Redox pathways in electron - transfer proteins : Correlations between reactivities , solvent exposure , and unpaired - spin - density distributions ( cytochromes / iron - sulfur proteins / copper proteins )

The relative reactivities toward reduction by free flavin semiquinones of cytochromes (c-type cytochromes, cytochrome b5, c'-type cytochromes) iron-sulfur proteins (high-redox-potential ferredoxins, rubredoxins, low-redoxpotential ferredoxins), and blue copper proteins (plastocyanin, azurins) are shown to correlate with calculations of the solvent exposure of the various prosthetic groups. In the case of the c-type cytochromes, one of the major centers of exposure is the sulfur atom of the thioether bridge that covalently links heme ring C to the protein. Charge-iterative extended Huckel calculations on a heme c model indicate that both porphyrin u and Fe(III) dfr orbitals can delocalize onto the bridging sulfur atom. Unpaired spin densities are comparable to those obtained for individual aromatic porphyrin ring carbon atoms. Thus, the exposed sulfur of ring C may act to facilitate electron transfer. a 1CH3 .C---SGUCH3 -OlH Previous work from this laboratory (1-5) has demonstrated that rate constants for reduction of a homologous series of redox proteins [cytochromes c or c', high-redox-potential ferredoxins (HiPIPs), and blue copper proteins] by free flavin semiquinones can be related to redox-potential differences. This has allowed the establishment of relative intrinsic reactivities for proteins with the same type of prosthetic group, which were shown to correlate qualitatively with the degree of solvent exposure of the redox center at the protein surface (1). The present work represents an effort to place this latter correlation on a more quantitative basis by using atomic coordinates derived from x-ray crystallographic analysis to calculate the extent of solvent exposure of various atoms of the heme centers, iron-sulfur clusters, and copper sites of several electron-transfer proteins. In the case of the c-type cytochromes, we have also evaluated the extent of delocalization of porphyrin ring i and iron d orbitals using molecular orbital calculations of unpaired spin densities. As will be demonstrated below, the results of these calculations are in good agreement with our kinetic measurements and suggest a possible role in the electron-transfer mechanism of the sulfur atom of the thioether bridge that covalently links heme ring C to the protein in the cytochromes c.