Effects of Solvent Viscosity on Conformational Dynamics of Heme-pocket in Myoglobin and Hemoglobin

Abstract

The influence of solvent viscosity on conformational dynamics of the heme-pocket, a small vacant site near the binding site of myoglobin (Mb) and hemoglobin (Hb), and playing a functionally important role by serving as a station in ligand binding and escape, was studied by probing time-resolved vibrational spectra of CO photodissociated from MbCO and HbCO in D 2 O, 75 wt% glycerol/D 2 O, and trehalose at 283 K. Two absorption bands (B 1 and B 2 ) of the sample in viscous solvents, arising from CO in the heme pocket, are very similar to those in D 2 O. Two bands in Mb and Hb under all three solvents exhibit very similar nonexponential spectral evolution (B 1 band; blue shifting and broadening, B 2 band; narrowing with a negligible shifting), suggesting that in the present experimental time window of 100 ps, the extents of the spectral shift and narrowing is much influenced neither by the viscosity of solvent nor by the quaternary contact of Hb. Spectral evolution can be described by a biexponential function with a fast universal time constant of 0.52 ps and a slow time constant ranging from 13 to 32 ps. For both proteins in all three solvents majority of spectral evolution occurs with the fast universal time constant. The magnitude of the slow rate in the spectral shift of B 1 band decreases with increasing solvent viscosity, indicating that it is influenced by global conformational change which is retarded in viscous solvent, thereby serve as a reporter of global conformational change of heme proteins after deligation.