Copper proteins are proteins that contain one or more copper ions as prosthetic groups. Copper proteins are found in all forms of air-breathing life. These proteins are usually associated with electron-transfer with or without the involvement of oxygen (O2). Some organisms even use copper proteins to carry oxygen instead of iron proteins. A prominent copper proteins in humans is in cytochrome c oxidase (cco). The enzyme cco mediates the controlled combustion that produces ATP. Classes The metal centers in the copper proteins can be classified into several types: Type I copper centres (T1Cu) are characterized by a single copper atom coordinated by two histidine residues and a cysteine residue in a trigonal planar structure, and a variable axial ligand. In class I T1Cu proteins (e.g. amicyanin, plastocyanin and pseudoazurin) the axial ligand is the sulfur of methionine, whereas aminoacids other than methionine (e.g. glutamine) give rise to class II T1Cu copper proteins. Azurins contain the third type of T1Cu centres: besides a methionine in one axial position, they contain a second axial ligand (a carbonyl group of a glycine residue). T1Cu-containing proteins are usually called "cupredoxins", and show similar three-dimensional structures, relatively high reduction potentials (> 250 mV), and strong absorption near 600 nm (due to S→Cu charge transfer), which usually gives rise to a blue colour. Cupredoxins are therefore often called "blue copper proteins". This may be misleading, since some T1Cu centres also absorb around 460 nm and are therefore green. When studied by EPR spectroscopy, T1Cu centres show small hyperfine splittings in the parallel region of the spectrum (compared to common copper coordination compounds). Type II copper centres (T2Cu) exhibit a square planar coordination by N or N/O ligands. They exhibit an axial EPR spectrum with copper hyperfine splitting in the parallel region similar to that observed in regular copper coordination compounds. Since no sulfur ligation is present, the optical spectra of these centres lack distinctive features. T2Cu centres occur in enzymes, where they assist in oxidations or oxygenations. Type III copper centres (T3Cu) consist of a pair of copper centres, each coordinated by three histidine residues. These proteins exhibit no EPR signal due to strong antiferromagnetic coupling (i.e. spin pairing) between the two S = 1/2 metal ions due to their covalent overlap with a bridging ligand. These centres are present in some oxidases and oxygen-transporting proteins (e.g. hemocyanin and tyrosinase). Binuclear Copper A centres (CuA) are