Conformational relaxation in heme proteins : ligand rebinding above the glass transition

Below the glass-transition temperature of the solvent, heme proteins are frozen into static conformations, and ligand rebinding is well described with a time- and temperature-independent distribution of enthalpic barriers, g(H). This work addresses the relaxation of the enthalpic barriers that sets in near the glass transition of the solvent as significant protein motions occur on the time scale of rebinding. As the heme domes fully toward its equilibrium deoxy structure, the distribution of enthalpies changes with time and temperature. A simple phenomenological model is used to describe MbCO rebinding at all temperatures above 50 K. Like the glassy relaxations observed in MbCO upon a sudden pressure release, the relaxation of all barriers to higher enthalpy is nonexponential in time and does not obey an Arrhenius relation. The extent of the enthalpic shift is consistent with a prediction based upon the inhomogeneous rebinding of band III, the charge transfer band observed in unligated Mb near 13100 cm-1 ??? Geminate rebinding from 160 to 290 K is well described in MbCO by a relaxation of all barriers by 9.3 kJ /mol without invoking any wells along the reaction coordinate representing ligand migration into the protein matrix. A simple argument is used to estimate the rate coefficient for ligand escape into the solvent. It, too, is found to have a non-Arrhenius temperature dependence. A numerical inversion technique is used to obtain the distribution of dissociated lifetimes, f(logr), from kinetics at a single temperature. The Maximum Entropy Method is outlined, tested with synthetic data, and applied to MbCO kinetics at various temperatures. The f distribution obtained at 90 K indicates that the tail of the g( H) distribution measured in the Soret is a manifestation of rebinding to A3, the slowest of the three predominant bound conformations to rebind. Several features are resolved in the f(logr) distributions obtained above the glass transition of the solvent.U of I OnlyThesi