to be measured in these experiments. Among the protons that have measurable hydrogen-deuterium exchange rates, two additional classes may be distinguished. The second class of protons are in elements of secondary structure that are mostly on the surface of the protein, and exchange linearly with increasing base concentration (log kex versus pH). The third class of amide protons is characterized by much higher protection against exchange at higher pH. These protons are located in the core of the protein, in helices B and C. The exchange rate in the core region does not increase linearly with pH, but rather goes through a minimum around pH 6. The mechanism of exchange for the slowly exchanging core protons is interpreted in terms of the two-process model of Hilton and Woodward (1979, Biochemistry 18:5834-5841), i.e., exchange through both a local mechanism that does not require unfolding of the protein, and a mechanism involving global unfolding of the protein. The increase in exchange rates at low pH is attributed to a partial unfolding of the repressor. It is concluded that the formation of secondary structure alone is insufficient to account for the high protection factors seen in the core of native proteins at higher pH, and that tertiary interactions are essential to stabilize the structure.