Cytochrome c peroxidase

Cytochrome c peroxidase, or CCP 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 Compound I and then a Compound II intermediate:

CCP + ROOH → Compound I + ROH + H₂O CCP-compound I + e⁻ + H⁺ → Compound II 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 (usually hydrogen peroxide), it becomes oxidised to a formal oxidation state of +5 (Fe^V, commonly referred to as ferryl heme However, both low-temperature magnetic susceptibility measurements and Mössbauer spectroscopy show that the iron in Compound I of CCP is a +4 ferryl iron, with the second oxidising equivalent existing as a long-lived free-radical on the side-chain of the tryptophan residue (Trp-191). This is different to most peroxidases, which have the second oxidising equivalent on the porphyrin instead. Compound I of CCP is fairly long-lived, decaying to CCP-compound II with a half-life at room temperature of 40 minutes to a couple hours.

CCP has high sequence identity to the closely related ascorbate peroxidase enzyme.