SPECTRAL AND KINETIC PROPERTIES OF CHLOROBIUM THIOSULFATOPHILUM CYTOCHROME C-555 (ELECTROSTATICS, REDOX KINETICS, CIRCULAR DICHROISM).

Abstract

The spectral and kinetic properties of Chlorobium thiosulfatophilum cytochrome c-555 have been investigated and are compared to analogous properties of procaryotic and eucaryotic c-type cytochromes. The circular dichroism spectra are similar for c-type cytochromes of a given class and effects arising from specific amino acid substitutions are determined. The oxidation-reduction state dependence of the spectra in the near-ultraviolet region is interpreted in terms of changes in hydrogen bonding and polarity of the environment of aromatic amino acid residues in close proximity to heme iron resulting from small structural changes. An electrostatic model of bimolecular rate constants is developed to adequately describe the electric field effects for accurate computation of intrinsic rate constants. Structural models of bimolecular complexes of cytochrome c and flavodoxin or cytochrome b₅, developed by other investigators, are used to constrain the dielectric coefficients of the electrostatic model. Using these constraints, the relative contributions of ionic and dipolar interactions to the intermolecular electrostatic potential energies are estimated for several c-type cytochrome reactions. The ionic contribution is adequately described by the charged residue interactions in the region of intermolecular contact, and the dipolar contribution is adequately described by the molecular dipole moment. Oxidation-reduction reactions of cytochrome c-555 with small molecule reagents suggest a mechanism of electron transfer that is similar to other c-type cytochromes. In general, electron transfer may be preceded by formation of a bimolecular collision complex or may occur during the collision. Further, the ionic strength dependence of second-order rate constants indicates that molecular electric fields significantly affect the bimolecular reaction rates. Calculated intrinsic rate constants are dependent on the difference between oxidation-reduction potentials of the reactants as predicted by Marcus outer sphere electron transfer theory. The mechanism of reduction of c-type cytochromes by flavodoxin semiquinone is similar to the mechanism of electron transfer reactions involving small molecules. Thus, principles involved in protein-small molecule reactions are also involved in protein-protein reactions.