Plastocyanin ligands

Furthermore, based on earlier calculations (39) for the type 1 copper protein plastocyanin, ligand-field parameters for the blue copper in laccase have been derived. These reports (37,38) also include a structural representation of the type 1 center composed of a flattened tetrahedron (D2d symmetry) with two imidazole side-chains, a cysteine sulfur, and a fourth ligand (which probably is methionine sulfur), bound to the metal ion. Although no such low-temperature experiments have been performed with ascorbate oxidase, one might anticipate similar structural features for the blue type 1 centers. 

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,... 

Figure 3. Plastocyanin orientations. The Cu(N-Imid)2(S-Cys)(S-Met) unit has approximately trigonal pyramidal symmetry, with the S(Met) ligand at the apex of the pyramid. Key top, orientation giving predominantly Cu-S(Met) EXAFS and bottom, orientation giving no Cu-S(Met) EXAFS. (Reproduced from Ref. 30. Copyright 1982, American Chemical Society.)...

Figure 5.9 Ligands (A) N,S-mpy, (B) N4-mpy, (C) N2S2-mpy, and (D) N2S2-inim designed to mimic azurin and plastocyanin active sites. (Adapted with permission of The Royal Society of Chemistry from Malachowski, M. R. Adams, M. Elia, N. Rheingold, A. L. Kelly, R. S. J. Chem. Soc., Dalton Trans., 1999, 2177-2182.

The nature of the ligand donor atom and the stereochemistry at the metal ion can have a profound effect on the redox potential of redox-active metal ions. The standard redox potentials of Cu2+/Cu+, Fe3+/Fe2+, Mn3+/Mn2+, Co3+/Co2+, can be altered by more than 1.0 V by varying such parameters. A simple example of this effect is provided by the couple Cu2+/Cu+. These two forms of copper have quite different coordination geometries, and ligand environments, which are distorted towards the Cu(I) geometry, will raise the redox potential, as we will see later in the case of the electron transfer protein plastocyanin. 

Figure 14.1 (Left) X-ray structure of plastocyanin from poplar leaves as a ribbon diagram with the metal ion and its ligands highlighted, PDB code 1PLC (right) Type 1 Cu site in Cu(II)-nitrite reductase from Alcaligenes faecalis, PDB code 1AS6. (From Messerschmidt et al., 2001. Reproduced with permission from John Wiley Sons., Inc.)...

The oxidized T1 site In laccase Is spectroscopically similar to that of plastocyanin and azurln. Indicating that exogenous ligands can coordinate only to the T2 and T3 coppers at the native active site, (see Splra, D.J. Co, M.S. Solomon, E.I. Hodgson, K.O. Blochem. Blophys. Res. Commun. 1983, 112, 746.)... 

In contrast to azurin, the plant plastocyanins have a conserved negative patch of residues adjacent to a putative redox partner-binding site. Plastocyanin has, in addition, the hydrophobic face into which the edge of the second histidine ligand is inserted. 

The location of the copper with respect to the Greek key fold is interesting when compared to that of the cupredoxins. While the copper in the cupredoxins lies in the interior of the /8 barrel bound by three interior-facing residues of the carboxy-terminal loop in the )8 barrel, and by a histidine in an adjacent strand, the copper in SOD lies on the outside of its jS barrel, bound by one residue from the carhoxy-terminal loop and three from the adjacent strand (cf. Figs. 2c-5c with Fig. 8c.) A structural comparison of plastocyanin and SOD, coupled with sequence alignment of plastocyanin and ceruloplasmin (Ryden, 1988), showed that three of the SOD ligands correspond to putative copper ligands in ceruloplasmin. Why this is so will become more evident after the description of the ascorbate oxidase structure and its relationship to ceruloplasmin. 

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... 

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