Copper proteins
Encyclopedia
Copper proteins are proteins that contain one or more copper
ions as prosthetic groups. The metal centres in the copper proteins can be classified into several types:
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
ions as prosthetic groups. The metal centres 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 histidineHistidineHistidine Histidine, an essential amino acid, has a positively charged imidazole functional group. It is one of the 22 proteinogenic amino acids. Its codons are CAU and CAC. Histidine was first isolated by German physician Albrecht Kossel in 1896. Histidine is an essential amino acid in humans...
residues and a cysteineCysteineCysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
residue in a trigonal planarTrigonal planarIn chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of a triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands are identical and all bond angles are 120°. Such species belong to...
structure, and a variable axial ligandLigandIn coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
. In class I T1Cu proteinsPlastocyanin family of copper binding proteinsPlastocyanin/azurin family of copper-binding proteins is a family of small proteins that bind a single copper atom and that are characterised by an intense electronic absorption band near 600 nm...
(e.g. amicyaninAmicyaninAmicyanin is a type I copper protein that plays an integral role in electron transfer. In bacteria such as Paracoccus denitrificans, amicyanin is part of a three-member redox complex, along with methylamine dehydrogenase and cytochrome c-551i....
, plastocyaninPlastocyaninPlastocyanin is an important copper-containing protein involved in electron-transfer. The protein is monomeric, with a molecular weight around 10,500 Daltons, and 99 amino acids in most vascular plants...
and pseudoazurin) the axial ligand is the sulfur of methionineMethionineMethionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...
, whereas aminoacids other than methionine (e.g. glutamineGlutamineGlutamine is one of the 20 amino acids encoded by the standard genetic code. It is not recognized as an essential amino acid but may become conditionally essential in certain situations, including intensive athletic training or certain gastrointestinal disorders...
) 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 groupCarbonylIn organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
of a glycineGlycineGlycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...
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 SSulfurSulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
→CuCopperCopper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
charge transferLuminophoreA luminophore is an atom or atomic grouping in a chemical compound that manifests luminescence. There exist organic and inorganic luminophores. It should be stressed that the correct, textbook terminology is luminophore, not lumophore, although the latter term has been frequently but erroneously...
), 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 planarSquare planarThe square planar molecular geometry in chemistry describes the stereochemistry that is adopted by certain chemical compounds...
coordination by N or N/O ligandLigandIn coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s. They exhibit an axial EPR spectrum with copper hyperfine splittingHyperfine structureThe term hyperfine structure refers to a collection of different effects leading to small shifts and splittings in the energy levels of atoms, molecules and ions. The name is a reference to the fine structure which results from the interaction between the magnetic moments associated with electron...
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 enzymeEnzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s, 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 ligandBridging ligandA bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually restricted to small ligands such as pseudohalides or to ligands that are...
. These centres are present in some oxidases and oxygen-transporting proteins (e.g. hemocyaninHemocyaninHemocyanins are respiratory proteins in the form of metalloproteins containing two copper atoms that reversibly bind a single oxygen molecule . Oxygenation causes a color change between the colorless Cu deoxygenated form and the blue Cu oxygenated form...
and tyrosinaseTyrosinaseTyrosinase also known as monophenol monooxygenase is an enzyme that catalyses the oxidation of phenols and is widespread in plants and animals...
). - Binuclear Copper A centres (CuA) are found in cytochrome c oxidaseCytochrome c oxidaseThe enzyme cytochrome c oxidase or Complex IV is a large transmembrane protein complex found in bacteria and the mitochondrion.It is the last enzyme in the respiratory electron transport chain of mitochondria located in the mitochondrial membrane...
and nitrous-oxide reductaseNitrous-oxide reductaseIn enzymology, a nitrous oxide reductase is an enzyme that catalyzes the chemical reactionThe three substrates of this enzyme are nitrous oxide and reduced cytochrome c; its three products are nitrogen, H2O, and oxidized cytochrome c...
. The two copper atoms are coordinated by two histidines, one methionine, a protein backbone carbonyl oxygen, and two bridging cysteine residues. - Copper B centres (CuB) are found in cytochrome c oxidaseCytochrome c oxidaseThe enzyme cytochrome c oxidase or Complex IV is a large transmembrane protein complex found in bacteria and the mitochondrion.It is the last enzyme in the respiratory electron transport chain of mitochondria located in the mitochondrial membrane...
. The copper atom is coordinated by three histidines in trigonal pyramidal geometry. - Tetranuclear Copper Z centre (CuZ) is found in nitrous-oxide reductase. The four copper atoms are coordinated by seven histidine residues and bridged by a sulfur atom.