Structural dynamics of myoglobin.

Conformational fluctuations have been invoked to explain the observation that the diffusion of small ligands through a protein is a global phenomenon, as suggested (for example) by the oxygen induced fluorescence quenching of buried tryptophans. In enzymes processing large substrates, a channel to the catalytic site is often seen in the crystal structure; on the other hand in small globular proteins, it is not known if the cavities identified in the interior space are important in controlling their function by defining specific pathways in the diffusion to the active site. This point is addressed in this paper, which reports some relevant results obtained on myoglobin, the hydrogen atom of molecular biology. Protein conformational relaxations have been extensively investigated with myoglobin because the photosensivity of the adduct with CO, O2 and NO allows us to follow events related to the migration of the ligand through the matrix. Results obtained by laser photolysis, molecular dynamics simulations, X-ray diffraction of intermediate states of wt type and mutant myoglobins are briefly summarized. Crystallographic data on the photochemical intermediate of a new triple mutant of sperm whale myoglobin (Mb-YQR) show, for the first time, the photolyzed CO* sitting in one of the Xe-binding cavities, removed from the heme group. These results support the viewpoint that pre-existing 'packing defects' in the protein interior play a major role in controlling the dynamics of ligand binding, including oxygen, and thereby acquire a survival value.