Amicyanin is a type I copper protein that plays an integral role in electron transfer. In

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

such as ''

Paracoccus denitrificans ''Paracoccus denitrificans'', is a coccoid bacterium known for its nitrate reducing properties, its ability to replicate under conditions of hypergravity and for being a relative of the eukaryotic mitochondrion (endosymbiotic theory). Descript ...

'', amicyanin is part of a three-member

redox Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...

complex Complex commonly refers to: * Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe ** Complex system, a system composed of many components which may interact with each ...

, along with methylamine dehydrogenase (MADH) and

cytochrome Cytochromes are redox-active proteins containing a heme, with a central iron (Fe) atom at its core, as a cofactor. They are involved in the electron transport chain and redox catalysis. They are classified according to the type of heme and its ...

c-551i.

Function

In the electron transfer mechanism from MADH to

heme Heme (American English), or haem (Commonwealth English, both pronounced /Help:IPA/English, hi:m/ ), is a ring-shaped iron-containing molecule that commonly serves as a Ligand (biochemistry), ligand of various proteins, more notably as a Prostheti ...

, amicyanin acts as an electron-accepting intermediate. In this reaction, MADH catalyzes the oxidative deamination of

methylamine Methylamine, also known as methanamine, is an organic compound with a formula of . This colorless gas is a derivative of ammonia, but with one hydrogen atom being replaced by a methyl group. It is the simplest primary amine. Methylamine is sold ...

formaldehyde Formaldehyde ( , ) (systematic name methanal) is an organic compound with the chemical formula and structure , more precisely . The compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde. It is stored as ...

plus

ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...

. The

tryptophan tryptophylquinone Tryptophan tryptophylquinone (TTQ) is an enzyme cofactor, generated by posttranslational modification of amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino ...

(TTQ) group of MADH then donates electrons to the copper centre of amicyanin, which in turn gives the electrons to the heme of the cytochrome c. In ''P. denitrificans'', amicyanin is absolutely required for electron transfer from MADH to c-type cytochromes. It has been shown that inactivation of amicyanin by gene replacement in vivo results in complete loss of ability to grow on methylamine.

Structure

As a type I copper protein, amicyanin contains one copper atom coordinated by two

histidine Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an Amine, α-amino group (which is in the protonated –NH3+ form under Physiological condition, biological conditions), a carboxylic ...

residues and a

cysteine Cysteine (; symbol Cys or C) is a semiessential proteinogenic amino acid with the chemical formula, formula . The thiol side chain in cysteine enables the formation of Disulfide, disulfide bonds, and often participates in enzymatic reactions as ...

residue in a

trigonal planar In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of an equilateral triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands a ...

structure along with an axial

methionine Methionine (symbol Met or M) () is an essential amino acid in humans. As the precursor of other non-essential amino acids such as cysteine and taurine, versatile compounds such as SAM-e, and the important antioxidant glutathione, methionine play ...

residue

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

. Alterations from this particular coordination of the copper centre are found to negatively alter the redox potential of amicyanin. In ''P. denitrificans'', amicyanin exists in a three-part complex along with MADH and cytochrome c-551i. This is the only redox complex composed of three weakly associated proteins naturally observed.

References

1. Victor L. Davidson and Limei Hsu Jones, ''Biochemistry'' 1996, 35, 8120-8125.
2. Arnout P. Kalverda, Jesus Salgado, Christopher Dennison, and Gerard W. Canters, ''Biochemistry'' 1996, 35, 3085-3092.
3. Victor L. Davidson and Dapeng Sun, ''J. Am. Chem. Soc.'' 2003, 125, 3224-3225. Bacterial proteins Copper proteins