Review TRENDS in Biochemical Sciences Vol.27 …gunner/Pages-422/PDF/bioenergetics/photos… · types of reaction centre ... Reaction centres:the structure and evolution of biological

TRENDS in Biochemical Sciences Vol.27 No.2 February 2002

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79Review

Paul K. Fyfe

Michael R. Jones*

Dept of Biochemistry,School of MedicalSciences, University ofBristol, University Walk,Bristol, UK BS8 1TD.*e-mail: [email protected]

Peter Heathcote

School of BiologicalSciences, Queen Mary,University of London,Mile End Road, London,UK E1 4NS.

Light is used as a source of energy by eukaryotes,prokaryotes and archaea. Photosynthetic archaeasuch as Halobacterium harness light energy througha protein called bacteriorhodopsin, which operates asa light-driven proton pump [1]. In eukaryotes andprokaryotes, pigment–protein complexes use lightenergy to drive a series of electron transfer reactionsthat are coupled to the translocation of protons acrossa charge-impermeable membrane. The protonelectrochemical gradient established by this processis used to drive the synthesis of ATP and otherenergy-dependent processes.

In the photosynthetic apparatus of prokaryotesand eukaryotes, antenna proteins are responsible forharvesting light energy. These complexes consist of amixture of proteins and pigments such aschlorophylls, carotenoids and bilins, and come in avariety of forms that are either embedded in themembrane or attached to its surface. Light isabsorbed by the pigments and the energy is initiallystored in the form of an excited electronic state. Theantenna pigments then transfer this excitationenergy to specialist chlorophyll cofactors in a secondtype of photosynthetic complex called the reactioncentre, where a photochemical reaction traps theenergy. Reaction centres are integral to thephotosynthetic membrane and are classifiedaccording to the identity of the terminal electronacceptors [2,3]. In one group, these are four-iron–four-sulfur clusters (Fe4S4) (Fe–S type or Type I), whereasthe other group has (bacterio)pheophytins andquinones (pheophytin–quinone type or Type II).

In an early (mid-1980s) breakthrough inphotosynthesis research, the structures of the Type IIreaction centres from two species of purplephotosynthetic bacteria, Rhodopseudomonas viridisand Rhodobacter sphaeroides, were determined byX-ray crystallography [4,5] (Fig. 1). These complexeshave since been subjected to a detailed structural andfunctional analysis [6,7], and they exhibit several

design features that appear to be common to allreaction centres. In particular, the cofactorsresponsible for catalysing light-driven transmembraneelectron transfer are arranged in two membrane-spanning branches around an axis of twofoldsymmetry (vertical line in Fig. 1c) that is orientedperpendicular to the plane of the membrane. Thesecofactors are bound at the extensive interface betweenthe L and M polypeptides, the five transmembrane αhelices of which are also arranged around the sameaxis of twofold symmetry (Fig. 1b). Our understandingof the details of this complex has guided much of thethinking on the structure and mechanism of othertypes of reaction centre [8–10] (Box 1).

Photosystem II reaction centre

A combination of spectroscopy, biochemistry andbioinformatics has shown that the Type II reactioncentre, located at the core of the photosystem II (PSII)complex of plant, algal and cyanobacterial thylakoidmembranes, has many features in common with the purple bacterial reaction centre. Thisconclusion has received support from electron andX-ray diffraction studies of the PSII complex [11–14],including most recently an X-ray structure for thePSII complex from the thermophilic cyanobacteriumSynechococcus elongatus, at a resolution of 3.8 Å[14,15] (Fig. 2a,b shows the organization of amonomer of the PSII antenna–reaction-centrecomplex; Fig. 2c,d focuses on some of the keystructural components of the actual reaction centre).

Although these new data are of relatively modestresolution, they confirm earlier proposals [11] thatthe D1 and D2 polypeptides of the PSII reactioncentre have a similar arrangement to the L andM polypeptides in the bacterial complex, with aheterodimeric arrangement of two sets of fivemembrane-spanning α helices arranged around anaxis of macroscopic twofold symmetry (Fig. 2c). Theredox cofactors of the PSII reaction centre resemblethose of the bacterial complex, with fourchlorophyll a, two pheophytin a and twoplastoquinone molecules arranged in two membrane-spanning branches (Fig. 2d). However, an importantdifference is that both the D1 and D2 polypeptidesalso bind an additional chlorophyll α (Fig. 2b,asterisks) that might act as a conduit for energytransfer from the chlorophylls of the PSII antenna tothose of the reaction centre. Flanking the reactioncentre complex are two light-harvesting antenna

Reaction centres: the structure and

evolution of biological solar power

Peter Heathcote, Paul K. Fyfe and Michael R. Jones

Reaction centres are complexes of pigment and protein that convert the

electromagnetic energy of sunlight into chemical potential energy. They are

found in plants, algae and a variety of bacterial species, and vary greatly in their

composition and complexity. New structural information has highlighted

features that are common to the different types of reaction centre and has

provided insights into some of the key differences between reaction centres

from different sources. New ideas have also emerged on how contemporary

reaction centres might have evolved and on the possible origin of the first

chlorophyll–protein complexes to harness the power of sunlight.

proteins, CP43 and CP47 (Fig. 2a, yellow and red).Each consists of a bundle of six membrane-spanningα helices that are associated with 12 or 14 moleculesof chlorophyll α, respectively [11,14]. Although purplebacteria contain light-harvesting proteins, they donot possess direct equivalents of the CP43 or CP47proteins.

On the acceptor side, the PSII reaction centreoperates in a similar way to its purple bacterialcounterpart (Box 1), with transmembrane electrontransfer from the primary donor species to the QAquinone being followed by the formation of quinol atthe QB site (Figs 1c,2d). Two of the most markeddifferences between the reaction centres of purplebacteria and PSII concern the redox chemistry on thedonor side of the complex. The first is the identity ofthe species that re-reduces the photo-oxidizedprimary electron donor, which is termed P680 in thePSII reaction centre. In purple bacteria, thephoto-oxidized P870+ is re-reduced by an electrondonated by a cytochrome (Box 1). However, in thePSII reaction centre, the photo-oxidized P680+ speciesis re-reduced by an electron derived from water in areaction that liberates molecular oxygen as aby-product. Water oxidation involves a cluster of fourmanganese atoms (Fig. 2d) in a region known as theoxygen-evolving complex (or the water-oxidizingcomplex) that is unique to the PSII reaction centre.The structural and mechanistic bases for wateroxidation have been examined through a combinationof spectroscopy, mutagenesis and studies of modelcompounds, and have been a topic of intense interestfor many years given that one of the products of water

oxidation is atmospheric oxygen (for example seeRef. [16]). The crystallographic data on themanganese cluster are a particularly exciting aspectof this new structural information on PSII [17]because much of the discussion about the mechanismof water oxidation and oxygen production is focusedaround the structural arrangement of the manganesecluster, and its interactions with the protein residuesand ions such as Ca2+ and Cl− that make up itsimmediate environment [16].

The second major difference between the reactioncentres from purple bacteria and PSII lies in the veryhigh redox potential (~ +1.1 V) of the primary electrondonor in PSII (Box 1). How this extreme of redoxpotential is achieved is not fully clear, and there isdebate over the identity of the P680 species in thePSII reaction centre. An intriguing feature of thestructural model of PSII in this regard is the spacingof the two chlorophylls located closest to the lumenalside of the membrane. In the purple bacterial reactioncentre, the P870 bacteriochlorophylls are sufficientlyclose together that they behave as a dimer (Fig. 1c).However, in the structural information on the PSIIreaction centre [11,14], the two chlorophylls closest tothe lumenal side of the membrane are further apartthan their counterparts in the bacterial complex,consistent with the generally accepted view that P680is not a chlorophyll dimer [18] (Box 1).

Type I reaction centres

Photogeneration of reducing powerIn oxygenic photosynthetic organisms – plants, algaeand cyanobacteria – the PSII reaction centres operatein tandem with a Type I reaction centre termedphotosystem I (PSI). The core PsaA and PsaBpolypeptides of these Type I reaction centres areconsiderably larger than their counterparts in Type IIcomplexes and accommodate ~90 light-harvestingchlorophyll molecules in addition to six chlorophyllsthat are associated with the electron transfer chain.The electron transfer chain in Type I reaction centresis more extensive than in Type II centres, with threebound Fe–S clusters being present, in addition to thesix chlorin and two quinone cofactors that arecommon to all known reaction centres (for examplesee Ref. [19]).


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80 Review

(a) (b) (c)

L-polypeptide M-polypeptide Activebranch

Inactivebranch

QA Fe

P870

HA

BA

QB

HB

BBCrt

Ti BS

Fig. 1. Structure of the Rhodobacter sphaeroides reaction centre. (a) The complex consists of threepolypeptides and ten cofactors [4]. The L and M polypeptides each have five transmembrane α helices(maroon and green, respectively). The H polypeptide (purple) caps the cytoplasmic faces of the L andM polypeptides, and has a single transmembrane α helix. The cofactors are shown in yellow in stickformat, and the approximate position of the membrane is indicated by the cyan boxes. (b) Thesymmetrically arranged transmembrane α helices of the L and M polypeptides encase the reactioncentre cofactors. (c) The cofactors are composed of four molecules of bacteriochlorophyll a (termedBA and BB and the P870 dimer), two molecules of bacteriopheophytin a (HA and HB), two molecules ofubiquinone 10 (QA and QB), a single photoprotective carotenoid (Crt) and a non-haem iron atom (Fe).Bacteriopheophytin is a bacteriochlorophyll in which the central magnesium atom (green circle) isreplaced by two protons. The bacteriochlorophyll, bacteriopheophytin and ubiquinone cofactors havehad their hydrocarbon sidechains removed for clarity. The non-haem iron atom is located on thesymmetry axis at the interface between the L and M polypeptides, and plays a structural role. Figurewas prepared using Molscript [47] and Raster3D [48].



81Review

At the heart of the reaction centres illustrated in this article aretwo symmetrically arranged branches of cofactors that span themembrane. In the Type II reaction centres, each branch consists oftwo molecules of (bacterio)chlorophyll, one (bacterio)pheophytinand one quinone (Figs 1c,2d), but only one of the branches isactive in catalysing transmembrane electron transfer. In theType I reaction centres, the electron transfer chain is extendedthrough the addition of an Fe–S cluster (FX), which is located onthe symmetry axis in a position to accept electrons from eitherquinone, and two further Fe–S clusters (FA and FB) (Fig. 3d).Recent evidence suggests that both cofactor branchesmight catalyse transmembrane electron transfer in the Type I complexes.

The detailed mechanism by which light energy is used in aproductive manner is best understood in purple bacteria. In theRhodobacter sphaeroides reaction centre, the cofactor branchesconverge at the periplasmic end, such that the terminalbacteriochlorophylls form a dimer that straddles the symmetryaxis. This dimer or ‘special pair’ is the primary electron donorand is termed P870 (because its lowest energy absorbance bandhas a maximum at ~870 nm). Photochemistry is triggered byformation of the first singlet excited state of the P870 pair(termed P870*). This can occur by direct absorption of a photonby the P870 bacteriochlorophylls in isolated reaction centres.However, in the intact system, light energy is mainly absorbedby the numerous antenna bacteriochlorophylls and carotenoidsthat reside in the light-harvesting pigment–protein complexessurrounding the reaction centre in the membrane. Followinglight absorption, excitation energy migrates through thepigments of the light-harvesting complexes by resonanceenergy transfer, on a timescale of tens of picoseconds, beforefinally arriving at the P870 bacteriochlorophylls in the reactioncentre.

Following acquisition of excitation energy, the P870* statedrives the reduction of, sequentially, the BA monomericbacteriochlorophyll, the HA bacteriopheophytin and the QA

ubiquinone, along the so-called ‘active’ cofactor branch [a](Fig. 1c). This process, termed light-driven transmembraneelectron transfer, occurs in ~200 picoseconds at roomtemperature and forms the radical pair state P870+QA

− (via theintermediate states P870+BA

− and P870+HA−). Following

transmembrane electron transfer, the P870+ cation oxidizes amolecule of cytochrome c2 that docks to the periplasmic face ofthe reaction centre, and QA

− reduces the QB ubiquinone, bothreactions occurring on a microsecond timescale. A secondlight-driven electron transfer along the active cofactor branchcauses double reduction of the QB ubiquinone, with two protonsbeing taken up from the cytoplasmic side of the membrane toform ubiquinol. The products of reaction centre action, oxidizedcytochrome c2 and ubiquinol, are used as oxidant and reductant,respectively, by a second integral membrane protein, thecytochrome bc1 complex. Because the reduction of ubiquinoneand the oxidation of ubiquinol occur on opposite sides of themembrane, the light-driven cycle of electron transfer reactionscatalysed by this system is coupled to the translocation ofprotons across the membrane and the establishment of aproton electrochemical gradient.

Transmembrane electron transfer is also a stronglyasymmetric process in the photosystem II (PSII) reaction centre(Fig. 2d). On the ‘acceptor side’, the mechanism of light-drivenelectron transfer and quinone reduction is thought to be similar tothat of the bacterial reaction centre, except that the P680* excitedstate that drives electron transfer is probably a multimer ofchlorophyll a rather than a dimer [b]. In addition, whereas theredox potential of the P870–P870+ couple in the purple bacterialcomplex is of the order of +400 mV to +500 mV, dependent onspecies, the P680–P680+ couple in the PSII reaction centre has asufficiently high redox potential (~ +1100 mV) to be able to oxidizewater, liberating molecular oxygen as a by-product. The detailedmechanism of water oxidation and oxygen production is a subjectof considerable interest. In very simplistic terms, this involves theremoval of four electrons from two molecules of water, a processthat requires four photons of light and involves a redox-activetyrosine residue (YZ) that might play a role in hydrogenabstraction during oxidation of water bound to the manganesecluster [c,d].

In the photosystem I (PSI) reaction centre, thetransmembrane electron transfer is more rapid than that inType II reaction centres, with electrons from the primaryelectron donor (P700) reaching the phylloquinone electronacceptor (A1) in ~20–30 picoseconds (Fig. 3d). This very rapidelectron transfer is necessary because a feature of PSI is thatexcitation energy can move back from the chlorophylls of theelectron transfer chain to those of the antenna, because of theirclose proximity in the same protein scaffold. Another particularfeature of Type I reaction centres is that they function at morereducing redox potentials than Type II complexes, because theyneed to generate electrons with a sufficiently low redoxpotential to reduce ferredoxin and/or NADP+. New structuraldata on the PSI reaction centre reveal that the axial ligand toboth chlorophylls that are candidates for the A0 primary electronacceptor is provided by the sulfur of a methionine sidechain [e].The lack of a strong fifth ligand to the central magnesium of thischlorophyll might, together with the hydrogen bonding of atyrosine to the keto oxygen of ring V, account for the relativelylow redox potential (~ −1050 mV) at which the A0 acceptoroperates. The precise mechanism by which the phylloquinoneelectron acceptor A1 can function at an estimated EM of −800 mVis unclear, but π–πstacking with a conserved tryptophan residueor asymmetric hydrogen bonding to the phyllosemiquinonemight play a role [e].

References

a Hoff, A.J. and Deisenhofer, J. (1997) Photophysics of photosynthesis. Structure and spectroscopy of reaction centers of purple bacteria. Phys. Rep.287, 1–247

b Durrant, J.R. et al. (1995) A multimer model for P680, the primary electron-donor of photosystem-II. Proc. Natl. Acad. Sci. U. S. A. 92,4798–4802

c Rutherford, A.W. and Faller, P. (2001) The heart of photosynthesis in glorious 3D. Trends Biochem. Sci. 26, 341–344

d Nugent, J. (2001) Photosynthetic water oxidation. Biochim. Biophys. Acta1503, 1

e Jordan, P. et al. (2001) Three-dimensional structure of cyanobacterialphotosystem-I at 2.5 angstrom resolution. Nature 411, 909–917

Box 1. Mechanism of light-energy use by reaction centres

During the past ten years, much work has beendone to resolve the X-ray crystal structure of the PSIcomplex from Synechococcus elongatus (Fig. 3). Initialreports of the structure at 6 Å and 4 Å resolutionsshowed that the complex is organized into threeprincipal domains [20–24]. The C-terminal regions ofthe PsaA and PsaB polypeptides were each observedto contribute five membrane-spanning α helices to aheterodimeric central core domain that houses theelectron transfer cofactors and additional antennapigments (Fig. 4), and exhibits a twofold symmetry

(Fig. 3c). The N-terminal regions of the PsaA andPsaB polypeptides were each observed to contributesix membrane-spanning α helices to antenna domainsthat are located on either side of the core domains(highlighted in Fig. 4). A new structure of a trimericform of this complex at 2.5 Å resolution has recentlybeen published [25], and the structure of a monomer issummarized in Fig. 3. The determination of thisstructure is a considerable achievement, because eachmonomer of the trimer contains 12 polypeptides(Fig. 3a) and a total of 127 cofactors (Fig. 3b).



82 Review

Cyt b559

Cyt c550

PheoD2

QBQA

ChlD2

PheoD1

ChlD1

PD2PD1Tyrz

Mn cluster

D1 D2

?

(a) (b)

(c) (d)

* *

Ti BS

Fig. 2. New crystallographic information on the structure of thephotosystem II (PSII) complex. (a) The PSII complex from thethermophilic cyanobacterium Synechococcus elongatus crystallizesas a dimer; only one monomer is shown. At the resolution obtained(3.8 Å), only major protein elements such as α helices and β sheetswere modelled as a Cα trace [14,17], and the chlorophyll a, pheophytina and haem cofactors were modelled as porphyrin rings. The majorsubunits of PSII [D1, D2, CP43, CP47 and cytochrome (Cyt) b559]account for 24 of the transmembrane α helices that were modelled(maroon, green, yellow, red and cyan, respectively). Five additionalminor subunits, each with a single transmembrane α helix, were alsomodelled (all shown in grey), as were two of the three membrane-extrinsic subunits (PsbO in dark green and Cyt c550 in red). Threesubunits intrinsic to the membrane were not assigned, andpresumably correspond to some or all of the seven unassignedtransmembrane α helices evident from the electron density data (alsoshown in grey). (b) Arrangement of the cofactors of the PSII complex.The structural model includes 36 porphyrin rings per monomer,corresponding to 32 chlorophyll a, two pheophytin a and two

cytochrome haems, and one plastoquinone modelled as a benzenering. (c) The D1 and D2 polypeptides each have five transmembraneα helices arranged around an axis of twofold symmetry (vertical line).(d) The cofactors that catalyse the electron transfer reaction arearranged in two membrane-spanning branches and are labelled as inRef. [14]. Each branch consists of two chlorophyll a, one pheophytin aand one quinone, but only the most tightly bound quinone (QA on theA branch) is resolved. The approximate position of the QB quinone isindicated. A bulge in the electron density corresponding to the C helixof the D1 polypeptide was assigned to the redox-active residue Tyr161,which is known as Tyrz. An important aspect of the complexcrystallized by Zouni and co-workers is that the water-oxidizing centrewas intact, because the crystals produce oxygen and protons whenilluminated [49]. The electron density map included an intensity-richY-shaped feature located at the lumenal side of the D1 subunit that wasmodelled as four atoms of manganese (magenta circles). Oxygenproduction also involves Ca2+ and Cl− ions but these could not belocated from the electron density data [14,17]. Figure was preparedusing Molscript [47] and Raster3D [48].

The new structural information on the PSIreaction centre provides some intriguing insights into

longstanding questions about the complex, as well asrevealing some surprising features, such as novelbinding interactions between the antennachlorophylls and the surrounding proteins. In most ofthe 79 antenna chlorophylls coordinated by the PsaAand PsaB polypeptides, the central magnesium atomengages in a fifth (axial) bonding interaction with ahistidine imidazole sidechain, as is seen for thebacteriochlorophylls of the bacterial reaction centre.However, in the remaining antenna chlorophylls, thisaxial ligand is formed by an oxygen atom of aglutamine, glutamate, aspartate or tyrosinesidechain, by a peptide carbonyl oxygen or by a watermolecule [25]. In one particularly intriguinginteraction, the axial ligand for one of the antennachlorophylls is provided by the head group phosphateof a molecule of phosphatidylglycerol. This lipid islocated on the surface of the PsaA polypeptide, at theinterface between monomers in the PSI trimer.

Turning to the electron transfer cofactors (Fig. 3d),the new structural data confirm the proposal that theP700 primary donor is, in fact, a heterodimer ofchlorophyll a and the carbon C132 epimer of chlorophylla (termed chlorophyll a′) [26–28]. Epimerization causesa large change in the conformation of the substituentgroup attached to C13 of the chlorophyll, and selectiveincorporation of chlorophyll a′ on the PsaA side of the



83Review

PsaAC-terminal

domain

PsaBC-terminal

domain

FB

FA

FX

Qk–B Qk

–A

Chl aChl a

Chl aChl a

Chl aChl a

(a) (b)

(c) (d)

(A1)

(A0)

(A)

(P700)

Ti BS

′

Fig. 3. Structural organization of a monomer of the trimeric form of thephotosystem I reaction centre from Synechococcus elongatus. (a) Eachmonomer consists of 12 polypeptides that are associated with 127cofactors [25]. A main structural element of the complex is aheterodimer of two polypeptides, PsaA and PsaB (maroon and green,respectively), each of which has 11 transmembrane α helices. (b) Thecofactors consist of (per monomer) 96 molecules of chlorophyll a,22 carotenoids (cyan), four lipids, three Fe–S clusters (orange and greencircles) and two phylloquinones (red). (c) The cofactors that areengaged in transmembrane electron transfer are encased by the fiveC-terminal membrane-spanning α helices of the PsaA (maroon) andPsaB (green) polypeptides. As in the Type II reaction centres, thesehelices are arranged around an axis of twofold symmetry. (d) The site oftransmembrane electron transfer contains six molecules of chlorophylla (Chl a) and two phylloquinones (QK) organized in two membrane-spanning branches around the axis of twofold symmetry. P700 is theprimary donor of electrons, and A, A0 and A1 are the electron acceptorsthat form the two electron transfer chains. These merge at the FX Fe–Scentre, located on the axis of twofold symmetry at the stromal side ofthe membrane, and both might be active (grey arrows). The FA and FB

Fe–S clusters transfer electrons from the FX centre to the soluble proteinferredoxin and are located in the PsaC subunit, which is bound to thestromal surface of the complex (cyan ribbons at the top of a). The PsaCprotein is similar in structure to a bacterial 2 (Fe4–S4) ferredoxin, andacts as a connection between the electron transfer chain in theheterodimeric core of the reaction centre and the soluble ferredoxinreduced in the stroma. The binding and function of PsaC is assisted bytwo other small subunits (PsaD and PsaE) located on the stromalsurface of the complex (red and orange ribbons, respectively, at the topof (a). Figure was prepared using Molscript [47] and Raster3D [48].

P700 dimer appears to result from a steric effect; thatis, the structure of the protein precluding the binding ofchlorophyll a at this position. As discussed in Box 1, thenew structural data also indicate how the protein mighttune the electrochemical properties of the electronacceptor cofactors of the PSI reaction centre in such away that they can operate at the very low potentialnecessary to reduce the terminal Fe–S redox centres.

One branch of electron transfer or two?The role of the phylloquinones of the PSI reactioncentre is to pass electrons to the Fe–S centres, whichare single-electron redox centres, rather than toprovide an exit for electrons from the complex via theformation of a doubly reduced (and doublyprotonated) quinol, as in the Type II complexes. As aresult, there has been some debate over which of thetwo branches of cofactors catalyses transmembraneelectron transfer from P700 to the Fe–S centres, andover which of the symmetrically arrangedphylloquinones is the so-called A1 electron transferintermediate (Box 1; Fig. 3d).

Recently, evidence has been presented suggestingthat electron transfer might take place along bothbranches of the PSI reaction centre (Fig. 3d). Tworates attributed to reoxidation of the phylloquinoneanion (18 ns and 160 ns) have been detected by opticaltechniques in PSI from the alga Chlorella sorokiniana

[29]. The 160 ns rate is modified followingmutagenesis of the conserved tryptophan from thePsaA polypeptide, which engages in π–πstackingwith the QK–A phylloquinone [30,31]. Mutagenesis ofthe symmetry-related tryptophan from the PsaBsubunit that interacts with the second (QK–B)phylloquinone modifies the 18 ns rate [30]. Theconclusion drawn from these observations is that, onany given turnover of the reaction centre, the A1electron transfer component is either of the twosymmetrically arranged phylloquinones (Fig. 3d),although this is still controversial. Attempts arecurrently in progress to identify ‘symmetry-breaking’structural elements that could account for thedifference in energetics on the two branches, andsuggestions include a carotenoid close to QK–A,different phospholipid molecules bound near the twophylloquinone binding sites, and a conservedtryptophan residue found only in PsaB [25].

Homodimeric Type I reaction centresAs with the Type II complexes, the Type I reactioncentre from PSI has a bacterial counterpart, found inheliobacteria and green sulfur bacteria. However,these bacteria have a Type I reaction centre thatappears to contain a homodimeric core of twoidentical polypeptides, rather than the PsaA–PsaBheterodimer found in the PSI reaction centre. Geneticstudies have shown that these bacteria contain onlyone gene for the core polypeptides of the reactioncentre rather than the two genes seen incyanobacteria, algae and higher plants [32–34].Sequence analyses indicate that theantenna–core–antenna domain structure of the mainsubunits observed in the heterodimeric PSI reactioncentre (Fig. 4) is conserved in the homodimeric Type Icomplexes. The polypeptide composition of thehomodimeric Type I complexes is simpler than that ofPSI, consisting of only the core polypeptide (anequivalent of the PsaC protein) and one other smallpolypeptide that is probably equivalent to PsaD.



84 Review

L-polypeptide

M-polypeptide

CP47antenna

CP43antenna

D1

D2

PsaB N-terminalantenna

PsaA N-terminalantenna

PsaA C-terminal

PsaB C-terminal

Cyanobacterialphotosystem II

Purple bacterialreaction centre

Cyanobacterialphotosystem I

Ti BS

Fig. 4. Structural homology in the Type I and Type II reaction centres.Despite considerable differences in their sizes and amino acidsequences, the main polypeptides of the Type I and Type II reactioncentres show considerable structural homology, indicating that allextant reaction centres have a common ancestor [3,10,25]. Inphotosystem I, the antenna (N-terminal) and core (C-terminal) domainsare formed by a heterodimer of two large polypeptides but, inphotosystem II, these domains are formed by separate polypeptides(CP43, CP47 and the D1–D2 heterodimer) that have probably arisen bygene fission. One point about photosystem I is that, in addition to thechlorophylls of the electron transfer chain, the core domain includesseveral antenna chlorophylls, some of which are in a suitable positionto act as conduits for energy transfer from the antenna to the reactioncentre. Figure was prepared using Molscript [47] and Raster3D [48].

The homodimeric reaction centres also bind fewer antenna bacteriochlorophylls than the PSIreaction centre.

Focusing on the electron transfer system, in aninteresting parallel with the PSI reaction centre, thehomodimeric reaction centres contain C132 epimers ofbacteriochlorophyll a (green sulfur bacteria) orbacteriochlorophyll g (heliobacteria) that mightconstitute half or all the primary donor dimer (thereare estimated to be two molecules of the epimer perreaction centre) [35]. Given the homodimericorganization of Type I reaction centres, it seemsprobable that electron transfer will occur along bothcofactor branches in these highly symmetric reactioncentres. Although the primary electron donor andantenna pigments are bacteriochlorophylls in theType I reaction centres of heliobacteria and greensulfur bacteria, the primary electron acceptor A0 is achlorophyll rather than a bacteriochlorophyll. This ispresumably because the redox potential of thechlorophyll a–a− couple is significantly more negativethan that of a bacteriochlorophyll a–a− couple,making a chlorophyll anion a more suitable electrondonor to the bound Fe–S clusters.

One possible difference between the homodimericbacterial Type I reaction centres and the PSI reactioncentre is the function of the naphthoquinone cofactorsand how tightly they are bound to the reaction centrecomplex [36]. At present, the weight of evidencesuggests that these naphthoquinones are notinvolved in forward electron transfer from the A0chlorophyll to the FX Fe–S centre [35]. It is alsopossible that they are free to exchange withnaphthoquinones in the intramembrane pool, as isthe case for the QB quinone in the Type II reactioncentres. This might be caused by the absence oftryptophan residues equivalent to those that form π–πstacking interactions with the phylloquinones in PSIfrom the bacterial Type I reaction centres [37].

Common structural blueprint

The crystallographic information summarized inFig. 4 highlights structural features that are commonto all types of reaction centre [3,10,25]. At the heart ofeach complex is a core domain consisting of anarrangement of two sets of five transmembraneα helices. This protein scaffold encases six(bacterio)chlorin and two quinone cofactors that arearranged in two pseudosymmetric membrane-spanning branches. These cofactors catalyse thephotochemical transmembrane electron transferreaction that is the key to the photosynthetic process.Added to this basic structural blueprint are a varietyof protein–cofactor structures, such as antennacomplexes, the oxygen-evolving complex or Fe–Scentres, which represent further adaptations. Inparticular, in the PSII reaction centre and all knownType I reaction centres, the core electron transferdomain is flanked by two homologous antennadomains, each consisting of a bundle of six

membrane-spanning α helices binding antennapigments [24], and antenna chlorophylls are alsobound to the ten-helix core (Fig. 4). These antennadomains are not present in purple bacteria such asRhodobacter sphaeroides or green filamentousbacteria such as Chloroflexus.

Which is the oldest reaction centre?

The realization that all reaction centres are based ona common design has provoked much discussion overthe evolutionary links between the differentcomplexes and the nature of the ancestral reactioncentre. This is a challenging topic because it is clearthat chlorophyll-based photosynthesis is a very oldprocess that appeared during the first few hundredmillion years of evolution [38]. One approach to thisproblem has been to examine which of the five distinctgroups of photosynthetic bacteria represents theoldest photosynthetic lineage, through phylogeneticstudies of both photosynthetic andnon-photosynthetic proteins. However, such studieshave produced conflicting results, with greenfilamentous bacteria, heliobacteria and purplebacteria all being identified as the oldest lineage indifferent studies [39–42]. The problem of tracing theevolutionary development of modern dayphotosystems is not helped by some of the variety andcomplexity exhibited by photosynthetic organisms,which indicates some interchange of photosyntheticcomponents by lateral gene transfer between groupsduring the course of evolution [41,43]. At present, it isprobably prudent to conclude that the use of thisapproach requires additional data and a moreextensive analysis.

Primordial reaction centre: Type I, Type II or both?

Setting aside the question of which is the oldestphotosynthetic organism, several models have beenproposed to account for the development of modernday reaction centres from simpler ancestors [41].Most recently, a new evolutionary scheme forcontemporary reaction centres has been proposedthat envisages the ancestral reaction centre ashomodimeric, with the three-domainantenna–core–antenna organization seen in extantType I complexes [37]. It is proposed that thisancestral reaction centre had two membrane-spanning electron transfer chains, each terminatingin a loosely bound quinone that could dissociatewhen reduced and move into the membrane pool,and that it occupied a membrane that had alreadydeveloped a fully functional anaerobic respiratorychain, in accordance with the ‘respiration early’hypothesis [44]. Therefore, the ancestral reactioncentre proposed had a mixed character, with thethree-domain organization and (possibly) symmetricelectron transfer characteristic of contemporaryType I reaction centres but a capacity to reduce theintramembrane quinone pool, as seen incontemporary Type II reaction centres [37].



85Review

The future …… and the dim, distant past

The increasingly detailed crystallographicinformation now available for the cyanobacterialType I and Type II reaction centres is provokingrenewed interest in the detailed mechanism of theseelegant transducers of energy. In particular, the firstcrystallographic glimpses of the machinery for oxygenevolution are both intriguing and exciting, and willtrigger much re-evaluation of our currentunderstanding of a reaction that is of obviousimportance to aerobes such as ourselves. It is alsobecoming apparent that a detailed understanding ofquinone chemistry of the homodimeric reactioncentres from heliobacteria and green sulfur bacteriamight help to focus ideas about the nature of theancestral reaction centre and the evolutionary routethat has led to contemporary complexes.

Finally, peering even further back in evolutionarytime, an intriguing question that remains relativelyunexplored concerns the origins of the ancestralreaction centre. What was the function of this(bacterio)chlorophyll-containing membrane proteinbefore it evolved into a system capable of harnessinglight energy? One suggestion is that early organismsused pigment–protein complexes to protect

themselves against the ultraviolet (UV) radiationthat bathed the surface of the planet before thedevelopment of the atmospheric ozone layer [45].Such proteins might originally have operated byabsorbing high-energy UV photons and dissipatingthe energy through internal conversion between the(bacterio)chlorophyll Soret absorbance transition andthe visible-region absorbance bands, before emittingthe energy as a much more benign visible or near-infrared photon [45]. Light-activated electrontransfer might originally have developed as anextension to this photoprotective function, excitedstate energy being converted first into the energy of acharge separated state (similar to the P870+HA

− stateformed in the purple bacterial reaction centre) andsubsequently lost as heat as the charge-separatedstate recombines (as occurs in purple bacterialreaction centres when forward electron transfer fromHA

− is blocked). Another suggestion is thatphotosynthetic function evolved frombacteriochlorophyll-containing proteins involved ininfrared thermotaxis [46]. Whatever the truth,addressing these questions requires a journey back toan early stage in the evolution of life, and presents afascinating challenge.



86 Review

Acknowledgements

We thank Norbert Kraußfor kindly supplying thecoordinates of thecyanobacterial PSI aheadof publication. We aresupported by the BBSRC,and P.H. alsoacknowledges supportfrom the TMR Programmeof the European Union.

References

1 Oesterhelt, D. (1998) The structure andmechanism of the family of retinal proteins fromhalophilic archaea. Curr. Opin. Struct. Biol. 8,489–500

2 Blankenship, R.E. (1992) Origin and earlyevolution of photosynthesis. Photosynth. Res. 33,91–111

3 Allen, J.P. and Williams, J.C. (1998) Photosyntheticreaction centers. FEBS Lett. 438, 5–9

4 Allen, J.P. et al. (1987) Structure of the reactioncenter from Rhodobacter sphaeroides R-26 – thecofactors. Proc. Natl. Acad. Sci. U. S. A. 84,5730–5734

5 Deisenhofer, J. et al. (1985) Structure of theprotein subunits in the photosynthetic reactioncenter of Rhodopseudomonas viridis at3 angstrom resolution. Nature 318, 618–624

6 Hoff, A.J. and Deisenhofer, J. (1997) Photophysicsof photosynthesis. Structure and spectroscopy ofreaction centers of purple bacteria. Phys. Rep.287, 1–247

7 Fyfe, P.K. and Jones, M.R. (2000) Re-emergingstructures: continuing crystallography of thebacterial reaction centre. Biochim. Biophys. Acta1459, 413–421

8 Barber, J. (1987) Photosynthetic reaction centers– a common link. Trends Biochem. Sci. 12,321–326

9 Michel, H. and Deisenhofer, J. (1988) Relevance ofthe photosynthetic reaction center from purplebacteria to the structure of photosystem-II.Biochemistry 27, 1–7

10 Nitschke, W. and Rutherford, A.W. (1991)Photosynthetic reaction centres: variations on acommon structural theme. Trends Biochem. Sci.16, 241–245

11 Rhee, K-H. et al. (1998) Three-dimensionalstructure of photosystem-II reaction centre at 8 Åresolution. Nature 396, 283–286

12 Hankamer, B. et al. (1999) Revealing thestructure of the oxygen-evolving core dimer of

photosystem-II by electron crystallography.Nat. Struct. Biol. 6, 560–564

13 Nield, J. et al. (2000) 3D map of the plantphotosystem-II supercomplex obtained bycryoelectron microscopy and single particleanalysis. Nat. Struct. Biol. 7, 44–47

14 Zouni, A. et al. (2001) Crystal structure ofphotosystem-II from Synechococcus elongatus at3.8 Å resolution. Nature 409, 739–743

15 Shen, J.R. and Kamiya, N. (2000) Crystallizationand the crystal properties of the oxygen-evolvingphotosystem II from Synechococcus vulcanus.Biochemistry 39, 14739–14744

16 Nugent, J. (2001) Photosynthetic water oxidation.Biochim. Biophys. Acta 1503, 1

17 Rutherford, A.W. and Faller, P. (2001) The heart ofphotosynthesis in glorious 3D. Trends Biochem.Sci. 26, 341–344

18 Durrant, J.R. et al. (1995) A multimer model forP680, the primary electron-donor ofphotosystem-II. Proc. Natl. Acad. Sci. U. S. A. 92,4798–4802

19 Heathcote, P. (2001) Type I photosyntheticreaction centres. Biochim. Biophys. Acta 1507, 1–2

20 Krauss, N. et al. (1993) 3-dimensional structure ofsystem-I of photosynthesis at 6 angstromresolution. Nature 361 326–331

21 Krauss, N. et al. (1996) Photosystem-I at 4 Åresolution represents the first structural model ofa photosynthetic reaction centre and core antennasystem. Nat. Struct. Biol. 3, 965–973

22 Fromme, P. et al. (1996) Structure ofphotosystem-I at 4.5 Å resolution: a short reviewincluding evolutionary aspects. Biochim. Biophys.Acta 1275, 76–83

23 Schubert, W-D. et al. (1998) A common ancestorfor oxygenic and anoxygenic photosyntheticsystems: a comparison based on the structuralmodel of photosystem-I. J. Mol. Biol. 280,297–314

24 Klukas, O. et al. (1999) Localisation ofphylloquinones, QK and QK′, in an improved

electron density map of photosystem-I at 4 Åresolution. J. Biol. Chem. 274, 7361–7367

25 Jordan, P. et al. (2001) Three-dimensionalstructure of cyanobacterial photosystem-I at2.5 angstrom resolution. Nature 411, 909–917

26 Watanabe, T. et al. (1985) Evidence that achlorophyll-a′ dimer constitutes thephotochemical-reaction center-1 (P700) inphotosynthetic apparatus. FEBS Lett. 191,252–256

27 Maroc, J. and Tremolieres, A. (1990) Chlorophyll-a′ and pheophytin-a, as determined by HPLC, inphotosynthesis mutants and double mutants ofChlamydomonas-reinhardtii. Biochim. Biophys.Acta 1018, 67–71

28 Maeda, H. et al. (1992) Presence of 2 chlorophyll-a′ molecules at the core of photosystem-I.Biochim. Biophys. Acta 1099, 74–80

29 Joliot, P. and Joliot, A. (1999) In vivo analysis ofthe electron transfer within photosystem-I: arethe two phylloquinones involved? Biochemistry38, 11130–11136

30 Guergova-Kuras, M. et al. (2001) Evidence for twoactive branches for electron transfer inphotosystem-I. Proc. Natl. Acad. Sci. U. S. A. 98,4437–4442

31 Purton, S. et al. (2001) Site-directed mutagenesisof PsaA residue W693 affects phylloquinonebinding and function in the photosystem Ireaction center of Chlamydomonas reinhardtii.Biochemistry 40, 2167–2175

32 Buttner, M. et al. (1992) Photosynthetic reactioncenter genes in green sulfur bacteria and inphotosystem-I are related. Proc. Natl. Acad. Sci.U. S. A. 89, 8135–8139

33 Buttner, M. et al. (1992) The photosystem-I-likeP840-reaction center of green S-bacteria is ahomodimer. Biochim. Biophys. Acta 1101,154–156

34 Liebl, U. et al. (1993) Single core polypeptide inthe reaction-center of the photosyntheticbacterium Heliobacillus-mobilis – structural

implications and relations to other photosystems.Proc. Natl. Acad. Sci. U. S. A. 90, 7124–7128

35 Hauska, G. et al. (2001) The reaction center ofgreen sulfur bacteria. Biochim. Biophys. Acta1507, 260–277

36 Muhiuddin, I.P. et al. (1999) ENDOR and specialtriple resonance spectroscopy ofphotoaccumulated semiquinone electron acceptorsin the reaction centers of green sulfur bacteria andheliobacteria. Biochemistry 38, 7159–7167

37 Baymann, F. et al. (2001) Daddy, where did PS(I)come from? Biochim. Biophys. Acta 1507, 291–310

38 Nisbet, E.G. and Sleep, N.H. (2001) The habitatand nature of early life. Nature 409, 1083–1091

39 Olsen, G.J. et al. (1994) The winds of(evolutionary) change: breathing new life intomicrobiology. J. Bacteriol. 176, 1–6

40 Gupta, R.S. et al. (1999) Evolutionary relationshipsamong photosynthetic prokaryotes (Heliobacteriumchlorum, Chloroflexus aurantiacus, cyanobacteria,Chlorobium tepidum and proteobacteria):implications regarding the origin of photosynthesis.Mol. Microbiol. 32, 893–906

41 Xiong, J. et al. (1998) Tracking molecularevolution of photosynthesis by characterization ofa major photosynthesis gene cluster fromHeliobacillus mobilis. Proc. Natl. Acad. Sci.U. S. A. 95, 14851–14856

42 Xiong, J. et al. (2000) Molecular evidence for theearly evolution of photosynthesis. Science 289,1724–1730

43 Blankenship, R.E. (2001) Molecular evidence forthe evolution of photosynthesis. Trends Plant Sci. 6, 4–6

44 Castresana, J. et al. (1994) Evolution ofcytochrome oxidase, an enzyme older thanatmospheric oxygen. EMBO J. 13, 2516–2525

45 Mulkidjanian, A.Y. and Junge, W. (1997) On theorigin of photosynthesis as inferred from sequenceanalysis. Photosynth. Res. 51, 27–42

46 Nisbet, E.G. et al. (1995) Origins ofphotosynthesis. Nature 373, 479–480

47 Kraulis, P.J. (1991) Molscript – a program toproduce both detailed and schematic plots ofprotein structures. J. Appl. Crystallogr. 24, 946–950

48 Merritt, E.A. and Bacon, D.J. (1997) Raster3D:photorealistic molecular graphics. MethodsEnzymol. 277, 505–524

49 Zouni, A. et al. (2000) First photosystem-IIcrystals capable of water oxidation. Biochim.Biophys. Acta 1457, 103–105


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87Review

Microorganisms such as the budding yeastSaccharomyces cerevisiae must elicit coordinated,accurate and robust responses to changes in acomplex extracellular environment. They cancoordinate such responses by re-using individualfactors and motifs for related responses, althoughthey must then ensure that the correct response istriggered by the appropriate stimulus. The cyclin-dependent kinase (CDK) Pho85 is an example of akinase that is shared between several pathways,having multiple functions and cyclin-bindingpartners [1].

The past 15 years have seen the extensivecharacterization of a wide variety of CDKs. Thesesignaling molecules have proven to be central toseveral cellular processes in addition to regulation ofthe cell cycle, the context in which they were firstidentified. Our basic understanding of CDKs andtheir activity is by no means complete. Weunderstand that the binding of a cyclin partnerconfers substrate specificity on the kinase, but wehave no systematic way to predict the substrates,

the regulators or the physiological function targetedby a given cyclin–CDK complex. This is certainlytrue of the budding yeast CDK Pho85, which hasproven useful for approaching these generalquestions because of its non-essential nature, thenumber of cyclin partners with which it associates,and the experimental tractability of yeast ingeneral.

As we learn more about the functions of Pho85,one theme emerges repeatedly. Many functions ofPho85 appear to be involved in transducing signalsrelated to changes in the extracellular environment(Table 1). Furthermore, the kinase activity of therelevant Pho85 complex appears to signal that thecurrent environmental situation is satisfactory;when environmental conditions become stressful,the relevant kinase activities of Pho85 are switchedoff, resulting in activation of the appropriateresponse.

Pho80 and phosphate metabolism

Pho85 is best known for its pivotal role in the PHOpathway, a signaling pathway that coordinates theresponses of yeast to phosphate starvation [2,3].Pho85 is directed to this function by its associationwith the cyclin Pho80 [4]. Pho80–Pho85 kinaseactivity is regulated in response to phosphate levelsby the CDK inhibitor (CKI) Pho81, which remainsbound to Pho80–Pho85 in both high and lowphosphate conditions [5,6]. When high levels ofinorganic phosphate are present in the environment,the Pho80–Pho85 kinase is active, phosphorylatingand inactivating the transcription factor, Pho4 [7].

Pho85 and signaling environmental

conditions

Adam S. Carroll and Erin K. O’Shea

Through its association with a family of ten cyclins, the Pho85 cyclin-

dependent kinase is involved in several signal transduction pathways in the

yeast Saccharomyces cerevisiae. The responses mediated by Pho85 include

cell-cycle progression and metabolism of nutrients such as phosphate and

carbon sources. Although these responses require the phosphorylation of

different substrates, and have different mechanistic consequences as a result of

this phosphorylation, all appear to be involved in responses to changes in

environmental conditions. Few of the activating signals or regulated targets

have been unambiguously identified, but the kinase activity of Pho85 appears

to inform the cell that the current environment is satisfactory.

Adam S. Carroll

Erin K. O’Shea*

Dept of Biochemistry andBiophysics and theHoward Hughes MedicalInstitute, University ofCalifornia San Francisco,San Francisco,CA 94143-0448, USA.*e-mail: [email protected]

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