The complex of cytochrome c and cytochrome c peroxidase: The end of the road?

Cytochrome c (Cc) and cytochrome c peroxidase (CcP) form a physiological complex in the inter-membrane space of yeast mitochondria, where CcP reduces hydrogen peroxide to water using the electrons provided by ferrous Cc. The Cc–CcP system has been a popular choice of study of interprotein biological electron transfer (ET) and in understanding dynamics within a protein–protein complex. In this review we have charted seven decades of research beginning with the discovery of CcP and leading to the latest functional and structural work, which has clarified the mechanism of the intermolecular ET, addressed the putative functional role of a low-affinity binding site, and identified lowly-populated intermediates on the energy landscape of complex formation. Despite the remarkable attention bestowed on this complex, a number of outstanding issues remain to be settled on the way to a complete understanding of Cc–CcP interaction.

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► We review the protein complex between cytochrome c and cytochrome c peroxidase.
► X-ray crystallographic studies of the 1:1 complex are highlighted.
► The ‘encounter state’ of the complex determined by NMR is illustrated.
► The electron transfer mechanisms for the 1:1 and 2:1 complexes are discussed.