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Any iron-porphyrin containing electron-carrying protein forming part of the electron transport chains of respiration and photosynthesis. The porphyrin prosthetic group of cytochromes contains a chelated Fe ion, which changes oxidation state from ferric, Fe (III), to ferrous, Fe(II), when the cytochrome is reduced. Most cytochromes are tightly membrane bound. Cytochrome aa3 (also called cytochrome c oxidase or the respiratory enzyme) is responsible for the final step in the respiratory chain in which water is formed from oxygen and hydrogen ions.
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