Ligand migration in sperm whale myoglobin.

Geminate oxygen rebinding to myoglobin was followed from a few nanoseconds to a few microseconds after photolysis for more than 25 different oxymyoglobin point mutants in the presence and absence of 12 atm of xenon. In all cases, two relaxations were observed: an initial fast phase (half-time 20 ns) and a slower, smaller phase (half-time 0.5-2 micros). Generally, xenon accelerates the fast reaction but slows the slower reaction and diminishes its amplitude. The rates and proportions of the two components and the effects of xenon on them vary widely for different mutants. The locations of specific xenon binding sites [Tilton, R. F., Kuntz, I. D. Jr., and Petsko, G. A. (1984) Biochemistry 23, 2849-2857], the effects of point mutations on the geminate reactions, and molecular dynamics simulations were used to suggest locations in the protein interior occupied by ligands on the nanosecond to microsecond time scale. Photodissociated ligands may occupy xenon site 4 in the distal pocket and xenon site 1 below the plane of the heme. Rebinding from these positions corresponds to the slower geminate phase for O2 rebinding. The rapid geminate component is determined by competition between rebinding from a position closer to the iron atom and escape to solvent or more distant locations in the protein.