CYTOCHROME-C PEROXIDASE COMPLEXED WITH CYTOCHROME-C HAS AN UNPERTURBED HEME MOIETY

Transient resonance Raman, Raman difference, circular dichroism (CD), and optical absorption studies have been carried out on the electrosta tic complexes formed by yeast cytochrome c peroxidase (CCP) with horse cytochrome c (Cytc) in low ionic strength solutions. In all the compl exes examined [e.g., CCP(II)/Cytc(II), CCP(III)/Cytc(II), CCP(III)/Cyt c(III)], the local heme environments of both proteins are largely unpe rturbed upon complexation. Specifically, CCP preserves a completely pe ntacoordinate high-spin heme in both its ferric and ferrous forms in C CP/Cytc complexes and uncomplexed mixtures. We found no evidence corro borating the previously reported increase in the low-spin fraction of CCP heme upon complexation with Cytc [Hildebrandt et al. (1992) Bioche mistry 31, 2384-2392]. Instead, our Raman data strongly suggest that t he II-bonding networks in the distal and proximal pockets of CCP are w ell maintained in the complexes. On the other hand, CD spectra of CCP( III)/Cytc(III) complexes showed substantial variations (relative to th e uncomplexed mixtures) in the far-UV region, reflecting some protein conformational rearrangements. In addition, the spectral data suggest that complexation with Cytc affects the previously observed pH-depende nt flexibility of the heme structure of CCP and thus influences the ph otodynamics of the CCP active site.