Ligand binding to heme proteins: connection between dynamics and function

Ligand binding to heme proteins is studied by using flash photolysis over wide ranges in time (10ns-1ks) and temperature (10-320K). Below about 200K in 75% glycerol/water solvent, ligand rebinding occurs from the heme pocket and is nonexponential in time. The kinetics is explained by a distribution, g(H),of the enthalpic barruier of height H between the pocket and the bound state. Above 170K rebinding slows markedly. Previously we interpreted the slowing as a «matrix process» resulting from the ligand entering the protein matrix before rebinding. Experiments on band III, and inhomogeneously broadened charge-transfer band near 760 nm (13 000cm −1 ) in the photolyzed state (Mb*) of (carbon-monoxy)myoglobin (MbCO), force us to reinterpret the data