Time-resolved studies of carbonmonoxy myoglobin photolysis: protein relaxation and ligand trajectory

Summary form only given.We are conducting nanosecond time-resolved crystallographic studies at the ID9 beamline of the ESRF on two heme proteins: sperm whale myoglobin (Mb) and dimeric hemoglobin (HbI) from the clam Scapharca inaequivalvis. The goals of these studies are to understand the evolution of the photo-induced structural changes and their propagation from the active site through the protein and to determine the trajectories and the docking sites of the photo-dissociated ligand and how they are affected by mutations of side chains known to affect ligand binding properties. We use intense, focused, white, 150-ps and 1-/spl mu/s X-ray pulses in a pump-probe type of measurement to investigate structural changes in carbonmonoxy complexes of Mb and HbI. Changes are photo-initiated by 10-ns laser pulses. The complete reversibility of the reaction simplifies the use of the signal averaging to improve the data quality and allows us to obtain complete and even multiple data sets from one crystal. Departure of the CO ligand upon photolysis and its subsequent rebinding, and the iron atom displacement from the heme plane are clearly observed in both molecules. We also identify a possible docking site for the photo-dissociated CO molecule in the MbCO heme pocket.