Tetranuclear Cuz sites

The partial delocalized description of die resting Cuz site is supported by DFT calculations, with the spin density dominantly on the Cui center (42%) and delocalized over the Cun center (16%), while Cum and Cuiv centers have little spin density [31,32,122]. The calculated spin distribution at the Cui and Cun centers has a 5 2 ratio that is consistent with the Q-/X-band EPR results. This is consistent with the Cu K-edge XAS experiment [31], which demonstrated diat, of the two possible combinations of the oxidation state of the four Cu eenters in the Cuz site (i.e., one Cu(II) + three Cu(I) vs. three Cu(II) + one Cu(I)), only die lCu(II)/3Cu(I) model 

Since only Cui is dominantly oxidized in the resting Cuz, fiie observed d-d transitions in absorption and MCD speetra are attributed to this Cu center. Importantly, the Cui dxy— transition is high in energy (at 18000 cm ) relative to 

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In addition to the Tl, T2, and T3 sites, recent studies on cytochrome c oxidase [23,24] and nitrous oxide reductase [25,26] have revealed new classes of eopper sites. These are the binuclear Cua and the tetranuclear Cuz sites, both of whieh have mixed-valent Slot = 1/2 ground states. (A Cub center is also found in the eyto-chrome c oxidase. It forms a binuclear heme a3-CuB active site where 4e reduction of O2 occurs. However, due to the lack of a distinctive spectral feature, studies on the Cub center have been limited. Interestingly, it has a covalently linked Tyr residue bound to a His ligand, which is believed to have an important role in flic reactivity [27])... 

Chen, P., Gorelsky, S.I., Ghosh, S. and Solomon, E.I. (2004) NzO reduction by the i4-sulfide-bridged tetranuclear Cuz cluster active site, Angew. Chem. Int. Ed., 43, 4132-4140. 

Fig. 11. The binuclear CuA (A) and tetranuclear CuZ (B) copper-binding sites of nitrous oxide reductase from Pseduomonas nautica (PDB Accession Code IQNl). The sulfur atom in the tetranuclear copper site is marked with an S.

A new representative of a multicopper cluster in a protein is Cuz in nitrous oxide reductase. As was discussed above this enzyme contains a binuclear CuA centre as in COX. While the latter in addition has CuB in the form of a copper-heme group, N20 reductase has Cuz which is the site of dinitrogen formation from the substrate N20. Recently a central inorganic sulfide has been found as a ligand to copper and multiple forms of Cuz were detected in the enzyme from Paracoccus pantotrophus.134 More recently a tetranuclear copper cluster with X-S bridges was proposed as structure for Cuz..135... 

The Cuz active site consists of four copper ions, arranged in a distorted tetrahedron and coordinated by seven histidine residues and one hydroxide anion. This site was detected in nitrous oxide reductase [16, 17] (Figure 5.1g) and is involved in the reduction of N20 to N2. The copper ions in the tetranuclear cluster are bridged by an inorganic sulfur ion [18], which until recently was believed to be a hydroxide anion. Three copper ions are coordinated by two histidine residues, whereas the fourth is coordinated by only one, thus leaving a binding site for the substrate. 

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