Copper proteins properties

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... 

Several copper enzymes will be discussed in detail in subsequent sections of this chapter. Information about major classes of copper enzymes, most of which will not be discussed, is collected in Table 5.1 as adapted from Chapter 14 of reference 49. Table 1 of reference 4 describes additional copper proteins such as the blue copper electron transfer proteins stellacyanin, amicyanin, auracyanin, rusticyanin, and so on. Nitrite reductase contains both normal and blue copper enzymes and facilitates the important biological reaction NO) — NO. Solomon s Chemical Reviews article4 contains extensive information on ligand field theory in relation to ground-state electronic properties of copper complexes and the application of... 

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 general properties of simple electron transfer proteins (e.g., the ferredoxins, the blue, or type 1, copper proteins, cytochrome c, and... 

Copper proteins have been classified according to their spectroscopic properties (Malkin and Malmstrbm, 1970 Fee, 1975) as type I, II, or III. Type I blue copper proteins are characterized by an extraordinarily intense absorption near 600 nm and by unusually small hyperfine coupling constants for the paramagnetic [oxidized Cu(II)] form of the protein. 

Fig. 1. Protein factors which may affect type I copper site properties. Schematic view from the protein surface closest to the copper center.

Extensive analysis of the EPR and redox behavior of this unusual copper protein led to the hypothesis that the protein might contain a Cu(A) site similar to that in cytochrome oxidase (Riester et ai, 1989) and that the unusual seven-line EPR is due to the Cu(A)-type site. An alternative interpretation of this EPR is based on electron spin-echo spectroscopy as well, and that is that the seven-line EPR is due to a half-met Cu—Cu pair and to unusual type I sites (Jin et ai, 1989). Three sets of spin-echo peaks can be attributed to nitrogens on imidazole ligands to a CuA-type site and to another imidazole on the half-met site. The electron spin-echo spectra of cytochrome oxidase are similar, although there is not enough copper in cytochrome oxidase for a half-met site. Conceivably, the property of delocalization of the paramagnetic electron could be effected by the proposed bridging between Cub and heme as (nomenclature summarized by Capaldi, 1990), which are proposed to be 3-4 A apart. 

Blue copper proteins, 36 323, 377-378, see also Azurin Plastocyanin active site protonations, 36 396-398 charge, 36 398-401 classification, 36 378-379 comparison with rubredoxin, 36 404 coordinated amino acid spacing, 36 399 cucumber basic protein, 36 390 electron transfer routes, 36 403-404 electron transport, 36 378 EXAFS studies, 36 390-391 functional role, 36 382-383 occurrence, 36 379-382 properties, 36 380 pseudoazurin, 36 389-390 reduction potentials, 36 393-396 self-exchange rate constants, 36 401-403 UV-VIS spectra, 36 391-393 Blue species... 

The current chapter focuses on the electrochemistry of the ionic forms of copper in solution, starting with the potentials of various copper species. This includes the effect of coordination geometry, donor atoms, and solvent upon the electrochemical potentials of copper redox couples, specifically Cu(II/I). This is followed by a discussion of the various types of coupled chemical reactions that may contribute to the observed Cu(II/I) electrochemical behavior and the characteristics that may be used to distinguish the presence of each of these mechanisms. The chapter concludes with brief discussions of the electrochemical properties of copper proteins, unidentate and binuclear complexes. 

Blue copper proteins in their oxidized form contain a Cu2+ ion in the active site. The copper atom has a rather unusual tetra-hedral/trigonal pyramidal coordination formed by two histidine residues, a cysteine and a methionine residue. One of the models of plastocyanin used in our computational studies (160) is pictured in Fig. 7. Among the four proteins, the active sites differ in the distance of the sulfur atoms from the Cu center and the distortion from an approximately trigonal pyramidal to a more tetrahedral structure in the order azurin, plastocyanin, and NiR. This unusual geometrical arrangement of the active site leads to it having a number of novel electronic properties (26). 

Table I lists isomorphous replacements for various metalloproteins. Consider zinc enzymes, most of which contain the metal ion firmly bound. The diamagnetic, colorless zinc atom contributes very little to those physical properties that can be used to study the enzymes. Thus it has become conventional to replace this metal by a different metal that has the required physical properties (see below) and as far as is possible maintains the same activity. Although this aim may be achieved to a high degree of approximation [e.g., replacement of zinc by cobalt(II)], no such replacement is ever exact. This stresses the extreme degree of biological specificity. The action of an enzyme depends precisely on the exact metal it uses, stressing again the peculiarity of biological action associated with the idiosyncratic nature of active sites. (The entatic state of the metal ion is an essential part of this peculiarity.) Despite this specificity, the replacement method has provided a wealth of information about proteins that could not have been obtained by other methods. Clearly, there will often be a compromise in the choice of replacement. Even isomorphous replacement that should retain structure will not necessarily retain activity at all. However, it has become clear that substitutions can be made for structural studies where the substituted protein is inactive (e.g., in the copper proteins and the iron-sulfur proteins). It is also possible to substitute into metal coenzymes. Many studies have been reported of the...

Active-Site Properties of the Blue Copper Proteins A. G. Sykes... 

The multicomponent type IV copper proteins are usually coloured (Figure 95) as they generally contain at least one blue type I copper centre, but as there are usually more than one of each type present, the physical properties of the different types I—III environments are difficult to separate and, for ESR-silent type III centres, difficult to detect.1203 No crystallographic data are yet available on a type IV copper protein. 

Copper-deficient cells of Ps. perfectomarinus give N20 rather than dinitrogen from nitrite. This has led to the interesting suggestion that the dinitrogen monoxide reductase is a copper protein,15361537 but it appears that the properties of the copper protein differ from those of the dinitrogen monoxide reductase.1538 The copper protein may well lie on the electron-transfer pathway to the N20 reductase. 

Among the coordination compounds obtained on the basis of polypyrazolyl-borates, it is worth emphasizing the copper chelates 235 which are still the only biomimetic model of blue copper proteins, reproducing all their physical (UV-and EPR-spectral) properties [441,446-448], Compound 236 [449] is also an example of complexes of this kind of system ... 

Copper(II) sites in proteins can be classified into three types based on their spectral properties. The blue (Type I) copper proteins are characterised by a visible absorption... 

The plastocyanins are blue copper proteins found in the chloroplasts of higher plants and algae where they mediate electron transport between cytochrome f and P-700 (Barber, 1983 Haehnel, 1984, 1986 Cramer etal., 1985 Sykes, 1985 Andersen et al., 1987). Plastocyanins each contain one copper bound by a single polypeptide chain of molecular weight around 10500 (Sykes, 1985). The spectroscopic properties of the copper are those of a typical blue site. The properties of the plastocyanins have been the subject of detailed reviews (Sykes, 1985 Haehnel, 1986 Chapman, 1991). 

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