Cytochromes c' in their reaction with ethyl isocyanide

Abstract

The binding of ethyl isocyanide (EIC) to a representative number of cytochromes c'' is demonstrated. Spectroscopic and equilibrium constants were measured and compared for the binding of EIC to cytochromes c'' from the photosynthetic bacteria chromatium vinosum, Rhodopseudomonas palustris, Rhodospirillum rubrum and Rhodopseudomonas sphaeroides. While the absorption spectra of the EIC complexes resemble those of EIC complexes of other high-spin hemoproteins, the Soret half band widths and extinction coefficients per heme exhibit > 2-fold difference, with the values of C. vinosum being most similar to those of R. sphaeroides and of R. palustris similar to those of R. rubrum. The cytochromes exhibit binding equilibria consistent with the ligation of 1 molecule of EIC/heme in contrast to the reported binding of > 1 molecule of CO/heme. The binding constants exhibit > 1000-fold difference with the values of C. vinosum being closely similar to those of R. sphaeroides and of R. palustris similar to those of R. rubrum. The lack of correlation between EIC and CO binding properties indicates that electronic factors do not determine the difference in EIC binding properties. The observed correlation between the extinction coefficients, half band widths and equilibrium constants for EIC complex formation provides the first spectroscopic evidence that the differences in binding properties are associated with sterically hindered ligation to the heme. Although the differences in binding properties provide evidence of steric hindrance, the EIC binding constants for particular cytochromes c'' indicate that the distal heme binding site is more accessible than previously indicated. The differences in spectroscopic and binding properties are discussed in terms of structural differences between amino acids thought to be associated with the distal heme binding site.