Picosecond dynamics of ligand interconversion in the primary docking site of heme proteins

We have used femtosecond IR spectroscopy to probe interconversion dynamics of ligand in the primary docking site of heme proteins under physiological conditions. The docking site, fashioned with highly conserved amino acid residues, modulates ligand-binding activity by mediating the passage of ligand to and from the active binding site. Ligands in two states of the docking site interconvert on the picosecond time scale, and the rates are about 4 times slower in hemoglobin than that in myoglobin. The accurate interconversion rates on the time scale readily accessible by MD simulations can be used to refine computer simulations, which could in turn provide a detailed mechanistic picture of ligand binding in heme proteins.