Plastocyanin, electron-transfer

Electron transfer plastocyanin and azurin redox transport... 

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

F.A. Armstrong, A.M. Bond, H.A.O. Hill, B.N. Oliver, and I.S.M. Psalti, Electrochemistry of cytochrome c, plastocyanin, and ferredoxin at edge- and basal-plane graphite electrodes interpreted via a model based on electron transfer at electroactive sites of microscopic dimensions in size. J. Am. Chem. Soc. 111,91859189 (1989). 

Metalloproteins fall into three main structure categories depending on whether the active site consists of a single coordinated metal atom, a metal-porphyrin unit, or metal atoms in a cluster arrangement. In the context of electron-transfer metalloproteins, the blue Cu proteins, cytochromes, and ferre-doxins respectively are examples of these different structure types. Attention will be confined here mainly to a discussion of the reactivity of the blue Cu protein plastocyanin. Reactions of cytochrome c are also considered, with brief mention of the [2Fe-2S] ferredoxin, and high potential Fe/S protein [HIPIP]. 

It is timely to review the reactivity of plastocyanin in the light of recent aqueous solution studies, and the elegant structural work of Freeman and colleagues on both the PCu(I) and PCu(II) forms (1 2) Plastocyanin now ranks alongside cytochrome c (3) as the electron-transfer metalloprotein for which there is most structural information. 

Rate Constants and Reactivity. Electron-transfer reactions of plastocyanin (and other metalloproteins) are so efficient that only a narrow range of redox partners (having small driving force) can be employed. Rates are invariably in the stopped-flow range, Table I. Unless otherwise stated parsley plastocyanin... 

N. Sutin, Brookhaven National Laboratory The 16% decrease in the rate of oxidation of reduced plastocyanin by Co(phen)33 resulting from the attachment of the chromium(III) label seems rather small if the chromium is indeed bound at or near the protein site used for electron transfer to Co(phen)33. ... 

PS1 The PS 1-prep, introduced in this communication is the first reported with a polyhistidine tag fused to the N-terminus of the PsaF subunit. This construct was possible due to the fact that cyanobacterial PsaF-deletion mutants show no impact on photoautotrophic growth - in contrast to Chlamydomonas reinhardtii, where inactivation of PsaF results in a severe reduction of electron transfer from plastocyanin to PS 1 [Hippier et al. 1997], Also, the N-terminus of the F-subunit which was decorated by the tag is located towards the lumen side which enables an attachment of the isolated PS1 with the lumen-exposed /donor-side to the electrode surface in our hydrogen-producing device. 

Hippier, M., F. Drepper, J. Farah and J. D. Rochaix (1997) Fast electron transfer from cytochrome c6 and plastocyanin to photosystem I of Chlamydomonas reinhardtii requires PsaF. Biochemistry, 36 6343-6349... 

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. 

For the cytochrome c-plastocyanin complex, the kinetic effects of cross-linking are much more drastic while the rate of the intracomplex transfer is equal to 1000 s in the noncovalent complex where the iron-to-copper distance is expected to be about 18 A, it is estimated to be lower than 0.2 s in the corresponding covalent complex [155]. This result is all the more remarkable in that the spectroscopic and thermodynamic properties of the two redox centers appear weakly affected by the cross-linking process, and suggests that an essential segment of the electron transfer path has been lost in the covalent complex. Another system in which such conformational effects could be studied is the physiological complex between tetraheme cytochrome and ferredoxin I from Desulfovibrio desulfuricans Norway the spectral and redox properties of the hemes and of the iron-sulfur cluster are found essentially identical in the covalent and noncovalent complexes and an intracomplex transfer, whose rate has not yet been measured, takes place in the covalent species [156]. 

There have been many studies on the reactivity of plastocyanin. Two sites (or regions) have been identified on the surface of the molecule as relevant to electron transfer. One is the adjacent site at or near His87, and the other a more... 

While there is at present no full understanding as to why plastocyanin should require two sites for reaction, there is now much evidence detailing this two-site reactivity. Moreover, the recent X-ray crystal structure of ascorbate oxidase (which has 4 Cu atoms per molecule) has indicated a plastocyanin-like domain, with the two type 3 Cu s (in close proximity with the type 2 Cu) located at the remote site. Fig. 2 [5]. Since electrons are transferred, from the type 1 Cu to O2 bound at the type 3 center this structure defines two very similar through-bond routes for biological electron transfer. 

Earlier suggestions that the two uncoordinated and invariant residues His35 (inaccessible to solvent and covered by polypeptide) and His83 (remote and 13 A from Cu) are, from effects of [H ] on rate constants (and related pKg values), sites for electron transfer may require some re-examination. Thus, it has been demonstrated in plastocyanin studies [50] that a surface protonation can influence the reduction potential at the active site, in which case its effect is transmitted to all reaction sites. In other words, an effect of protonation on rate constants need not necessarily imply that the reaction occurs at the site of protonation. His35 is thought to be involved in pH-dependent transitions between active and inactive forms of reduced azurin [53]. The proximity of... 

Table 7. Partitioning of electron transfer between adjacent (k ) and remote (ke) binding sites on spinach plastocyanin PCu(I) at 25°C, pH7.5, I=0.10M(NaCl), using redox inactive [(NH3)5CoNHjCo(NH3)5]5+ [100, 117]...

Binding at the remote site has also been detected in studies on the quenching of the excited states [Cr(phen)3] and [Ru(bipy)3] by French bean plastocyanin [103]. The model adopted allows for electron transfer from the remote and adjacent sites, where at low protein concentrations the adjacent pathway is 10 times faster. At the higher concentrations of protein, up to 4 X 10 M, an interesting feature is the evidence for an adduct in which two PCu(I) molecules are associated with one inorganic complex. The oxidant is believed to be sandwiched between two PCu(I) s. 

Whatever the explanation, the sensitivity of the remote pK to oxidation state of the Cu is of potential importance in relation to the functional role of plastocyanin. Plastocyanin and its physiological electron transport partner cytochrome f are believed to have complementary surfaces which lead to efficient interaction prior to electron transfer. As will be seen below there is substantial evidence for cytochrome f(II) (as reductant) reacting at the remote site of PCu(II). One problem which may be anticipated here is how dissociation of the product... 

The recent X-ray crystal structure of ascorbate oxidase [6] has indicated the relative positions of type 1, 2 and 3 Cu centers. The type 1 center is in a plastocyanin like domain, and is the primary acceptor of electrons from substrate. The shortest pathway for electron transfer from the type 1 to type 3 Cu s is the bifurcated path via Cys508 and either His 507 or His509. The two histidines are part of the plastocyanin-like domain, and serve also to coordinate the type 3 Cu s, Fig. 2. The His507 to Cys508 bonding is similar to that of Tyr83... 

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