Type 2 Copper Proteins

NOH is changed to nitrous oxide under oxygen-deficient conditions (Hooper and Terry, 1979), though the reduction of nitrite (or nitrous acid) once fronted to the gas may also occur (Poth and Focht, 1985 Yoshida, 1988). Indeed, the bacterium has a copper protein-type nitrite reductase which catalyzes the reduction of nitrite to nitric oxide and/or nitrous oxide (Hooper, 1968 Ritchie and Nicholas, 1972, 1974 Miller and Wood, 1983 DiSpirito et al., 1985). However, as Beaumont et al. (2002) have recently found that nitrous oxide is produced even by N. europaea in which the DNA encoding nitrite reductase has been destroyed, the formation of nitrous oxide by the bacterium seems attributable to the decomposition of NOH. Camera and Stein (2007) have recently reported similar results about the formation of nitrous oxide by the nitrite reductase-deficient mutant of the bacterium. 

Next, nitrite is reduced to nitric oxide by the catalysis of nitrite reductase. Two kinds of nitrite reductases are known in the denitrifying bacteria (N2 forming) cytochrome cdx-type enzyme (Yamanaka et al., 1960, 1961, 1963 Yamanaka and Okunuki, 1963a,b,c Yamanaka, 1964) and copper protein-type enzyme (Iwasaki and Matsubara, 1972). However, no case has been found in which one species of the denitrifying bacterium has both types of the enzymes simultaneously (Coyne et al., 1989). 

The copper-protein type nitrite reductases have been purified from several denitrifying bacteria. They have 2 1 copper atoms in the molecule one copper per subunit (30 10 kDa). The enzymes catalyze the reduction of nitrite to nitric oxide, but the physiological electron donors for the enzymes have not been clarified, though they are said to be cytochrome c in some cases and a copper-protein in some cases (see Otsuka and Yamanaka, 1988). 

Electron transfer copper proteins usually belong to the blue copper proteins (Type 1) azurin is a simple example. This family of proteins are also called cupredoxins, and they participate in many redox reactions involved in processes fundamental to biology, such as respiration or photosynthesis. The striking electron transfer capabilities of blue copper proteins have been studied extensively. Plastocyanin, with a tetrahedral CUN2S2 core, acts as the electron donor to Photo System I in photosynthesis in higher plants and some algae. 

It is known, from the crystal structures of some natural proteins, such as plastocyanin [1] and azurin [2], that the so-called blue-copper proteins. Type 1, have one copper(II) ion, with two Cu-S and two Cu-N coordinate bonds, in their active sites. 

Copper proteins, 1,168 5,720 models, 2,85 nonblue, 5,723 type 111, 5,724 Copper salts cellulose dyes, 6,38 Copper(I) salts stabilization, 6,786 Copper(II) salts ammoniacal leaching, 6,787 oxidant... 

In the blue, Type I copper proteins plastocyanin and azurin, the active-site structure comprises the trigonal array [CuN2S] of two histidine ligands and one cysteine ligand about the copper,... 

From the standpoint of modeling Type I copper proteins,4,5,59,60 a variety of imidazole-based ligands containing thioether sulfurs and imidazole groups have been synthesized.61,62 The structures and spectroscopic properties of their copper(II) complexes (51)-(53) and (55)-(60) were investigated.65,79-82 To characterize apical copper(II)-thioether bonding, the complex (51) was... 

In their pursuit of modeling Type I copper proteins, Kitajima et al. reported112 a rare, tetrahedrally coordinated complex (105), which displayed an EPR spectrum consistent with the presence of the unpaired electron in the dz2 orbital.1 They also isolated a square-pyramidal DMF adduct (complex (106)). They were successful in providing structural proof of a copper(II) complex (trigonal pyramidal) with C6F5S -coordinated complex (107), with CuN3S chromo-phore.113 The X-ray analysis (poor data set) of a closely similar complex with Ph3CS as the... 

Model systems for Type I copper proteins structures of copper coordination compounds with thioether and azole-containing ligands.17... 

Copper, Cu+(d10), Cu2+ (d9) 4, tetrahedral N-Thiolate, thioether, AMmidazole Electron transfer in Type I blue copper proteins... 

This discussion of copper-containing enzymes has focused on structure and function information for Type I blue copper proteins azurin and plastocyanin, Type III hemocyanin, and Type II superoxide dismutase s structure and mechanism of activity. Information on spectral properties for some metalloproteins and their model compounds has been included in Tables 5.2, 5.3, and 5.7. One model system for Type I copper proteins39 and one for Type II centers40 have been discussed. Many others can be found in the literature. A more complete discussion, including mechanistic detail, about hemocyanin and tyrosinase model systems has been included. Models for the blue copper oxidases laccase and ascorbate oxidases have not been discussed. Students are referred to the references listed in the reference section for discussion of some other model systems. Many more are to be found in literature searches.50... 

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

Since the first EPR work on Cu(II) ions in proteins in the late fifties193, a great many EPR investigations on copper-containing proteins have been reported194-198. For a classification of the copper proteins into type I (blue copper), type II (non-blue copper) and type III (binuclear cupric pair), the reader is referred to Fee197. ... 

In frozen solution EPR spectra of copper proteins, ligand and copper hf interactions are only partially resolved, or not at all. It is therefore striking that only few ENDOR studies on copper proteins have been published so far17,199-201). Nevertheless, these few ENDOR results already demonstrate the power of the double resonance technique to probe the coordination environment of the copper-containing sites in these types of proteins. 

The second class of dioxygen carriers is that of haemocyanins. These proteins, which contain a binuclear Cu(I) site (thus in the oxidized Cu(II) met form they belong to the so-called Type 3 copper proteins , which contain an EPR-silent dicopper active site), regulate dioxygen transport in the respiration of arthropods and molluscs. Figures 7 and 8 show... 

Blue (or type 1 ) copper proteins (or cupredoxins) are important components of biological electron transfer processes in many organisms ranging from bacteria to animals, from fungi to plants.56 They are characterized by ... 

Copper, Cu (d °), Cu " (d ) 4, tetrahedral Y-Thiolate, thioether, A-imidazoIe Electron transfer in Type I bine copper proteins and Type III heme-copper oxidases (Cua in cytochrome c oxidase, for example)... 

When copper is bound to one sulfur atom of a cysteine and two nitrogens of two histidines in an essentially tetrahedrally distorted - trigonal ligand environment (type I copper proteins), the excited levels are low in energy, and the values are reduced to about 5 x 10 ° s (29). Examples are blue copper proteins, like ceruloplasmin and azurin, and copper(II) substituted liver alcohol dehydrogenase (30-32). 

The blue, or type 1, copper proteins, azurin from Pseudomonas aeruginosa (Adman et ai, 1978 Adman and Jensen, 1981) and from Al-caligenes denitrificans (Norris et al., 1983, 1986) and poplar plastocyanin (Guss and Freeman, 1983 Guss et al., 1986), have been studied by X-ray diffraction. These involve a Cu(I)/Cu(II) redox system. Cu(I) d ) is... 

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