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The Cytochrome F∓Plastocyanin Complex As a Model to Study

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The Cytochrome F∓Plastocyanin Complex As a Model to Study 1 2 3 The cytochromef–plastocyanin complex as a model to study transient interactions 4 between redox proteins ⇑ 5 Q1 Isabel Cruz-Gallardo, Irene Díaz-Moreno, Antonio Díaz-Quintana, Miguel A. De la Rosa 6 Instituto de Bioquímica Vegetal y Fotosíntesis, Centro de Investigaciones Científicas, Isla de la Cartuja (cicCartuja), Universidad de Sevilla-CSIC, Avda. Américo Vespucio 49, 7 Sevilla 41092, Spain 8 2910 11 Transient complexes, with a lifetime ranging between microseconds and seconds, are essential30 for 12 biochemical reactions requiring a fast turnover. That is the case of the interactions between31 proteins 13 engaged in electron transfer reactions, which are involved in relevant physiological processes32 such 14 as respiration and photosynthesis. In the latter, the copper protein plastocyanin acts as 33a soluble 15 34 16 carrier transferring electrons between the two membrane-embedded complexes cytochromeb6f 17 and photosystem I. Here we review the combination of experimental efforts in the literature35 to 18 unveil the functional and structural features of the complex between cytochrome f and plastocya-36 19 nin, which have widely been used as a suitable model for analyzing transient redox Q2inter37actions. 20 Ó 2011 Published by Elsevier B.V. on behalf of the Federation of European Biochemical 3Societies.8 21 22 Keywords: 23 Cytochrome f 24 Electron transfer 25 Plastocyanin 26 Transient complex 27 Photosynthesis 28 39 40 41 42 1. Introduction recognition as an encounter that can be transiently stabilized54 to yield a productive complex as an outcome. 55 43 Protein–protein interactions are key processes in the properElectron transfer (ET) reactions are excellent examples of 56tran- 44 operation of living cells. Different kinds of interactions cansient be complexes. In fact, soluble proteins mediate redox exchange57 45 distinguished on the basis of protein binding affinities. Complexesbetween large membrane complexes in the photosynthetic 58and 46 characterized by low binding affinities occur in transient protein–respiratory electron transport chains via short-lived interaction59s. 47 protein interactions, which display high dissociation equilibriumFor instance, in oxygenic photosynthesis there is a soluble metal-60 48 constants ðKDÞ, within the range lM–mM,of or lifetimes in theloprotein that shuttles electrons from cytochromef (Cf), which is 61 49 order of milliseconds [1,2]. a component of the membrane-embedded cytochrome6f (C b6bf) 62 50 Transient complexes are typical of physiological processes thatcomplex, to P700, which is the special chlorophyll pair of 63Photo- 51 require a compromise between binding specificity and turnoversystem to I (PSI) [5–7]. Either plastocyanin (Pc) or cytochrome6 64 c 52 run at an appropriate rate [3]. On the basis of a two-step model(Cc6) can[4], play such a role of electron carrier betweenf and 65C 53 the formation of a final complex first entails a primary unspecificPSI. Higher plants only contain Pc, whereas most cyanobacte66 ria and green algae synthesize either Pcc 6 ordepending C on the 67 relative availability of copper and iron, their respective cofactor68 Abbreviations: BD, Brownian dynamics;b fC, cytochromeb f;Cc , cytochrome 6 6 6 metals [8,9]. 69 c6; Cf, cytochrome f; CSP, chemical-shift perturbations; ET,electron transfer; Cf is anchored to the thylakoid membrane, withinC b6thef 70 EXAFS, extended X-ray absorption fine structurek2, bimolecular; rate constant;KD, dissociation equilibrium constant; MD, molecular dynamics; NIR, nitrite reductascomplex,e; by a C-terminal transmembrane helix leaving a 7128- NMR, nuclear magnetic resonance; Pc, Plastocyanin; PCS, pseudocontact shifts;kDa pI, N-terminal soluble portion exposed to the lumen with72 a isoelectric point; PRE, paramagnetic relaxation enhancement; PSI, Photosystemclear I; two-domain structure. The large domain harbors the heme73 RMSD, root mean square deviation; WT, wild type; XANE, X-ray absorption near 74 edge structure; XAS, X-ray absorption spectrometry group, and the small domain possesses a patch of charged resi- ⇑ Corresponding author. Fax: +34 954460065. dues. Cf is considered an unusualc-type cytochrome because 75 E-mail address:[email protected] (M.A. De la Rosa). of itsb-sheet-based structure, elongated form and particular76 f 2 I. Cruz-Gallardo et al. / FEBS Letters xxx (2011) xxx–xxx 77 heme axial coordination with the N-terminus Tyr1 [10,11]. Pc,with in spinach and pea Pc [21]. Those charged residues at the122 basic 78 its turn, is an 11-kDa cupredoxin with a P-barrel structureCf patch are crucial for the in vitro interaction with Pc. 123Lysines 79 formed by eight P-strands and a small a-helix, along with58, 65a and 187 fofdirectly C interact with acidic residues at site124 80 copper centre coordinated by two histidines, one methionine2 of Pc. The electrostatic attraction between the two partners125 81 and one cysteine [12]. results in a bell-shaped reaction rate dependence on ionic strength126 82 The overall structures of bothf and C Pc are highly conserved[16] (Fig. 1, upper panel). This behavior has been explained127 by 83 from cyanobacteria to higher plants [13–15], but striking differ-assuming that the complex gets ‘locked’ in a non-produ128ctive 84 ences occur in some of their physical properties. The surfaceelectros near tatic orientation at low ionic strength. As the ionic strength129 85 the heme moiety inf is C mainly hydrophobic for all organisms.increases, the rearrangement of both proteins into the complex130 86 However, a remarkable basic ridge is found in the small domaintakes place in order to achieve a well-oriented and productive131 87 for higher plants and green algae turning to acidic in cyanobacomplex,cte- which corresponds to the maximum of the bell. Further132 88 ria. On the other hand, Pc relies on two functional sites: aincrease hydro- in ionic strength leads to complex dissociation [6]. 133It has 89 phobic patch surrounding the solvent-accessible histidine copperrecently been proposed that the expression ‘locked complex’134 90 ligand, or the so-called ‘‘site 1’’, and an electrostatically chargedshould be avoided as such a non-productive state at low135 ionic 91 surface area, the so-called ‘‘site 2’’, whose nature varies fromstrength one could rather correspond to a set of non-productive 136orien- 92 organism to another. Actually, site 2 is mainly acidic in tationsplants slightly different in energy [4]. 137 93 and green algae, whereas it ranges from acidic to basic in cyano-In vitro kinetic analyses of the interaction betweenf and PcC in 138 94 bacteria. This feature is a key for the dynamics of the complexcyanobacteria have mainly been addressed in the mesophi139lic 95 and is thus herein reviewed. Nostoc [22,24] and thermophiliPhormidiumc [23,25,26] species. 140 96 The Cf–Pc complex has extensively been studied in the Thelast electron transfer rate constant between theNostoc twoproteins 141 97 years as a model to understand the nature of protein–protein monotointer- nically decreases with increasing ionic strength (Fig. 1421, 98 actions in ET chains. The main goal of this article is tomiddle briefly panel). The charge mutations at site 2 reduce the 143ability 99 review not only our current understanding of the mechanismof Pcof to oxidizef [22 C], whereas the mutations at the hydrophobic144 100 ET in transient complexes [16], but also the different techniquespatch decrease its reactivity towards the heme protein. It is 145worth 101 used to analyze the structural features of such complexes [17]to mentionin that the charge mutation of Arg93 – a residue146 of Pc 102 an ample set of prokaryotic and eukaryotic organisms. located at the interface between sites 1 and 2 – drastically 147dimin- ishes the reaction rate, so revealing the crucial role of such148 amino 103 2. Kinetics within the–Pc C f complex acid in ET within the complex (Table 1). In contrast, the 149charge replacements at the small domain f hardlyof C affect the interac- 150 104 Kinetic analyses provide information about the ET reactiontion with Pc [24] (Table 1), thereby suggesting that the specificity151 105 mechanism, namely the limiting step and the nature of the partnerin the f–PcC interaction is mostly determined by the cupredoxin.152 106 interactions. Some years ago, our group proposed three differentThis has been corroborated by NMR studies of thef –Pcmixed 153C 107 kinetic mechanisms to analyze the well-related interactions complexesof betweenNostoc and Phormidium cyanobacterial pro- 154 155 108 PSI with Pc andc 6 fromC a wide range of photosynthetic organ-teins [27]. 109 isms [18–20]. These kinetic models can also be applied to the redoxThe ET rate constant between the wild-type (WT) forms 156of the 157 110 interactions between f Cand the two soluble carriers Pc c6and twoC Phormidium proteins slightly depends on ionic strength 111 [21–23]. Some of the currently available experimental data (Fig.for 1, lower panel), but site-directed mutagenesis of certain158 112 the interaction between f andC Pc are summarized in Table charged1, residues of both proteins revealed a clear influence 159over 113 where the values for the following constants are presented: bimo-the reaction rate (Table 1) [23,25]. Actually, a deeper analysis160 of 161 114 lecular rate constant for complex formationðk2Þ, equilibrium con- the Cf–Pc complex by continuum electrostatics shows that net 162 115 stant for association between partnersðKAÞ, and effective electron coulombic forces between protein charges are slightly repulsive 0 [28]. Assuming a diffusion
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