The Copper Site

The zinc oxide component of the catalyst serves to maintain the activity and surface area of the copper sites, and additionally helps to reduce light ends by-product formation. Selectivity is better than 99%, with typical impurities being ethers, esters, aldehydes, ketones, higher alcohols, and waxes. The alumina portion of the catalyst primarily serves as a support. 

Cytochrome c oxidase contains two heme centers (cytochromes a and %) as well as two copper atoms (Figure 21.17). The copper sites, Cu and Cug, are associated with cytochromes a and respectively. The copper sites participate in electron transfer by cycling between the reduced (cuprous) Cu state and the oxidized (cupric) Cu state. (Remember, the cytochromes and copper sites are one-electron transfer agents.) Reduction of one oxygen molecule requires passage of four electrons through these carriers—one at a time (Figure... 

Ndi g5Ceo.i5Cu04 Substitution of Ce" for Nd " ions involves the formation of charge compensating electrons distributed among the copper sites... 

On the other hand a direct hydrogen transfer through a Meerwein-Ponndorf mechanism, involving coordination of both the donor alcohol and the ketone to the copper site may also be considered. In this case, by using alcohols other than 2-propanol, we could expect some difference in stereochemistry. This would also imply the possibility of carrying out the enantioselective reduction of a prochiral ketone with a chiral alcohol as donor. 

The highly rhombic nuclear quadrupole tensor of the Cu(II) ion indicates that the total electron density at the copper site deviates considerably from any idealized symmetry. 

A further cupredoxin we consider is mavicyanin from green zucchini peelings (FW = 12 700),70 a positively charged phytocyanin (p/= 8.85) the copper site of which is very similar to that of the above discussed... 

In moist conditions, the presence of water hinders the adsorption of N02 on the copper sites and physical adsorption in small micropores is the main mechanism of the removal process. 

Norris, G. E., Anderson, B. F., and Baker, E. N. (1986). Blue copper proteins. The copper site in azurin from Alcaligenes denitr cans.J. Am. Chem. Soc. 108, 2784-2785. 

Fig. 2. Copper site in azurin. In this and subsequent figures the following conventions have been used, (a) The copper site is generally an enlargement of (b). The copper site is a dotted sphere, the ligand residues are represented by bonds connecting atoms in the side chain, and, where possible, the atoms bonded to the copper atom are identified by atom type. Ribbons represent portions of the backbone structure near the copper. In (b) of each figure, the main-chain polypeptide is represented by a ribbon fit to the main-chain coordinates, and the amino and carboxy termini are indicated by N and C, respectively, (c) A schematic version drawn from (b). Solid arrows represent main-chain regions participating in the /3 sheet roughly above the plane of the paper, while dotted or light arrows are the...

Spectra, but, in general, leaves the copper site the most exposed of the four cupredoxins. The sequence of Cbp is quite similar to that of stella-cyanin. Stellacyanin is a plant protein, also of unknown function, having visible spectra characteristic of type I copper, but lacking the methionine ligand found in all other type I proteins. A disulfide bond has been suggested as a potential copper ligand in stellacyanin the Cbp has both a methionine and the disulfide, so that prior to the structure determina-... 

Although the details of the structure of superoxide dismutase (SOD), one form of which is a Cu,Zn-enzyme (Cu,Zn-SOD), have been well described previously (Getzoff etal., 1983), a brief description is repeated here, for the geometry of the copper site as well as the topology of its fold is relevant to all copper enzymes. 

Fig. 8. (a) Copper and zinc sites in superoxide dismutase (SOD). The small sphere is waterbound to the copper site, (b) Ribbon drawing of the SOD backbone, (c and d) Schematic of SOD topology. 

In addition to the fascinating and complex properties of the subunit architecture of hemocyanins [described, for example, by Preaux and Gielens (1984) and Herskovits (1988)], the nature of the copper site has... 

It is noteworthy that the proximity of the copper sites in ceruloplasmin, and, indeed, the involvement of most of the correct ligand histidines, were predicted some time ago by Ryden (1982, 1984) strictly on the basis of sequence homologies to plastocyanin. A similar prediction was made for laccase based on sequence similarities around the cysteine regions (Briving et al, 1980). Proximity of the type II site to the type III site (e.g., a trinuclear site) was also predicted by Solomon and co-workers (Allen-dorf et al., 1985 Spira-Solomon et al, 1986) on the basis of spectroscopic analysis of azide binding to laccase. What could not have been foreseen... 

Fig. 7 Geometry of a typical type I copper site in plastocyanin. Bond lengths and bond angles are relatively constant upon reduction or oxidation of the copper site (J. M. Guss, P. R. Harrowell, M. Murata et al.,J. Mol. Biol. 1986, 792, 361-387).

Table 1. Crystallographic data are presented for the new phases which resulted from a 3d-metal substitution at the copper site in the n=l, 2 and 3 Bi-based cuprates. The periodicity of the structural modulation (p) is given by bold numbers when obtained from single crystal x-ray studies, otherwise the values were obtained from TEM studies. 

Sano, M., and Matsubara, T. (1988). Structural change in the one-electron oxidation-reduction at the copper site in nitrite reductase. Evidence from EXAFS. Inorg. Chem. Acta Bioinorg. Chem 152, 53-54. 

Having elucidated, in combination with X-ray structural data, the characteristics of the copper site coordination in blue proteins in extenso, the challenge for EPR spectroscopy (and other techniques) is now to find ways to model the electron transfer (ET) in a realistic fashion. At present EPR is, however, mostly used to ascertain that the coordination of copper in the experimental ET chain models employed is not disturbed prior to ET. Plastocyanin is the electron carrier in photosynthesis. Indications of structural origins of impaired ET in... 

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... 

Figure 92 (a) Structural mechanism for the hydroxylation of monophenolic substrates by oxytyrosinase (b) reaction coordinate diagram for associative ligand substitution at the copper site of tyrosinase... 

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