A Diffusion-Based Approach to Geminate Recombination of Heme Proteins with Small Ligands

Abstract

A model of postphotodissociative monomolecular (geminate) recombination of heme proteins with small ligands (NO, O2 or CO) is represented. The non-exponential decay with time for the probability to find a heme in unbound state is interpreted in terms of diffusion-like migration of ligabs physics/0212040 and between protein cavities. The temporal behavior for the probability is obtained from numerical simulation and specified by two parameters: the time \tau_{reb} of heme-ligand rebinding for the ligand localized inside the heme pocket and the time \tau_{esc} of ligand escape from the pocket. The model is applied in the analysis of available experimental data for geminate reoxygenation of human hemoglobin HbA. Our simulation is in good agreement with the measurements. The analysis shows that the variation in pH of the solution (6.0<pH<9.4) results in considerable changes for \tau_{reb} from 0.36 ns (at pH=8.5) up to 0.5 ns (pH=6.0) but effects slightly on the time \tau_{esc} (\tau_{esc} ~ 0.88 ns).Comment: 8 pages with 4 figures, submitted to Chem. Phy