Conformational dynamics of partially denatured myoglobin studied by time-resolved electrospray mass spectrometry with online hydrogen-deuterium exchange

This study demonstrates the use of electrospray mass spectrometry in conjunction with rapid online mixing ("time-resolved" ESI-MS) for monitoring protein conformational dynamics under equilibrium conditions. The hydrogen/deuterium exchange (HDX) kinetics of mildly denatured myoglobin (Mb) at pD 9.3, in the presence of 27% acetonitrile, were studied with millisecond time resolution. Analytical ultracentrifugation indicates that the average protein compactness under these solvent conditions is similar to that of native holomyoglobin (hMb). The mass spectrum shows protein ions in a wide array of charge and heme binding states, indicating the presence of multiple coexisting conformations. The experimental approach used allows the HDX kinetics of all of these species to be monitored separately. A combination of EX1 and EX2 behavior was observed for hMb ions in charge states 7+ to 9+, which predominantly represent nativelike hMb in solution. The EX1 kinetics are biphasic, indicating the presence of two protein populations that undergo conformational opening events with different rate constants. The EX2 kinetics observed for nativelike hMb are biphasic as well. All other charge and heme binding states represent non-native protein conformations that are involved in rapid interconversion processes, thus leading to monoexponential EX2 kinetics with a common rate constant. Burst phase labeling for these non-native proteins occurs at 125 sites. In contrast, the nativelike protein conformation shows burst phase labeling only for 88 sites. A kinetic model is developed which is based on the assumption of three distinct (un)folding units in Mb. The model implies that the free energy landscape of the protein exhibits a major barrier. The crossing of this barrier is most likely associated with slow, cooperative opening/closing events of the heme binding pocket. Rapid conformational fluctuations on either side of the barrier give rise to the observed EX2 kinetics. Simulated HDX kinetics based on this model are in excellent agreement with the experimental data.