Plastocyanin redox potential

Electron Transport Between Photosystem I and Photosystem II Inhibitors. The interaction between PSI and PSII reaction centers (Fig. 1) depends on the thermodynamically favored transfer of electrons from low redox potential carriers to carriers of higher redox potential. This process serves to communicate reducing equivalents between the two photosystem complexes. Photosynthetic and respiratory membranes of both eukaryotes and prokaryotes contain stmctures that serve to oxidize low potential quinols while reducing high potential metaHoproteins (40). In plant thylakoid membranes, this complex is usually referred to as the cytochrome b /f complex, or plastoquinolplastocyanin oxidoreductase, which oxidizes plastoquinol reduced in PSII and reduces plastocyanin oxidized in PSI (25,41). Some diphenyl ethers, eg, 2,4-dinitrophenyl 2 -iodo-3 -methyl-4 -nitro-6 -isopropylphenyl ether [69311-70-2] (DNP-INT), and the quinone analogues,... 

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. 

Fe "-OOH (ES) complex, 43 95-97 heme-bound CO, 43 115 lock-and-key model, 43 106-107 mutation in proximal heme cavity, 43 98 residue location, 43 101-102 van der Waals surfaces, 43 112-113 Velcro model, 43 107 zinc-substituted, 43 110-111 plastocyanin, cross-linked, cyclic voltammogram, 36 357-358 promoters, 36 345-346 protein-electrode complex, 36 345, 347 redox potential, 36 349 self-exchange rate constants, 36 402 stability at electrode/electrolyle interface, 36 349-350... 

Visible MCD spectra of plastocyanin, azurin, Rhus vernicifera laccase, ascorbate oxidase and ceruloplasmin are similar on a per copper basis, but show differences from those of stellacyanin and fungal laccase. This is of interest in view of the absence of methionine from the coordination sphere of copper in stellacyanin, and the very high redox potential of fungal laccase.925... 

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). 

In 1965 Hill elaborated the two-photosystem scheme further as shown in Fig. 15 (B). In this Z-shaped scheme, two groups of chloroplast components with known redox potentials were placed at the bends of the Z Cyt/, plastocyanin and P700, close to -1-0.4 V, and plastoquinone and Cyt b( close to 0 V. Ferredoxin, with a potential of-0.43 V, is close to the midpoint potential ofhydrogen electrode. For oxygen production, the midpoint potential of the unknown component must exceed that of the oxygen electrode. Over the past thirty years, a variety of Z-schemes have been published in the literature to illustrate the electron-transfer processes in green-plant photosynthesis, but their basic features have not deviated from that shown in Fig. 15 (B). For instance, we show a currently accepted, concise Z-scheme in Fig. 15 (C) it includes many more individual components than were originally envisioned, plus a representation of the operation of the so-called Q-cycle in the Cyi-b(,f complex. 

The oxidized P700 is reduced by the copper protein, plastocyanin, present in the lumenal space ofthe thylakoid. Reflecting the overall reaction it supports, the PS-1 reaction center is sometimes called plastocyanin ferredoxin oxidoreductase. Note that in cyanobacteria the corresponding electron donor is cytochrome c552 rather than plastocyanin and some algal species when in a copper-deficient medium synthesize a c-type cytochrome as a replacement for plastocyanin. The complete electron-transport chain ofphotosystem 1 is shown in Fig. 2. The approximate redox potentials and halftimes for forward electron transfers at ambient temperature are also indicated. Perhaps to assure wasteful back reactions are mini-... 

Reduction of plastocyanin by cytochrome / would presumably involve some direct interaction between them. Cyt/will be described in detail in Chapter 35, but some relevant information on the composition and structure of Cyt / is given here to provide a perspective on the PC/Cyt / interaction. Cyt /, a subunit of the Cyt complex, is a c-type cytochrome of 33 kDa molecular mass with an in situ redox potential of -370 mV. The amino-acid composition has been determined for Cyt/from several species, that for turnip being shown in Fig. 11. Turnip Cyt/, containing 252 amino-acid residues, was crystallized and its three-dimensional structure determined in 1994 by Martinez, Huang, Szczepaniak, Cramer and Smith " at 2.3 A resolution, now refined to 1.96 A (see Chapter 35). 

Until now, pseudoazurin has only been found in bacteria, e.g., in the denitrifying bacteria Alcaligenes faecalis and Achromobacter cycloclastes. It is the electron donor of the green copper-protein nitrite reductase which catalyses the reduction of nitrite (NO2) to nitrogen monoxide (NO) [86-89]. The physiological electron donor of pseudoazurin is as yet unknown [70]. Pseudoazurin has a molecular mass of approximately 13.5 kD and a chain length of about 123 amino acid residues [88]. The additional amino acids, as compared with plastocyanin, form the C-terminal end of the protein (Fig. 14). The pseudoazurins have redox potentials of about 230 mV [90,91]. 

In all three proteins, the type 1 copper is coordinated in a distorted tetrahedron in which the Cu2+ ion is situated 0.35 (Pcy), 0.43 (Paz), and 0.40 A (Acy) above the plane formed by both histidines and cysteine, towards the methionine residue [20]. In azurin, the distance between the methionine ligand and copper ion is markedly larger than for the members of the plastocyanin family, resulting in a more trigonal pyramidal conformation [20,21], a ligand stereochemistry which also occurs in halocyanin [18,94], Other factors influence the redox potentials as well, e.g., the hydrophobicity of the region surrounding the copper center. However, so little is known about the collaboration between the various factors that it is currently not possible to predict accurately the probable redox potentials of type 1 copper proteins. 

FIGURE 9.1 Plant-type ferredoxin (Fd) isoforms with diverse redox potential could donate or receive electrons involved in many metabolic processes (modified from the model of chlo-roplast described in Tognetti et al. 2006 and Hanke et al. 2004). PSI and PSII, photosystem I and II, respectively FNR, Fd NADP reductase FTR, Fd thioredoxin reductase Trx, thiore-doxin PGR5, proton gradient regulation protein 2-Cys Prx, 2-Cys peroxiredoxin PQ, plas-toquinone PC, plastocyanin. 

The redox potentials of a number of plastocyanins are given in Table 1, but only spinach plastocyanin has been studied at more than one pH. Katoh et al. 33) report that above pH 5.4 the redox potential is constant at 370 mV while below pH 5.4 the potential increases approximately 60 mV for each unit of pH. This is formally rationalized by the equilibrium... 

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