MECHANISM OF LIGAND-BINDING TO HEMES AND HEMOPROTEINS - A HIGH-PRESSURE STUDY

The effect of pressure on the recombination kinetics of small ligands binding to sperm whale myoglobin, protoheme dimethyl ester, and monochelated protoheme was studied with use of laser flash photolysis. The volumes of activation observed indicate that in both the protein and the models bond formation is the rate-determining step only for carbon monoxide, while for oxygen, isocyanides, and 1-methylimidazole almost no bond formation occurs in the transition state of the observed reaction. The effect of pressure on the escape of carbon monoxide, oxygen, and methyl isocyanide from the heme pocket of sperm whale myoglobin was also investigated. The volume increase observed for all ligands during this process is attributed to a “gatelike” conformational change in the protein. The results are discussed in terms of the previously proposed three- and four-state reaction schemes for model hemes and myoglobin, respectively. © 1990, American Chemical Society. All rights reserved.