Copper proteins plastocyanin

Structure and electron transfer reactivity of the blue copper protein, plastocyanin. A. G. Sykes, Chem. Soc. Rev., 1985,14, 283 (117). 

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,... 

More subtle factors that might affect k will be the sites structures, their relative orientation and the nature of the intervening medium. That these are important is obvious if one examines the data for the two copper proteins plastocyanin and azurin. Despite very similar separation of the redox sites and the driving force (Table 5.12), the electron transfer rate constant within plastocyanin is very much the lesser (it may be zero). See Prob. 16. In striking contrast, small oxidants are able to attach to surface patches on plastocyanin which are more favorably disposed with respect to electron transfer to and from the Cu, which is about 14 A distant. It can be assessed that internal electron transfer rate constants are =30s for Co(phen)3+, >5 x 10 s for Ru(NH3)jimid and 3.0 x 10 s for Ru(bpy)3 , Refs. 119 and 129. In the last case the excited state Ru(bpy)3 is believed to bind about 10-12 A from the Cu center. Electron transfer occurs both from this remote site as well as by attack of Ru(bpy)j+ adjacent to the Cu site. At high protein concentration, electron transfer occurs solely through the remote pathway. 

We have studied the blue copper proteins plastocyanin, azurin, cucumber basic protein ( ) and nitrite reductase (NiR) (159,160). We shah focus on these four in the remainder of this section. 

The simpler cytochrome bc] complexes of bacteria such as E. coli,102 Paracoccus dentrificans,116 and the photosynthetic Rhodobacter capsulatus117 all appear to function in a manner similar to that of the large mitochondrial complex. The bc] complex of Bacillus subtilis oxidizes reduced menaquinone (Fig. 15-24) rather than ubiquinol.118 In chloroplasts of green plants photochemically reduced plastoquinone is oxidized by a similar complex of cytochrome b, c-type cytochrome /, and a Rieske Fe-S protein.119 120a This cytochrome b6f complex delivers electrons to the copper protein plastocyanin (Fig. 23-18). 

The electron donor to Chl+ in PSI of chloroplasts is the copper protein plastocyanin (Fig. 2-16). However, in some algae either plastocyanin or a cytochrome c can serve, depending upon the availability of copper or iron.345 Both QA and QB of PSI are phylloquinone in cyanobacteria but are plastoquinone-9 in chloroplasts. Mutant cyanobacteria, in which the pathway of phylloquinone synthesis is blocked, incorporate plasto-quinone-9 into the A-site.345a Plastoquinone has the structure shown in Fig. 15-24 with nine isoprenoid units in the side chain. Spinach chloroplasts also contain at least six other plastoquinones. Plastoquino-nes C, which are hydroxylated in side-chain positions, are widely distributed. In plastoquinones B these hydroxyl groups are acylated. Many other modifications exist including variations in the number of iso-prene units in the side chains.358 359 There are about five molecules of plastoquinone for each reaction center, and plastoquinones may serve as a kind of electron buffer between the two photosynthetic systems. 

The chain of carriers between the two photosystems includes the cytochrome b6f complex and a copper protein, plastocyanin. Like the mitochondrial and bacterial cytochrome be i complexes, the cytochrome b(J complex contains a cytochrome with two b-type hemes (cytochrome b6), an iron-sulfur protein, and a c-type cytochrome (cytochrome /). As electrons move through the complex from reduced plastoquinone to cytochrome/, plastoquinone probably executes a Q cycle similar to the cycle we presented for UQ in mitochondria and photosynthetic bacteria (see figs. 14.11 and 15.13). The cytochrome bbf complex provides electrons to plastocyanin, which transfers them to P700 in the reaction center of photosystem I. The electron carriers between P700 and NADP+ and between H20 and P680 are... 

Figure 5.1 Schematic representations of selected active sites of the copper proteins plastocyanin [56] (type 1, a) galactose oxidase [57] (type 2, b) oxy hemocyanin [58] (type 3, c) ascorbate oxidase [10] (type 4, or multicopper site, d) nitrous oxide reductase [59] (CuA site, e) cytochrome c oxidase [15]...

The EPR spectrum of the blue copper protein plastocyanin (Figure 3C) has gu > g > 2.00, and thus the copper site must have a dx2 y2 ground state. First, we are interested in determining the orientation of the dx2 y2 orbital relative to the distorted tetrahedral geometry observed in the protein crystal structure. Single crystal EPR spectroscopy allowed us to obtain this orientation and located the unique (i.e., z) direction in this distorted site (29). Plastocyanin crystallizes in an orthorhombic space group with four symmetry related molecules in the unit cell. The orientation of the plastocyanin copper sites in the unit cell are shown in... 

B(C2H5)2The tridentate ligand [HB pz-3,5-(CH3)2 2(SCgH4-4-CH3)]" was synthesized intentionally from ArSH and K[H2B pz-3,5-(CH3)2 2]- It was converted to L MSR complexes (M = Cu or Co SR = 0-ethylcysteine,p-nitrobenzenethiolate or pentafluorophenylthiolate). These compounds were studied as synthetic approximations of the proposed active sites in the blue copper proteins (plastocyanin, azurin)... 

A number of other electron carriers are present in various photosynthetic systems. These include soluble carriers such as the blue copper protein plastocyanin and auracyanin see Copper Proteins with Type 1 Sites) and soluble cytochromes and ferredoxins see Iron-Sulfur Proteins), as well as additional membrane-bound complexes. The membrane-bound multisubunit cytochrome b f complex is discussed is Section 7. 

The photosynthetic apparatus of green plants and cyanobacteria oxidizes water and transfers electrons to NADP, with a net gain in electrochemical potential of 1.13 eV (at pH 7), utilizing the energy of two light quanta per electron. The complete system is contained in the chloroplasts, and is localized within the thylakoid membranes, with the exception of the electron carrier ferredoxin, which is in solution in the stroma, and serves to transfer electrons from the reducing end of photosystem I (PS I) to a membrane-bound flavoprotein which then reduces NADP, and of the copper protein plastocyanin (PC, the electron donor to PS I), which is in solution in the internal phase of thylakoids. 

After its photooxidation, P-700 stays oxidized for more than a few microseconds. It is re-reduced by the soluble copper protein plastocyanin or, in cyanobacteria and some algae, by the soluble cytochrome c-553. The relationship between plastocyanin and P-700 has been mainly studied through kinetic analysis of the P-700 ab-... 

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... 

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. 

Fig. 20. Structure of photosynthetic electron carriers and electron-transfer proteins (A) ATP, NAD and NADP (B) quinones (C) redox-active amino acids tyrosine and histidine (D) cytochromes and f (E) iron-sulfur proteins and (F) the copper protein plastocyanin.

As indicated, above, the two Z -hemes of the Cyt b f complex provide a pair of reacting sites spanning the thylakoid membrane, one near the stromal side and the other near the lumenal side of the thylakoid membrane. The plastohydroquinone is first oxidized by the Rieske FeS to a semiquinone, which is then oxidized by cytochrome/, which then releases the electron to the copper protein plastocyanin. After loss of one electron by the plastohydroquinone, the resulting semiquinone loses an electron to the two fc-hemes in series. The Z -hemes operate in the so-called Q-cycle, similar to that in the mitochondrial or bacterial cytochrome bc complex, and provide a translocation of additional protons across the membrane into the lumenal space. Discussion of the cytochrome b(,f complex and the Q-cycle will be presented in Chapter 35. 

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-... 

Although electron transfer rates within myoglobin appear to follow an exponential dependence on distance, the derived rate expression is not directly transferable to other electron transfer proteins. A particularly striking comparison is between the c cytochromes and the copper proteins plastocyanin and azurin. Intramolecular electron transfer rates are at least 10-100 times slower in the copper proteins compared to the c cytochromes, even though the distances and driving forces for the reactions are comparable. The origin of this behavior is unclear, but it does suggest caution in the quantitative transfer of rate expressions between different systems. 

Electronic difference spectra of the Hg(II)-substituted blue copper protein plastocyanin have been interpreted in terms of an unusually low energy charge transfer from a cysteine S atom to the central Hg(II) atom (188). The Hg(II)-plastocyanin affords a unique opportunity for investigating the coordination geometry via the UV spectrum since the three-dimensional structure of the Hg(II)-protein complex is known to high resolution (46), as is the structure of the native copper protein (48, 74). In plastocyanin. 

Figure 2 Low-temperature (a) absorption and (b) MCD of the blue copper protein plastocyanin. The use of...

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