Transient resonance Raman, Raman difference, circular dichroism (CD), and optical absorption studies have been carried out on the electrostatic complexes formed by yeast cytochrome c peroxidase (CCP) with horse cytochrome c (Cytc) in low ionic strength solutions. In all the complexes examined [e.g., CCP(II)/Cytc(II), CCP(III)/Cytc(II), CCP(III)/Cytc(III)], the local heme environments of both proteins are largely unperturbed upon complexation. Specifically, CCP preserves a completely pentacoordinate high-spin home in both its ferric and ferrous forms in CCP/Cytc complexes and uncomplexed mixtures. We found no evidence corroborating the previously reported increase in the low-spin fraction of CCP heme upon complexation with Cytc [Hildebrandt et al. (1992) Biochemistry 31, 2384-2392]. Instead, our Raman data strongly suggest that the H-bonding networks in the distal and proximal pockets of CCP are well maintained in the complexes. On the other hand, CD spectra of CCP(III)/Cytc(III) complexes showed substantial variations (relative to the 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-dependent flexibility of the heme structure of CCP and thus influences the photodynamics of the CCP active site.
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