A TEMPERATURE-DEPENDENT EFFECTIVE POTENTIAL EXPLAINS CO BINDING TO MYOGLOBIN

Up to the onset of ligand escape from the heme pocket, CO binding to m yoglobin can be explained by stochastic motion of the protein subject to an effective, temperature-dependent potential. Two ''temperature-mo dels'' for the effective potential are investigated. The quantitative solution of the transient Smoluchowski equation for these models shows inhomogeneous kinetics at short times and protein relaxation at inter mediate times. Additional two phases of ligand binding, ligand escape and bimolecular recombination, can be identified. Both models agree th at the protein ''diffusion coefficient'' increases in an Arrhenius fas hion through the solvent glass transition, indicating that protein rel axation occurs mainly in the interior of the protein. In contrast, the effective potential ''collapses'' above the glass transition temperat ure, indicating that it is determined also by interactions at the surf ace of the protein. In one of the models the parameters of the potenti al vary linearly with temperature, resembling the situation for rubber elasticity.