Cytochrome c peroxidase

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Cytochrome c peroxidase, or CCP (PDB 2CYP, EC 1.11.1.5) is a water-soluble heme-containing enzyme of the peroxidase family that takes reducing equivalents from cytochrome c and reduces hydrogen peroxide to water:

CCP + H₂O₂ + 2 ferrocytochrome c + 2H⁺ → CCP + 2H₂O + 2 ferricytochrome c

Cytochrome c peroxidase can react with hydroperoxides other than hydrogen peroxide, but the reaction rate is much slower than with hydrogen peroxide.

It was first isolated from baker's yeast by R. A. Altschul, Abrams, and Hogness in 1940, though not to purity. The first purified preparation of yeast CCP dates to Takashi Yonetani and his preparation by ion exchange chromatography in the early 1960s. The X-ray structure was the work of Thomas Poulos and coworkers in the late 1970s.

The yeast enzyme is a monomer of molecular weight 34,000, containing 293 amino acids, and contains as well a single noncovalently bound heme b. Unusual for proteins, this enzyme crystallizes when dialysed against distilled water. More so, the enzyme purifies as a consequence of crystallization, making cycles of crystallization an effective final purification step.

Much like catalase, the reaction of cytochrome c peroxidase proceeds through a three step process, forming first a CCP-compound I and then a CCP-compound II:

CCP + ROOH → CCP-compound I + ROH + H₂O

CCP-compound I + e^- + H⁺ → CCP-compound II

CCP-compound II + e^- + H⁺ → CCP

CCP in the resting state has a ferric heme, and after the addition of two oxidizing equivalents from a hydroperoxide, it becomes an enzyme of formal oxidation state V. However, both low temperature magnetic susceptibility measurements and Mössbauer spectroscopy show that the iron in CCP-compound I is a +4 ferryl iron, and not in oxidation state V. The other salient feature of CCP-compound I is a long lived free radical, whose signal suggests a species other than the porphyrin free radicals of other peroxidase compound I species. Early on it was recognized to be an organic free radical, with the bulk of evidence now linking it to the side chain of the tryptophan residue (Trp-191).

Unlike most peroxidases, CCP-compound I is fairly long lived, decaying to CCP-compound II with a half life at room temperature of 40 minutes to a couple hours.