Local Stability and Dynamics of Apocytochrome b562 Examined by the Dependence of Hydrogen Exchange on Hydrostatic Pressure,

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Abstract
Hydrostatic pressure is used to perturb the manifold of states available to apocytochrome b562 and to examine the energetics and dynamics of the protein using hydrogen exchange monitored in real-time by heteronuclear spectroscopy at pressures ranging up to 1.1 kbar. An analytical framework for interpreting the effects of hydrostatic pressure on the physical events leading to protein hydrogen exchange is presented. The protein is found to have three regions of subglobal cooperative stability. The most stable region, or core, is composed of the central two helices of the bundle. The dependence of the global unfolding free energy upon pressure is first order and associated with a negative volume change of −102 mL mol-1. Two additional regions of cooperative structure are identified, and both also have negative volume changes associated with their unfolding at high pressure. Surprisingly, one of the subglobal unfolding units shows a significant positive volume change at low pressures (<200 bar) suggesting the presence of a highly mispacked open state at ambient pressure. The three regions of cooperative stability are the same as identified by perturbation with chemical denaturant. The implications of these results for issues in protein folding and the form of the energy landscape of globular proteins are discussed.