Copper enzymes plastocyanin

There are a number of excellent sources of information on copper proteins notable among them is the three-volume series Copper Proteins and Copper Enzymes (Lontie, 1984). A review of the state of structural knowledge in 1985 (Adman, 1985) included only the small blue copper proteins. A brief review of extended X-ray absorption fine structure (EXAFS) work on some of these proteins appeared in 1987 (Hasnain and Garner, 1987). A number of new structures have been solved by X-ray diffraction, and the structures of azurin and plastocyanin have been extended to higher resolution. The new structures include two additional type I proteins (pseudoazurin and cucumber basic blue protein), the type III copper protein hemocyanin, and the multi-copper blue oxidase ascorbate oxidase. Results are now available on a copper-containing nitrite reductase and galactose oxidase. 

Most copper enzymes and proteins are found only in eukaryotes, but a few copper proteins, such as azurin and plastocyanin are also present in certain prokaryots... 

We have mentioned earlier the dissimilarities between the spectral properties of chromophoric metal ions at the active sites of metalloen-zymes and the properties of simple bidentate model complexes of the same metals. Cobalt phosphatase has served well to illustrate such a dissimilarity and, in Figure 9, the data for phosphatase, representative of a cobalt enzyme, are shown again along with those for plastocyanin, a copper enzyme, and ferredoxin, an iron enzyme. Each enzyme spectrum is unusual compared with the simple model complexes shown at the bottom of the figure. More detailed spectral data as well as comparison of other physical properties of metalloenzymes—e.g., electron paramagnetic resonance spectra—with those of model complexes have been summarized previously (10). 

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... 

Several copper-containing NiRs have been identified, but the most extensive structural and mechanistic studies have focused on the enzyme from Achromobacter cycloclastes (17-25). A 2.3-A resolution X-ray crystal structure for this NiR in its oxidized form at pH 5.2 has been reported (17), and a representation of the active site is shown in Figure 1. Each monomer in the trimeric protein contains two copper ions, one of which (Cu-1) is ligated to a cysteine, a methionine, and two histidine residues in a geometry similar to that of type 1 copper centers in proteins such as plastocyanin (26). The second type 2 copper ion in NiR (Cu-2) is only 12.5-A distant from the first and is bound to three histidine imidazoles (two from one monomer, the third from an associated subunit) and a fourth small ligand in an unusual tetrahedral arrangement. The... 

The type I copper sites function as electron transfer centers in the blue copper proteins and in multicopper enzymes, particularly oxidases (33). They are characterized by their intense blue color, their unusually small A values, and their very positive redox potentials (Table II). X-ray crystal structures of several blue copper proteins have been determined, notably plastocyanin (34), azurin (35), cucumber basic blue protein (36), and pseudoazurin (37). The active site structures show marked similarities but also distinct differences (Fig. 8). 

Copper is present in mammals in ceruloplasmin and superoxide dismutase and it is also part of a number of enzymes in plants and other organisms, including laccase, ascorbate oxidase, and plastocyanin. In these compounds, it is present in four different forms, listed in Table 16-3. 

For the Cu-Swet bond length, the results are less clear. In all cases, the bond is elongated, and for the oxidised structures, it is in excellent agreement with experimental structures. However, for reduced plastocyanin, the Cu-Smci bond becomes too long, 339 pm, compared to -290 pm. This is most likely due to the flexibility of the bond, combined with problems in the classical force field. Apparently, the molecular mechanics part of the calculations is not accurate enough to describe the fine-tuned interplay between methionine group, the copper ion, and the surrounding enzyme. 

Up to this point, we have considered the election carriers bound to the photosystem-I thylakoid membrane. They include the primary electron donor P700 and the series ofelectron acceptors Aq (Chi a). A, (phylloquinone), FeS-X, and FeS-A/B. We now turn to two mobile electron carriers around photosystem I called plastocyanin (PC), a copper protein and ferredoxin (Fd) a [2Fe-2S] irons-sulfur protein, plus the enzyme that catalyzes the reduction of NADP by ferredoxin and called the ferredoxin-NADP -reductase and abbreviated as FNR. 

The multicopper NiR is a homotrimer, in which each monomer contains two domains.On the other hand, AO and Lc contain three domains while human Cp contains six domains. Each domain of these enzymes has a typical cupredoxin fold. The blue copper center resides in the first domain of NiR, the third domain of AO and Lc, and the second, fourth, and sixth domains of human Cp. The blue copper centers in domain 1 of NiR (see Structure C in Figure 2), domain 3 of AO and in domains 4 and 6 of human Cp are quite similar to that in plastocyanin, consisting of Cu (NHis)2ScysSMet in a flattened tetrahedral geometry. While the distance of Cu -S (Met) is shorter for the blue copper center in NiR (2.62-2.64 A) than in plastocyanin, the distances in AO and Cp (2.8-3.0 A) are unusually longer (see Table 4). Further-... 

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