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.
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.
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Metalloprotein is a generic term for a protein that contains a metal ion cofactor. A large proportion of all proteins are part of this category. For instance, at least 1000 human proteins (out of ~20,000) contain zinc-binding protein domains although there may be up to 3000 human zinc metalloproteins. It is estimated that approximately half of all proteins contain a metal. In another estimate, about one quarter to one third of all proteins are proposed to require metals to carry out their functions.
Hemocyanins (also spelled haemocyanins and abbreviated Hc) are proteins that transport oxygen throughout the bodies of some invertebrate animals. These metalloproteins contain two copper atoms that reversibly bind a single oxygen molecule (O2). They are second only to hemoglobin in frequency of use as an oxygen transport molecule. Unlike the hemoglobin in red blood cells found in vertebrates, hemocyanins are not confined in blood cells but are instead suspended directly in the hemolymph.
Copper is a chemical element with the symbol Cu (from cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orange color. Copper is used as a conductor of heat and electricity, as a building material, and as a constituent of various metal alloys, such as sterling silver used in jewelry, cupronickel used to make marine hardware and coins, and constantan used in strain gauges and thermocouples for temperature measurement.
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