The role of viscosity in oxygen binding to respiratory proteins

Investigations of Myoglobin, Hemoglobin and, more recently of Hemerythrin, suggest that the inward and outward motion of the oxygen ligand is governed by fluctuations between open and closed paths in the protein and that the driving force is the local brownian motion of protein residues. Reaction rates involving protein internal motions are strongly influenced by the viscosity of the solvent, whereas the ultimate binding of oxygen at the protein's active site remains viscosity independent. The description of a protein reaction therefore requires the introduction of a free energy surface with a conformation coordinate accounting for the viscosity-dependent protein transitions in addition to the usual reaction coordinate which only describes the system of reactants in a fixed protein conformation.