Plastocyanin, blue copper center

A clear example of the contribution of inorganic spectroscopy in understanding the unique properties associated with an active site is the blue copper center (1-3) in plastocyanin. 

Research aimed at identifying the ligands comprising the flattened tetrahedral blue copper center has been particularly intense in the case of plastocyanin. Direct evidence for a sulfur ligand has come from x-ray photoelectron spectral (XPS) experiments on bean plastocyanin, where a large shift of the S2p core energy of the single cysteine (Cys-85) residue in the protein upon metal incorporation (164.5, apo 169.8, native 168.8 eV, Co(II) derivative) was observed (15). The two histidines in spinach plastocyanin exhibit pK values below 5 in NMR titration experiments,... 

Based on the Fe(EDTA)2- results, the blue copper center in stellacyanin appears to be much more accessible than that situated in either azurin or plastocyanin. Thus it should be profitable to compare the electron transfer reactivities of these three proteins with a variety of redox agents. Kinetic studies of the oxidation of the three blue proteins by Co(phen)33+ have been made (26), and the results together with those for other redox agents are set out in Table IV. The electrostatic corrections to the predicted kn values are modest both for the large charge on plastocyanin and the small one on azurin, as the protein selfexchange and the cross reaction work terms compensate. The reactivity... 

Figure 3 Examples of metal cofactors in proteins (a) the zinc center of carbonic anhydrase, (b) the blue-copper center of plastocyanin, (c) the iron center in 2,3-dihydroxybiphenil dioxygenase, (d) the iron binding site of transferrin, and (e) the dinuclear copper site of Cu/ in cytochrome c oxidase.

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-... 

The metal-binding site structures of pseudoazurin and auracyanin are similar to that of other cupredoxins, with one cysteine and two histidines forming a trigonal plane and a weak axial methionine to complete the distorted tetrahedral geometry. The Cu-S (Met) distance (2.76 A) in pseudoazurin is shorter than those of blue copper centers in plastocyanin and azurin, and is still longer than those in plantacyanin and NiR (see Table 4). 

In Section II we discussed the properties of certain proteins containing single or isolated and independent blue copper centers. With the exception of plastocyanin the biological function of these proteins is not known, but it is very likely that they participate in single electron transfer reactions. 

One biological pair that has been studied is that comprising cytochrome c [cyt(II)] with its heme iron in a divalent state, and plastocyanin [pc(II)] with its blue copper center also in a divalent state. " Because cyt(II) is positively charged and pc(II) is negatively charged, an encounter complex is obtained prior to electron transfer to give cyt(III) and pc(I)... 

Blue copper proteins. A typical blue copper redox protein contains a single copper atom in a distorted tetrahedral environment. Copper performs the redox function of the protein by cycling between Cu and Cu. Usually the metal binds to two N atoms and two S atoms through a methionine, a cysteine, and two histidines. An example is plastocyanin, shown in Figure 20-29Z>. As their name implies, these molecules have a beautiful deep blue color that is attributed to photon-induced charge transfer from the sulfur atom of cysteine to the copper cation center. 

Fig. 4. View of the blue copper and a5Rupis59) centers in ruthenated Anabaena variabilis plastocyanin. The edge-edge distance is 11.9 A [39]...

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). 

The copper proteins with a type 1 active site are commonly known as blue copper proteins due to their intense blue color in the Cu11 state. They are usually participants in electron transfer processes, and the best-known representatives of this class include plastocyanin, azurin and amicyanin [1]. The copper center in the type 1 active site is surrounded by two nitrogen donor atoms from two... 

Stellacyanin, the plastocyanins, and the azurins are the most widely studied copper-containing metalloproteins of the next active-site class, the Blue Copper sites. These proteins, which generally appear to be involved in redox chemistry, have quite unique spectral features32,33). The potential for complementary interaction between inorganic spectroscopy and protein crystallography is well demonstrated by the roles that they have played in generating fairly detailed geometric and electronic structural pictures of the Blue Copper metal centers. 

The blue copper proteins azurin, plastocyanin, stellacyanin, and umecyanin incorporate Cu bound to a combination of N/thiolate/thioether ligands. An important feature of these metalloenzymes is the facile copper(II)/(I) couple that these species exhibit, which is linked to the highly strained, asymmetric coordination geometry at the metal center. The synthesis of model complexes for these so-called Type 1 copper proteins has been reviewed. ... 

Redox potentials for the different copper centers in the blue oxidases have been determined for all members of the group but in each case only for a limited number of species. The available data are summarized in Table VI 120, 121). The redox potentials for the type-1 copper of tree laccase and ascorbate oxidase are in the range of 330-400 mV and comparable to the values determined for the small blue copper proteins plastocyanin, azurin, and cucumber basic protein (for redox potentials of small blue copper proteins, see the review of Sykes 122)). The high potential for the fungal Polyporus laccase is probably due to a leucine or phenylalanine residue at the fourth coordination position, which has been observed in the amino-acid sequences of fungal laccases from other species (see Table IV and Section V.B). Two different redox potentials for the type-1 copper were observed for human ceruloplasmin 105). The 490-mV potential can be assigned to the two type-1 copper sites with methionine ligand and the 580-mV potential to the type-1 center with the isosteric leucine at this position (see Section V.B). 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). 

Another impetus for the study of the coordination chemistry of crown thioethers stems from the role of thioether binding in biological systems such as d-biotin (involving tetrahydrothiophene) (145, 208) and blue copper proteins such as plastocyanin and azurin (involving methionine) (4,13, 73,109,124,185). The binding of Cu(II) and Cu(I) centers to macrocyclic thioethers has led to a greater understanding of Cu-S(thioether) interactions and the stereochemical preferences of these metal centers (91, 95, 99,121,180,181). 

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. 

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