Cytochrome oxidases dioxygen binding

Cytochrome a is usually low-spin and six-coordinate, and will therefore be involved in electron transfer. Cytochrome a3 is high-spin and five-coordinate, and is therefore able to bind small molecules such as 02 and CO in the sixth, axial position when present as Fe11. It appears therefore that cytochrome a3 is the component of cytochrome oxidase that binds dioxygen, and it is possible that at some stage the dioxygen species would bridge the a3 iron and CuB. 

A symmetrical reduction process would involve separate electron entry into each Cu-Fe pair from two cytochrome c binding sites. The symmetric process is represented in Fig. 6. If this process is in fact followed, it would be advantageous for both heme irons to go reduced virtually simultaneously and this introduces two possible mechanistic problems. Reduction would be slowed by the requirement of two cytochrome c molecules binding to one oxidase, and with both iron atoms reduced there is the possibility of one dioxygen molecule binding at each heme site. Both of these circumstances would tend to retard the reaction. If, despite these obstacles, the reaction does proceed with entry of electrons to both Cu-Fe pairs, a reasonable mechanism for Oj reduction becomes that of O2 binding to one iron (II) as to oxyHb or oxyMb 117,118), then, to a /t-peroxo... 

Copper Proteins Oxidases Copper Proteins with Dinuclear Active Sites Copper Proteins with Type 1 Sites Copper Proteins with Type 2 Sites Cytochrome Oxidase Iron Heme Proteins Dioxygen Transport Storage Iron Heme Proteins Electron Transport Iron Heme Proteins, Mono- Dioxygenases Iron Heme Proteins, Peroxidases, Catalases Catalase-peroxidases Iron Proteins with Dinuclear Active Sites Iron Proteins with Mononuclear Active Sites Iron-Sulfur Proteins Peptide Metal Interactions Zinc DNA-binding Proteins Zinc Enzymes. 

The terminal oxidase in an energy-transducing, cytochrome-based electron-transport system maintains electron flow by coupling cytochrome oxidation to dioxygen (O2) reduction. Members of this protein class are referred to as cytochrome oxidases they carry out Oj-binding and redox chemistry at transition metal-containing active sites. Although iron is the most commonly used metal and may occur as a protoheme or iron-chlorin species in the protein, this section is concerned only with mitochondrial cytochrome oxidase, which contains 2 mol of Cn and 2 mol of heme a bound Fe per function unit. Biochemistry of the protein will not be considered here, instead the focus will be on the stmcture of the metal centers, on the reactions they catalyze and on models for these centers. 

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