We have performed transient spectroscopic studies on component I of trout hemoglobin over a wide range of temperatures. We have found that the rates and amplitudes of the spectral changes in the photolyzed deoxy hemes attributed to tertiary and quaternary changes in the protein are all strongly temperature dependent. We have also found that the amplitude of geminate rebinding of carbon monoxide to trout I hemoglobin decreases with increasing temperature, whereas the rate of this process is temperature independent. A simple analysis of these results suggests that the temperature dependence of the ligand rebinding properties of this protein is associated primarily with the entry of the ligand into the protein. We have simulated the photodissociation of bound ligands from the hemoglobin tetramer using molecular dynamics in order to probe the structural responses of the system to ligand dissociation. The heme conformational change, with the iron atom moving out of the mean plane of the heme atoms, appears to take place on a sub- picosecond time scale, consistent with our earlier simulations of isolated subunits of the protein. Preliminary analysis of the tetramer simulation shows no evidence of conformational responses of the protein to ligand dissociation which might be related to the known quaternary structural change. We have also performed molecular dynamics simulations of atomic motions in sperm whale myoglobin. The simulations predict the existence of multiple distinct conformations accessible to each tryptophan sidechain in the protein. Further analysis has shown that this structural heterogeneity can account for the fluorescence intensity and anisotropy decays observed for the tryptophans in myoglobins from sperm whale and other species. Our final molecular dynamics study has addressed the dissipation into the protein matrix of excess vibrational energy deposited in the heme by photo-excitation. This study predicts that the excess heme energy passes into the protein on a time scale of tens of picoseconds, via channels that appear to increase the temperature of all parts of the protein simultaneously.
| Henry, Eric R; Bettati, Stefano; Hofrichter, James et al. (2002) A tertiary two-state allosteric model for hemoglobin. Biophys Chem 98:149-64 |
| Eaton, W A; Munoz, V; Hagen, S J et al. (2000) Fast kinetics and mechanisms in protein folding. Annu Rev Biophys Biomol Struct 29:327-59 |
| Eaton, W A; Henry, E R; Hofrichter, J et al. (1999) Is cooperative oxygen binding by hemoglobin really understood? Nat Struct Biol 6:351-8 |
