Abstract

The conformation of a protein often influences its activity, yielding a structure-function relationship. X-ray diffraction studies have shown that the tertiary structures of ligated and deligated myoglobin (Mb) are somewhat different¹. Consequently, dissociation of a ligand from Mb triggers a transition between the two tertiary conformations. The potential energy gradient causing this change is developed at the heme; the iron prefers to be in the plane of the porphyrin in ligated Mb but is displaced 0.5 Å from the plane of the porphyrin in deoxy Mb. The dynamics of this conformational transition may influence the dynamics of rebinding ligands, implying that protein dynamics are also functionally important. For example, the dynamics of ligand recombination with Mb following photolysis of MbCO or MbO₂ in low-temperature glasses are similar². In contrast, Mb expurgates CO with far greater efficiency than O₂ when photolysis is carried out at biologically important temperatures³. Since protein motion is inhibited at low temperatures, protein relaxation likely accounts for the temperature-dependent difference in the quantum yield of photodissociation. The ability to discriminate against the binding and storage of CO is functionally important as endogenously produced CO would otherwise compete effectively with O₂ for binding sites. A steric mechanism for discriminating against the binding of CO, involving the distal histidine, is well known. The dynamics of protein relaxation evidently provide a mechanism for discriminating against the storage of CO. We have investigated the dynamics of protein relaxation in order to probe this mechanism and thereby elucidate the relation between protein dynamics and function.

© 1992 The Author(s)