Plastocyanin electron-transfer reactions

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... 

A macromolecular complex that allows plants to harvest the sun s photic energy by absorbing photons and using their energy to catalyze photooxidation of plastocyanin, the copper protein situated in the lumen of thylakoid membranes, which undergoes subsequent electron transfer reactions. These reactions are illustrated in Fig. 1. 

Computational Simulation and Analysis of Dynamic Association Between Plastocyanin and Cytochrome F. Consequences for the Electron-Transfer Reaction. 

Ivkovic-Jensen MM, Kostic NM. Effects of viscosity and temperature on the kinetics of the electron-transfer reaction between the triplet state of zinc cytochrome c and cupri-plastocyanin. Biochemistry 1997 36 8135-44. 

Harris MR, Davis DJ, Durham B, Millett F. Temperature and viscosity dependence of the electron-transfer reaction between plastocyanin and cytochrome c labeled with a ruthenium(II) bipyridine complex. Biochim Biophys Acta 1997 1319 147-54. 

The first observation was reported in the electron transfer reaction between spinach plastocyanin with optically active iron(II) complexes, [Fe(S,S)-alamp] (A-form) and its (R,R)-isomer (A-form), where alamp is 2,6-bis[3-(S)- or 3-(R)-carboxy-2-azabutylpyridine (see Scheme 25) [56]. 

In this complex, there are two optically active sites. Spinach plastocyanin is a type I copper protein, in which two reactive sites have been identified on its surface, at least. The electron transfer reaction occurs with significantly large stereoselectivity the ratio of the observed reaction rate constant (k /k ) is 1.6 to 2.0. The difference in the activation enthalpy, AAH a, is 3.0 kJ mol-1, and the difference in the activation entropy, AS (a-a) is 15 J mol-1 K-1. This means that the stereoselectivity arises from the entropy term. 

The stereoselective reduction of spinach plastocyanin with several cobalt cage complexes (Scheme 26) has been reported, too [60]. These cage complexes are very useful for investigation of outer-sphere electron transfer reactions because of their inertness to hydrolysis and to loss of ligands in the redox reaction. 

As is indicated in Table 5-3, P680, P70o> the cytochromes, plastocyanin, and ferredoxin accept or donate only one electron per molecule. These electrons interact with NADP+ and the plastoquinones, both of which transfer two electrons at a time. The two electrons that reduce plastoquinone come sequentially from the same Photosystem II these two electrons can reduce the two >-hemes in the Cyt b(f complex, or a >-heme and the Rieske Fe-S protein, before sequentially going to the /-heme. The enzyme ferre-doxin-NADP+ oxidoreductase matches the one-electron chemistry of ferredoxin to the two-electron chemistry of NADP. Both the pyridine nucleotides and the plastoquinones are considerably more numerous than are other molecules involved with photosynthetic electron flow (Table 5-3), which has important implications for the electron transfer reactions. Moreover, NADP+ is soluble in aqueous solutions and so can diffuse to the ferredoxin-NADP+ oxidoreductase, where two electrons are transferred to it to yield NADPH (besides NADP+ and NADPH, ferredoxin and plastocyanin are also soluble in aqueous solutions). 

Quenching of excited-state [Ru(bipy)3] by reduced blue proteins involves electron transfer from the Cu with rate constants close to the diffusion limit for electron-transfer reactions in aqueous solution. It is suggested that the excited Ru complex binds close to the copper-histidine centre, and that outer-sphere electron transfer occurs from Cu through the imidazole groups to Ru. Estimated electron-transfer distances are about 3.3 A for plastocyanin and 3.8 A for azurin, suggesting that the hydrophobic bipy ligands of Ru " penetrate the residues that isolate the Cu-His unit from the solvent. ... 

S Niwa, H Ishikawa, S Nakai and T Takabe (1980) Electron transfer reactions between cytochrome fand plastocyanin from Brassica komatsuna. J Biochem 88 1177-1183... 

S He, S Modi, DS Bendall and JC Gray (1991) The surface exposed residue tyrosine Tyr83 of pea plastocyanin is involved in both binding and electron transfer reactions with cytochrome f EMBO J 10 4011-4016... 

The X-ray crystal structure of plastocyanin has recently been established (10), which indicated that the core of the molecule is hydrophobic and notably aromatic because six of the seven phenylalanine residues are clustered there. Polar side chains are distributed on the exterior of plastocyanin molecule. Many hypotheses have been proposed to explain the electron-transfer pathway to and from the metal center of plastocyanin, such as a tunnelling mechanism along hydrophobic channels (11). High reactivity and entropic favorability have been reported for the electron-transfer reaction of plastocyanin with Fe(II) complex (12). The Cu complex bound to the amphiphilic block copolymer is interesting as a metal compound of plastocyanin, because both polymer and apoprotein environments are considered to produce a hydrophobic environment and a large effect on the electron-transfer reaction through its entropic contribution. 

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. 

The kinetics of electron transfer reactions between spinach plastocyanin and [Fe(CN)6] ", [Co(phen)3] , and Fe(II) cytochrome c have been studied as a function of ionic strength. Applications of the equations of Van Leeuwen support the proposal of two sites of electron transfer, with [Co(phen)3] binding near residues 42-45 and the interaction of [Fe(CN)6] at a hydrophobic region near the copper ion. Pulse radiolysis has been employed to measure the rates of electron transfer from Ru(II) to Cu(II) in plastocyanins from Anabaena variabilis and Scenedesmus obliquus which have been modified at His-59 by [Ru(NH3)5] . The small intramolecular rates (<0.082 and <0.26 s , respectively) over a donor-acceptor distance of 12 A indicate that electron transfer from the His-59 site to the Cu center is not a preferred pathway. A more favorable route, via the acidic (residues 42-44) patch ( 14 A to Cu), is supported by the rate of >5 x 10 s for the reduction of PCu(II) by unattached [Ru(NH3)5im] . The intramolecular electron transfer from Fe(II) in horse cytochrome c to Cu(II) in French bean plastocyanin ( 12 A from heme edge to Cys-84 S), in a carbodiimide cross-linked covalent complex, proceeds with a rate of 1.05 x 10 s . The presence of the... 

The cation is, of course, susceptible to reduction reactions. It will be reduced, for instance, by metalloproteins, such as cytochrome C or plastocyanin. Particularly significant is its reduction by NADH. As a coenzyme for dehydrogenases, NADH plays a vital role in the control of biological redox systems. Oxidized by L, it becomes transformed into the radical cation NADH" -, which in turn, in the presence of base, converts to the neutral free radical NAD- and, further, to the NAD cation, with 1 involved as an electron acceptor in the last-named reaction step and 1 regenerated. This sequence of events, illustrated in Scheme 3, demonstrates the capability of oxidized ferrocenes to interfere rather drastically in enzymatically controlled electron transfer reactions. 

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

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]. 

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