Time-resolved optical spectroscopy and structural dynamics following photodissociation of carbonmonoxyhemoglobin

A summary is presented of our current understanding of the kinetics of ligand rebinding and conformational changes at room temperature following photodissociation of the carbon monoxide complex of hemoglobin with pulsed lasers. The events which occur subsequent to excitation have been followed over 12 decades in time, from about 100 fs to the completion of ligand rebinding at about 100 ms. Experiments with picosecond and subpicosecond lasers by others, together with molecular dynamics simulations, indicate that by 1 ns the deoxyhemoglobin photoproduct is in a thermally equilibrated ground electronic state, so that subsequent processes are unaffected by the initial laser excitation. The principal results have been obtained from time-resolved optical absorption spectroscopy using a sensitive nanosecond laser spectrometer. Five relaxations have been observed which are interpreted as geminate rebinding at about 50 ns that competes with motion of the ligand away from the heme which produces a tertiary conformational change, a second tertiary conformational change at 0.5-1 microseconds, transition from the R to T quaternary structure at about 20 microseconds, and overall bimolecular rebinding of ligands from the solvent to the R and T quaternary structures at about 200 microseconds and 10 ms. Assuming that the dissociation pathway in photolysis experiments is the reverse of the association pathway, we find that for the R state there is a 40% probability that the ligand will bind to the heme after entering the protein, and a 60% probability that it will return to the solvent. Studies on the alpha-subunit of an iron-cobalt hybrid hemoglobin indicate that carbon monoxide enters the protein at the same rate for both R and T quaternary structures. For the alpha-subunit in the T state the probability of binding after entry is much lower, and the ligand returns to the solvent more than 99% of the time, accounting for the 60-fold overall lower association rate. This decreased probability of binding results from a decreased rate of binding to the heme from within the protein, and not an increased rate of return to the solvent. There are still unresolved problems on the basic structural description of carbon monoxide binding and dissociation, particularly the functional significance of the tertiary relations in both the R and T states, and the precise number of kinetic barriers within the protein.