Cytochrome C Oxidase

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Chantal Edouard Cytochrome C Oxidase The cytochrome oxidase of eukaryotes is a very complex protein assembly containing from 8 to 13 polypeptide subunits, two hemes, a and a3, and two atoms of copper. The two hemes are chemically identical but are placed in different protein environments, so that heme a can accept an electron from cytochrome c and heme a3 can react with oxygen. When cytochrome oxidase has accepted four electrons, one from each of four molecules of reduced cytochrome c, both its hemes and both its copper atoms are in reduced form, and it can transfer the electrons in a series of reactions to a molecule of oxygen to yield two molecules of water. Cytochrome oxidase straddles the inner membrane of mitochondria, part of it on the matrix side, part within the membrane, and part on the outer surface or cytochrome c side of the inner membrane. Cytochrome c is the only protein member of the respiratory chain that is freely mobile in the mitochondrial intermembrane space. It is a small protein consisting of a single polypeptide chain of 104 to 112 amino acid residues, wrapped around a single heme prosthetic group. The cytochromes c of eukaryotes are all positively charged proteins, with strong dipoles, while the systems from which cytochrome c accepts electrons, cytochrome reductase, and to which cytochrome c delivers electrons, cytochrome oxidase, are negatively charged. There is good evidence that this electrostatic arrangement correctly orients cytochrome c as it approaches the reductase or the oxidase, so that electron transfer can take place very efficiently, even though the surface area at which the reaction occurs is less than 1% of the total surface of the protein. Source "Cytochrome." McGraw-Hill Concise Encyclopedia of Science and Technology. 5th ed. New York: McGraw-Hill Professional, 2005. 609. Gale Virtual Reference Library. Web. 5 May 2013.
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