Copper oxidases trinuclear

Electron transfer (continued) intramolecular, from type-1 copper center to trinuclear copper center blue copper oxidases, 40 175-178 iron-sulfur proteins, 47 405, 474-479 kinetic model, flavocytochrome bj, 36 282-283... 

Blackburn, N. J., Ralle, M., Hassett, R., and Kosman, D. J. (2000). Spectroscopic analysis of the trinuclear cluster in the Fet3 protein from yeast, a multinuclear copper oxidase. 

The mechanism of dioxygen reduction at the trinuclear cluster in MCO catalysis has been a strong focus for research on this class of copper oxidases. Dioxygen reduction has been most thoroughly investigated in Rhus laccase (a plant laccase, from the Japanese lacquer tree). The primary reason for using Rhus Lac is the availability of a metal-substituted form the enzyme, a TlHg form, in which the... 

Laccase, ascorbate oxidase, and ceruloplasmin are the classical members of the multicopper oxidase family also known as blue oxidases. Recently, a small number of bacterial members of this family have been characterized, including CueO from E. coli a spore-coat laccase (CotA) from Bacillus suhtilis and phenoxazinone synthase from Streptomyces antibioticus The catalyzed reaction of these enzymes except for phenoxazinone synthase is given in Equation (11). A comprehensive overview of the broad and active research on blue copper oxidases is presented in Messerschmidt. Recent results have been included in a review on the reduction of dioxygen by copper-containing enzymes. The nature and number of the different copper sites in blue oxidases has been described in the sections about the type-1 copper site and the trinuclear copper cluster. 

The active site of MCOs consists of a minimum of four Cu atoms, referred to as type 1 (Tl, blue copper), type 2 (T2, normal copper) and type 3 (T3, coupled dinuclear center). While AO is considered a homodimer of Lc, Cp has two additional Tl copper centers. The Tl site in the four-copper enzymes is the site of the substrate oxidation, and it is characterized by an intense S(cysteine)-to-Cu(II) charge-transition at 610 nm (thus, these are blue copper oxidases). The trinuclear cluster (TNC) is formed with the T2 and T3 sites, and the reduction of O2 to water is catalyzed here. The TNC is 13 A away from Tl site and coupled electronically through a Tl-Cys-His-T3 electron-transfer pathway (Fig. 13) (131). 

Laccases are used at the cathode of a BFC to catalyze the four-electron electroreduction of 02(which acts as the terminal electron acceptor) to water. They belong to a group of enzymes called blue copper oxidases," due to the presence of a type 1 (T1) copper site in the enzyme. This copper site acts as the primary electron acceptor and imparts laccases with a blue color. Additional copper ions in type 2 and type 3 (T2/T3) sites form a trinuclear cluster that acts as a binding site for molecular oxygen. Here, the electrons transferred firom the T1 site reduce oxygen to water. Laccases have been the subject of study for several decades, and have found use in fields ranging firom wastewater treatment to the paper industry [19,20], However, it is their DET ability and high redox potential that have led to their use in biosensors and BFCs. 

Blue copper oxidase (BCO) is a classification of oxidoreductase enzymes that contain at least one blue or T1 copper and a T2AT3 trinuclear cluster. These enzymes typically give rise to a characteristic blue color as a result of a strong absorption band around... 

Type III copper is characterized by antiferromagnetic coupling of a pair of copper atoms and strong absorbance at 330 nm. A single type III pair is found in hemocyanin, in which it is involved in O2 transport, and in tyrosinase, in which an oxygen is inserted into substrate. A pair of copper atoms is also found in the multi-copper ascorbate oxidase, but it is coupled to the type II copper in a trinuclear arrangement. 

With the structure of ascorbate oxidase in hand, a new structurally based alignment of the sequences of ascorbate oxidase, laccase, and ceruloplasmin has been performed (Messerschmidt and Huber, 1990). In brief, while gene triplication for ceruloplasmin is still revelant, its sequence can be further subdivided into two domains per unit of triplicated sequence, or six domains in total. Each of these sequences bears some resemblance to each of the three domains of ascorbate oxidase, as does each of the two domains in laccase. The coppers of the trinuclear site of ceruloplasmin then are predicted to be bound between domains 1 and 6, with a type I site also lying in both domains 6 and 4 (see Huber, 1990). The relative orientation of each of these domains is not predicted by this alignment, but it turns out that the structure of nitrite reductase may shed some light on this (see Section V,C). 

Usually, these metalloproteins contain both type 2 and type 3 copper centers, together forming a triangular-shaped trinuclear active site, such as found in laccase (polyphenol oxidase) [38-41] and ascorbate oxidase (3) [42]. Recent evidence for a related arrangement has been reported for the enzyme particulate methane monooxygenase as well [43], but in this case the Cu Cu distance of the type 2 subunit (2.6 A) appears to be unusually short and the third Cu ion is located far from the dinuclear site. 

Ceruloplasmin is involved in copper storage and transport as well as in iron mobilisation and oxidation. Among the blue oxidases it is unique since it contains, in addition to the usual motif of a type 1 combined with the trinuclear cluster, two other type 1 coppers. Electron transfer occurs, however, only between five of the six copper ions since one of the type 1 centres is not catalytically relevant due to its too high redox potential. The redox potentials of the centres were determined and possible electron transfer pathways among the copper sites were discussed.101... 

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