CuA site

Figure 14.10 (Left) The CuA site in bacterial cytochrome oxidase. (From Messerschmidt et al., 2001. Reproduced with permission from John Wiley Sons., Inc.) (Right) The haem-a3/CuB site in the resting form of oxidized bovine heart cytochrome c oxidase showing peroxide bound between the haem Fe and CuB. (From Bento et al., 2006. With kind permission of Springer Science and Business Media.)...

The disposition of the different metal centres of bovine heart CcOx is represented in Figure 14.9. The dimetallic CuA site receives electrons directly from cytochrome c, and is located in a globular domain of subunit II, which protrudes into the intermembrane space (the periplasmic space in bacteria). This centre, which was widely believed to be mononuclear is a dicopper site (Figure 14.10) in which the coppers are bridged by two cysteine sul-furs each copper in addition has two other protein ligands. In the one electron-reduced form,... 

Chemical Reviews paper. We can only discuss a small number of these here, but some important categories are (1) synthetic Fe(II)-Cu(I) complexes and their reactions with O2, (2) oxidized heme-copper models (Fe(III)-X-Cu(II) complexes, where X equals 0x0- and hydroxo-bridged complexes, cyanide-bridged complexes, or other X-bridged complexes), (3) crosslinked histidine-tyrosine residues at the heme-copper center, and (4) Cua site synthetic models. 

The Cua site, common in biology (inset in Fig. 5.42), is dinuclear with two copper atoms bridged by the thiolate sulfurs of two cysteine ligands. One unpaired electron is delocalized over two metals, which are thus Cul 5+. The NMR spectra show narrow lines from the copper ligands (Fig. 5.42) [120,121], corresponding to an electron relaxation time of 10 11 s, as in Cu2+-Cu2+ dimers (see Section 6.3.2). However, in Cua there is no magnetic coupling between the two centers, as they contain only one unpaired electron just as an isolated Cu2+ ion. Electron relaxation of Cua may be fast because the orbital overlap between the two copper centers provides new relaxation mechanisms not available to a monomer (as Orbach or Raman relaxation). 

Figure 5.1 Schematic representations of selected active sites of the copper proteins plastocyanin [56] (type 1, a) galactose oxidase [57] (type 2, b) oxy hemocyanin [58] (type 3, c) ascorbate oxidase [10] (type 4, or multicopper site, d) nitrous oxide reductase [59] (CuA site, e) cytochrome c oxidase [15]...

Wang K, Geren L, Zhen Y, et al. Mutants of the CuA site in cytochrome c oxidase of Rhodobacter sphaeroides II. Rapid kinetic analysis of electron transfer. Biochemistry 2002 41 2298-304. 

In addition to the mechanism of dioxygen reduction, an understanding of how Cco pumps protons is also desirable. Models have been proposed that allow for linkage of the proton pumping to the dioxygen reduction reaction (50). One attractive model involves the CuA site and is shown in Figure 6 (10). In this mechanism, the CuA center is ligated to two histidines and two thiolates and receives the initial electron from... 

Figure 6. Model for redox-linked proton pumping in cytochrome oxidase involving the CuA site. (Reproduced from reference 10. Copyright 1990 American Chemical Society.)...

Unlike the other ubiquinol binding sites presented above, the ubiquinol binding site in cytochrome bo% stabilizes a semiquinone during the oxidation of ubiquinol. A previous model placed the ubiquinone binding site of cytochrome bo within the extrinsic domain of subunit II, replacing the Cua site (Murray et al., 1999). However, several lines of evidence are available to refute this model. 

The CuA site may be of exceptional significance in the context of inorganic chemistry, since there is EXAFS evidence that it contains two copper atoms only about 2.50 A apart.39 Although this has been questioned,40 it is also supported by X-ray evidence.41 It has been suggested that at such a distance, one may postulate a Cu—Cu bond, the first metal-metal bond to be found in a biological context. That concept, however, is controversial. The site has two tetrahedra sharing an edge with /a-S atoms from cysteine residues, and it cycles between Cu+1/Cu+1 and Cu+1,5/Cu+1,5 states. 

At 2.8-A resolution, the electron density of Cua is spherical (Fig. 2A, see color insert) (Tsukihara et al., 1995). However, the three-dimensional organization of the amino acids surrounding the spherical electron density strongly suggests that the Cua site is a dinuclear copper center. The... 

Subunits II, Via, and VIb are arranged on the top of subunit I and form a concave surface including a surface atom near the Cua site (Fig. 7D). The concave surface is large enough to bind two cytochrome c molecules. However, a theoretical prediction of cytochrome c binding to the enzyme based on the X-ray structures of both proteins does not seems very straightforward, particularly for the weakly bound site. Crystallization of the enzyme-cytochrome c complex is desirable. 

Cua centers exist in two redox states [Cu(II)Cu(I)] and [Cu(I)Cu(I)]. The oxidized species is a fully delocalized mixed-valence pair (formally two Cu+ 1.5 ions), as revealed by EPR spectroscopy (Kroneck et al., 1988, 1990). Despite the similar coordination geometry around copper, these systems display sharper NMR lines than do the BCP due to a shorter electron relaxation time of the paramagnetic center (wlO "s) (dementi and Luchinat, 1998). NMR studies are available for the native Cua centers from the soluble fragments of the The. thermophilus, Paracoc-cus denitrificans, Paracoccus versutus, and Bacillus subtilis oxidases (Bertini et al., 1996 Dennison et al., 1995 Luchinat et al., 1997 Salgado et al., 1998a) and Pseudomonas stutzeri N2O reductase (Holz et al., 1999), as well as for engineered Cua sites in amicyanin (Dennison et al., 1997) and Escherichia coli quinol oxidase (Kolczak et al., 1999). 

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