Membrane potential-dependent reduction of cytochrome b-6 in an algal mutant lacking Photosystem I centers

Molecular basis of plastoquinone reduction in plant cytochrome b6f

A multi-subunit enzyme, cytochrome b6f (cytb6f), provides the crucial link between photosystems I and II in the photosynthetic membranes of higher plants, transferring electrons between plastoquinone (PQ) and plastocyanin. The atomic structure of cytb6f is known, but its detailed catalytic mechanism remains elusive. Here we present cryogenic electron microscopy structures of spinach cytb6f at 1.9 Å and 2.2 Å resolution, revealing an unexpected orientation of the substrate PQ in the haem ligand niche that forms the PQ reduction site (Qn). PQ, unlike Qn inhibitors, is not in direct contact with the haem. Instead, a water molecule is coordinated by one of the carbonyl groups of PQ and can act as the immediate proton donor for PQ. In addition, we identify water channels that connect Qn with the aqueous exterior of the enzyme, suggesting that the binding of PQ in Qn displaces water through these channels. The structures confirm large movements of the head domain of the iron-sulfur protein (ISP-HD) towards and away from the plastoquinol oxidation site (Qp) and define the unique position of ISP-HD when a Qp inhibitor (2,5-dibromo-3-methyl-6-isopropylbenzoquinone) is bound. This work identifies key conformational states of cytb6f, highlights fundamental differences between substrates and inhibitors and proposes a quinone-water exchange mechanism.

Unexpected Heme Redox Potential Values Implicate an Uphill Step in Cytochrome b6f

Cytochromes bc, key enzymes of respiration and photosynthesis, contain a highly conserved two-heme motif supporting cross-membrane electron transport (ET) that connects the two catalytic quinone-binding sites (Q_n and Q_p). Typically, this ET occurs from the low- to high-potential heme b, but in photosynthetic cytochrome b₆f, the redox midpoint potentials (E_ms) of these hemes remain uncertain. Our systematic redox titration analysis based on three independent and comprehensive low-temperature spectroscopies (continuous wave and pulse electron paramagnetic resonance (EPR) and optical spectroscopies) allowed for unambiguous assignment of spectral components of hemes in cytochrome b₆f and revealed that E_m of heme b_n is unexpectedly low. Consequently, the cross-membrane ET occurs from the high- to low-potential heme introducing an uphill step in the energy landscape for the catalytic reaction. This slows down the ET through a low-potential chain, which can influence the mechanisms of reactions taking place at both Q_p and Q_n sites and modulate the efficiency of cyclic and linear ET in photosynthesis.

Electron transfer via cytochrome b6f complex displays sensitivity to Antimycin A upon STT7 kinase activation

The cytochrome b6f complex (b6f) has been initially considered as the ferredoxin-plastoquinone reductase (FQR) during cyclic electron flow (CEF) with photosystem I that is inhibited by antimycin A (AA). The binding of AA to the b6f Qi-site is aggravated by heme-ci, which challenged the FQR function of b6f during CEF. Alternative models suggest that PROTON GRADIENT REGULATION5 (PGR5) is involved in a b6f-independent, AA-sensitive FQR. Here, we show in Chlamydomonas reinhardtii that the b6f is conditionally inhibited by AA in vivo and that the inhibition did not require PGR5. Instead, activation of the STT7 kinase upon anaerobic treatment induced the AA sensitivity of b6f which was absent in stt7-1. However, a lock in State 2 due to persisting phosphorylation in the phosphatase double mutant pph1;pbcp did not increase AA sensitivity of electron transfer. The latter required a redox poise, supporting the view that state transitions and CEF are not coercively coupled. This suggests that the b6f-interacting kinase is required for structure-function modulation of the Qi-site under CEF favoring conditions. We propose that PGR5 and STT7 independently sustain AA-sensitive FQR activity of the b6f. Accordingly, PGR5-mediated electron injection into an STT7-modulated Qi-site drives a Mitchellian Q cycle in CEF conditions.

Cytochrome b6f - Orchestrator of photosynthetic electron transfer

Cytochrome b₆f (cytb₆f) lies at the heart of the light-dependent reactions of oxygenic photosynthesis, where it serves as a link between photosystem II (PSII) and photosystem I (PSI) through the oxidation and reduction of the electron carriers plastoquinol (PQH₂) and plastocyanin (Pc). A mechanism of electron bifurcation, known as the Q-cycle, couples electron transfer to the generation of a transmembrane proton gradient for ATP synthesis. Cytb₆f catalyses the rate-limiting step in linear electron transfer (LET), is pivotal for cyclic electron transfer (CET) and plays a key role as a redox-sensing hub involved in the regulation of light-harvesting, electron transfer and photosynthetic gene expression. Together, these characteristics make cytb₆f a judicious target for genetic manipulation to enhance photosynthetic yield, a strategy which already shows promise. In this review we will outline the structure and function of cytb₆f with a particular focus on new insights provided by the recent high-resolution map of the complex from Spinach.

PGR5 is required for efficient Q cycle in the cytochrome b6f complex during cyclic electron flow

Proton gradient regulation 5 (PGR5) is involved in the control of photosynthetic electron transfer, but its mechanistic role is not yet clear. Several models have been proposed to explain phenotypes such as a diminished steady-state proton motive force (pmf) and increased photodamage of photosystem I (PSI). Playing a regulatory role in cyclic electron flow (CEF) around PSI, PGR5 contributes indirectly to PSI protection by enhancing photosynthetic control, which is a pH-dependent down-regulation of electron transfer at the cytochrome b6f complex (b6f). Here, we re-evaluated the role of PGR5 in the green alga Chlamydomonas reinhardtii and conclude that pgr5 possesses a dysfunctional b6f. Our data indicate that the b6f low-potential chain redox activity likely operated in two distinct modes - via the canonical Q cycle during linear electron flow and via an alternative Q cycle during CEF, which allowed efficient oxidation of the low-potential chain in the WT b6f. A switch between the two Q cycle modes was dependent on PGR5 and relied on unknown stromal electron carrier(s), which were a general requirement for b6f activity. In CEF-favoring conditions, the electron transfer bottleneck in pgr5 was the b6f, in which insufficient low-potential chain redox tuning might account for the mutant pmf phenotype. By attributing a ferredoxin-plastoquinone reductase activity to the b6f and investigating a PGR5 cysteine mutant, a current model of CEF is challenged.

The mechanism of cyclic electron flow

Apart from the canonical light-driven linear electron flow (LEF) from water to CO₂, numerous regulatory and alternative electron transfer pathways exist in chloroplasts. One of them is the cyclic electron flow around Photosystem I (CEF), contributing to photoprotection of both Photosystem I and II (PSI, PSII) and supplying extra ATP to fix atmospheric carbon. Nonetheless, CEF remains an enigma in the field of functional photosynthesis as we lack understanding of its pathway. Here, we address the discrepancies between functional and genetic/biochemical data in the literature and formulate novel hypotheses about the pathway and regulation of CEF based on recent structural and kinetic information.

Photosynthetic growth despite a broken Q-cycle

Central in respiration or photosynthesis, the cytochrome bc₁ and b₆f complexes are regarded as functionally similar quinol oxidoreductases. They both catalyse a redox loop, the Q-cycle, which couples electron and proton transfer. This loop involves a bifurcated electron transfer step considered as being mechanistically mandatory, making the Q-cycle indispensable for growth. Attempts to falsify this paradigm in the case of cytochrome bc₁ have failed. The rapid proteolytic degradation of b₆f complexes bearing mutations aimed at hindering the Q-cycle has precluded so far the experimental assessment of this model in the photosynthetic chain. Here we combine mutations in Chlamydomonas that inactivate the redox loop but preserve high accumulation levels of b₆f complexes. The oxidoreductase activity of these crippled complexes is sufficient to sustain photosynthetic growth, which demonstrates that the Q-cycle is dispensable for oxygenic photosynthesis.

The Q-cycle is thought to be an essential energetic component of the photosynthetic electron-transfer chain. Here, Chlamydomonas mutants with an inactive Q-cycle but normal levels of b₆f complexes are shown to display photosynthetic growth, demonstrating the dispensability of the Q-cycle in the oxygenic photosynthetic chain.

Conservation of lipid functions in cytochrome bc complexes

Lipid binding sites and properties are compared in two sub-families of hetero-oligomeric membrane protein complexes known to have similar functions in order to gain further understanding of the role of lipid in the function, dynamics, and assembly of these complexes. Using the crystal structure information for both complexes, we compared the lipid binding properties of the cytochrome b(6)f and bc(1) complexes that function in photosynthetic and respiratory membrane energy transduction. Comparison of lipid and detergent binding sites in the b(6)f complex with those in bc(1) shows significant conservation of lipid positions. Seven lipid binding sites in the cyanobacterial b(6)f complex overlap three natural sites in the Chlamydomonas reinhardtii algal complex and four sites in the yeast mitochondrial bc(1) complex. The specific identity of lipids is different in b(6)f and bc(1) complexes: b(6)f contains sulfoquinovosyldiacylglycerol, phosphatidylglycerol, phosphatidylcholine, monogalactosyldiacylglycerol, and digalactosyldiacylglycerol, whereas cardiolipin, phosphatidylethanolamine, and phosphatidic acid are present in the yeast bc(1) complex. The lipidic chlorophyll a and β-carotene (β-car) in cyanobacterial b(6)f, as well as eicosane in C. reinhardtii, are unique to the b(6)f complex. Inferences of lipid binding sites and functions were supported by sequence, interatomic distance, and B-factor information on interacting lipid groups and coordinating amino acid residues. The lipid functions inferred in the b(6)f complex are as follows: (i) substitution of a transmembrane helix by a lipid and chlorin ring, (ii) lipid and β-car connection of peripheral and core domains, (iii) stabilization of the iron-sulfur protein transmembrane helix, (iv) n-side charge and polarity compensation, and (v) β-car-mediated super-complex with the photosystem I complex.

The Q cycle of cytochrome bc complexes: a structure perspective

Aspects of the crystal structures of the hetero-oligomeric cytochrome bc(1) and b(6)f ("bc") complexes relevant to their electron/proton transfer function and the associated redox reactions of the lipophilic quinones are discussed. Differences between the b(6)f and bc(1) complexes are emphasized. The cytochrome bc(1) and b(6)f dimeric complexes diverge in structure from a core of subunits that coordinate redox groups consisting of two bis-histidine coordinated hemes, a heme b(n) and b(p) on the electrochemically negative (n) and positive (p) sides of the complex, the high potential [2Fe-2S] cluster and c-type heme at the p-side aqueous interface and aqueous phase, respectively, and quinone/quinol binding sites on the n- and p-sides of the complex. The bc(1) and b(6)f complexes diverge in subunit composition and structure away from this core. b(6)f Also contains additional prosthetic groups including a c-type heme c(n) on the n-side, and a chlorophyll a and β-carotene. Common structure aspects; functions of the symmetric dimer. (I) Quinone exchange with the bilayer. An inter-monomer protein-free cavity of approximately 30Å along the membrane normal×25Å (central inter-monomer distance)×15Å (depth in the center), is common to both bc(1) and b(6)f complexes, providing a niche in which the lipophilic quinone/quinol (Q/QH(2)) can be exchanged with the membrane bilayer. (II) Electron transfer. The dimeric structure and the proximity of the two hemes b(p) on the electrochemically positive side of the complex in the two monomer units allow the possibility of two alternate routes of electron transfer across the complex from heme b(p) to b(n): intra-monomer and inter-monomer involving electron cross-over between the two hemes b(p). A structure-based summary of inter-heme distances in seven bc complexes, representing mitochondrial, chromatophore, cyanobacterial, and algal sources, indicates that, based on the distance parameter, the intra-monomer pathway would be favored kinetically. (III) Separation of quinone binding sites. A consequence of the dimer structure and the position of the Q/QH(2) binding sites is that the p-side QH(2) oxidation and n-side Q reduction sites are each well separated. Therefore, in the event of an overlap in residence time by QH(2) or Q molecules at the two oxidation or reduction sites, their spatial separation would result in minimal steric interference between extended Q or QH(2) isoprenoid chains. (IV) Trans-membrane QH(2)/Q transfer. (i) n/p-side QH(2)/Q transfer may be hindered by lipid acyl chains; (ii) the shorter less hindered inter-monomer pathway across the complex would not pass through the center of the cavity, as inferred from the n-side antimycin site on one monomer and the p-side stigmatellin site on the other residing on the same surface of the complex. (V) Narrow p-side portal for QH(2)/Q passage. The [2Fe-2S] cluster that serves as oxidant, and whose histidine ligand serves as a H(+) acceptor in the oxidation of QH(2), is connected to the inter-monomer cavity by a narrow extended portal, which is also occupied in the b(6)f complex by the 20 carbon phytyl chain of the bound chlorophyll.

Heliobacterial Rieske/cytb complex

Data on structure and function of the Rieske/cytb complex from Heliobacteria are scarce. They indicate that the complex is related to the b (6) f complex in agreement with the phylogenetic position of the organism. It is composed of a diheme cytochrome c, and a Rieske iron-sulfur protein, together with transmembrane cytochrome b (6) and subunit IV. Additional small subunits may be part of the complex. The cofactor content comprises heme c (i), first discovered in the Q(i) binding pocket of b (6) f complexes. The redox midpoint potentials are more negative than in b (6) f complex in agreement with the lower redox midpoint potentials (by about 150 mV) of its reaction partners, menaquinone, and cytochrome c (553). The enzyme is implicated in cyclic electron transfer around the RCI. Functional studies are favored by the absence of antennae and the simple photosynthetic reaction chain but are hampered by the high oxygen sensitivity of the organism, its chlorophyll, and lipids.

Is the redox state of the ci heme of the cytochrome b6f complex dependent on the occupation and structure of the Qi site and vice versa?

Oxidoreductases of the cytochrome bc(1)/b(6)f family transfer electrons from a liposoluble quinol to a soluble acceptor protein and contribute to the formation of a transmembrane electrochemical potential. The crystal structure of cyt b(6)f has revealed the presence in the Q(i) site of an atypical c-type heme, heme c(i). Surprisingly, the protein does not provide any axial ligand to the iron of this heme, and its surrounding structure suggests it can be accessed by exogenous ligand. In this work we describe a mutagenesis approach aimed at characterizing the c(i) heme and its interaction with the Q(i) site environment. We engineered a mutant of Chlamydomonas reinhardtii in which Phe(40) from subunit IV was substituted by a tyrosine. This results in a dramatic slowing down of the reoxidation of the b hemes under single flash excitation, suggesting hindered accessibility of the heme to its quinone substrate. This modified accessibility likely originates from the ligation of the heme iron by the phenol(ate) side chain introduced by the mutation. Indeed, it also results in a marked downshift of the c(i) heme midpoint potential (from +100 mV to -200 mV at pH 7). Yet the overall turnover rate of the mutant cytochrome b(6)f complex under continuous illumination was found similar to the wild type one, both in vitro and in vivo. We propose that, in the mutant, a change in the ligation state of the heme upon its reduction could act as a redox switch that would control the accessibility of the substrate to the heme and trigger the catalysis.

Structure-function of the cytochrome b6f complex

The structure and function of the cytochrome b6f complex is considered in the context of recent crystal structures of the complex as an eight subunit, 220 kDa symmetric dimeric complex obtained from the thermophilic cyanobacterium, Mastigocladus laminosus, and the green alga, Chlamydomonas reinhardtii. A major problem confronted in crystallization of the cyanobacterial complex, proteolysis of three of the subunits, is discussed along with initial efforts to identify the protease. The evolution of these cytochrome complexes is illustrated by conservation of the hydrophobic heme-binding transmembrane domain of the cyt b polypeptide between b6f and bc1 complexes, and the rubredoxin-like membrane proximal domain of the Rieske [2Fe-2S] protein. Pathways of coupled electron and proton transfer are discussed in the framework of a modified Q cycle, in which the heme c(n), not found in the bc1 complex, but electronically tightly coupled to the heme b(n) of the b6f complex, is included. Crystal structures of the cyanobacterial complex with the quinone analogue inhibitors, NQNO or tridecyl-stigmatellin, show the latter to be ligands of heme c(n), implicating heme c(n) as an n-side plastoquinone reductase. Existing questions include (a) the details of the shuttle of: (i) the [2Fe-2S] protein between the membrane-bound PQH2 electron/H+ donor and the cytochrome f acceptor to complete the p-side electron transfer circuit; (ii) PQ/PQH2 between n- and p-sides of the complex across the intermonomer quinone exchange cavity, through the narrow portal connecting the cavity with the p-side [2Fe-2S] niche; (b) the role of the n-side of the b6f complex and heme c(n) in regulation of the relative rates of noncyclic and cyclic electron transfer. The likely presence of cyclic electron transport in the b6f complex, and of heme c(n) in the firmicute bc complex suggests the concept that hemes b(n)-c(n) define a branch point in bc complexes that can support electron transport pathways that differ in detail from the Q cycle supported by the bc1 complex.

A novel pathway of cytochrome c biogenesis is involved in the assembly of the cytochrome b6f complex in arabidopsis chloroplasts

We recently characterized a novel heme biogenesis pathway required for heme c(i)' covalent binding to cytochrome b6 in Chlamydomonas named system IV or CCB (cofactor assembly, complex C (b6f), subunit B (PetB)). To find out whether this CCB pathway also operates in higher plants and extend the knowledge of the c-type cytochrome biogenesis, we studied Arabidopsis insertion mutants in the orthologs of the CCB genes. The ccb1, ccb2, and ccb4 mutants show a phenotype characterized by a deficiency in the accumulation of the subunits of the cytochrome b6f complex and lack covalent heme binding to cytochrome b6. These mutants were functionally complemented with the corresponding wild type cDNAs. Using fluorescent protein reporters, we demonstrated that the CCB1, CCB2, CCB3, and CCB4 proteins are targeted to the chloroplast compartment of Arabidopsis. We have extended our study to the YGGT family, to which CCB3 belongs, by studying insertion mutants of two additional members of this family for which no mutants were previously characterized, and we showed that they are not functionally involved in the CCB system. Thus, we demonstrate the ubiquity of the CCB proteins in chloroplast heme c(i)' binding.

Structure of the cytochrome b6f complex: quinone analogue inhibitors as ligands of heme cn

A native structure of the cytochrome b(6)f complex with improved resolution was obtained from crystals of the complex grown in the presence of divalent cadmium. Two Cd(2+) binding sites with different occupancy were determined: (i) a higher affinity site, Cd1, which bridges His143 of cytochrome f and the acidic residue, Glu75, of cyt b(6); in addition, Cd1 is coordinated by 1-2 H(2)O or 1-2 Cl(-); (ii) a second site, Cd2, of lower affinity for which three identified ligands are Asp58 (subunit IV), Glu3 (PetG subunit) and Glu4 (PetM subunit). Binding sites of quinone analogue inhibitors were sought to map the pathway of transfer of the lipophilic quinone across the b(6)f complex and to define the function of the novel heme c(n). Two sites were found for the chromone ring of the tridecyl-stigmatellin (TDS) quinone analogue inhibitor, one near the p-side [2Fe-2S] cluster. A second TDS site was found on the n-side of the complex facing the quinone exchange cavity as an axial ligand of heme c(n). A similar binding site proximal to heme c(n) was found for the n-side inhibitor, NQNO. Binding of these inhibitors required their addition to the complex before lipid used to facilitate crystallization. The similar binding of NQNO and TDS as axial ligands to heme c(n) implies that this heme utilizes plastoquinone as a natural ligand, thus defining an electron transfer complex consisting of hemes b(n), c(n), and PQ, and the pathway of n-side reduction of the PQ pool. The NQNO binding site explains several effects associated with its inhibitory action: the negative shift in heme c(n) midpoint potential, the increased amplitude of light-induced heme b(n) reduction, and an altered EPR spectrum attributed to interaction between hemes c(n) and b(n). A decreased extent of heme c(n) reduction by reduced ferredoxin in the presence of NQNO allows observation of the heme c(n) Soret band in a chemical difference spectrum.

Transmembrane traffic in the cytochrome b6f complex

Crystal structures and their implications for function are described for the energy transducing hetero-oligomeric dimeric cytochrome b6f complex of oxygenic photosynthesis from the thermophilic cyanobacterium, Mastigocladus laminosus, and the green alga, Chlamydomonas reinhardtii. The complex has a cytochrome b core and a central quinone exchange cavity, defined by the two monomers that are very similar to those in the respiratory cytochrome bc1 complex. The pathway of quinol/quinone (Q/QH2) transfer emphasizes the labyrinthine internal structure of the complex, including an 11x12 A portal through which Q/QH2, containing a 45-carbon isoprenoid chain, must pass. Three prosthetic groups are present in the b6f complex that are not found in the related bc1 complex: a chlorophyll (Chl) a, a beta-carotene, and a structurally unique covalently bound heme that does not possess amino acid side chains as axial ligands. It is hypothesized that this heme, exposed to the cavity and a neighboring plastoquinone and close to the positive surface potential of the complex, can function in cyclic electron transport via anionic ferredoxin.

The ci/bH moiety in the b6f complex studied by EPR: a pair of strongly interacting hemes

X-band EPR features in the region of 90-150 mT have previously been attributed to heme ci of the b6 complex [Zhang H, Primak A, Bowman MK, Kramer DM, Cramer WA (2004) Biochemistry 43:16329-16336] and interpreted as arising from a high-spin species. However, the complexity of the observed spectrum is rather untypical for high-spin hemes. In this work, we show that addition of the inhibitor 2-n-nonyl-4-hydroxyquinoline N-oxide largely simplifies heme ci's EPR properties. The spectrum in the presence of 2-n-nonyl-4-hydroxyquinoline N-oxide is demonstrated to be caused by a simple S = 5/2, rhombic species split by magnetic dipolar interaction (A(xx )= 7.5 mT) with neighboring heme bH. The large spacing of lines in the uninhibited system, by contrast, cannot be rationalized solely on the basis of magnetic dipolar coupling but is likely to encompass strong contributions from exchange interactions. The role of the H2O/OH- molecule bridging heme ci's Fe atom and heme bH's propionate side chain in mediating these interactions is discussed.

Consequences of the structure of the cytochrome b6f complex for its charge transfer pathways

At least two features of the crystal structures of the cytochrome b6f complex from the thermophilic cyanobacterium, Mastigocladus laminosus and a green alga, Chlamydomonas reinhardtii, have implications for the pathways and mechanism of charge (electron/proton) transfer in the complex: (i) The narrow 11 x 12 A portal between the p-side of the quinone exchange cavity and p-side plastoquinone/quinol binding niche, through which all Q/QH2 must pass, is smaller in the b6f than in the bc1 complex because of its partial occlusion by the phytyl chain of the one bound chlorophyll a molecule in the b6f complex. Thus, the pathway for trans-membrane passage of the lipophilic quinone is even more labyrinthine in the b6f than in the bc1 complex. (ii) A unique covalently bound heme, heme cn, in close proximity to the n-side b heme, is present in the b6f complex. The b6f structure implies that a Q cycle mechanism must be modified to include heme cn as an intermediate between heme bn and plastoquinone bound at a different site than in the bc1 complex. In addition, it is likely that the heme bn-cn couple participates in photosytem I-linked cyclic electron transport that requires ferredoxin and the ferredoxin: NADP+ reductase. This pathway through the n-side of the b6f complex could overlap with the n-side of the Q cycle pathway. Thus, either regulation is required at the level of the redox state of the hemes that would allow them to be shared by the two pathways, and/or the two different pathways are segregated in the membrane.

Spectral and redox characterization of the heme ci of the cytochrome b6f complex

Absorption spectra of the purified cytochrome b(6)f complex from Chlamydomonas reinhardtii were monitored as a function of the redox potential. Four spectral and redox components were identified: in addition to heme f and the two b hemes, the fourth component must be the new heme c(i) (also denoted x) recently discovered in the crystallographic structures. This heme is covalently attached to the protein, but has no amino acid axial ligand. It is located in the plastoquinone-reducing site Q(i) in the immediate vicinity of a b heme. Each heme titrated as a one-electron Nernst curve, with midpoint potentials at pH 7.0 of -130 mV and -35 mV (hemes b), +100 mV (heme c(i)), and +355 mV (heme f). The reduced minus oxidized spectrum of heme c(i) consists of a broad absorption increase centered approximately 425 nm. Its potential has a dependence of -60 mV/pH unit, implying that the reduced form binds one proton in the pH 6-9 range. The Q(i) site inhibitor 2-n-nonyl-4-hydroxyquinoline N-oxide, a semiquinone analogue, induces a shift of this potential by about -225 mV. The spectrum of c(i) matches the absorption changes previously observed in vivo for an unknown redox center denoted "G." The data are discussed with respect to the effect of the membrane potential on the electron transfer equilibrium between G and heme b(H) found in earlier experiments.

An atypical haem in the cytochrome b6f complex

Photosystems I and II (PSI and II) are reaction centres that capture light energy in order to drive oxygenic photosynthesis; however, they can only do so by interacting with the multisubunit cytochrome b(6)f complex. This complex receives electrons from PSII and passes them to PSI, pumping protons across the membrane and powering the Q-cycle. Unlike the mitochondrial and bacterial homologue cytochrome bc(1), cytochrome b(6)f can switch to a cyclic mode of electron transfer around PSI using an unknown pathway. Here we present the X-ray structure at 3.1 A of cytochrome b(6)f from the alga Chlamydomonas reinhardtii. The structure bears similarities to cytochrome bc(1) but also exhibits some unique features, such as binding chlorophyll, beta-carotene and an unexpected haem sharing a quinone site. This haem is atypical as it is covalently bound by one thioether linkage and has no axial amino acid ligand. This haem may be the missing link in oxygenic photosynthesis.

Structure of the cytochrome b6f complex of oxygenic photosynthesis: tuning the cavity

The cytochrome b6f complex provides the electronic connection between the photosystem I and photosystem II reaction centers of oxygenic photosynthesis and generates a transmembrane electrochemical proton gradient for adenosine triphosphate synthesis. A 3.0 angstrom crystal structure of the dimeric b6f complex from the thermophilic cyanobacterium Mastigocladus laminosus reveals a large quinone exchange cavity, stabilized by lipid, in which plastoquinone, a quinone-analog inhibitor, and a novel heme are bound. The core of the b6f complex is similar to the analogous respiratory cytochrome bc1 complex, but the domain arrangement outside the core and the complement of prosthetic groups are strikingly different. The motion of the Rieske iron-sulfur protein extrinsic domain, essential for electron transfer, must also be different in the b6f complex.

Electrogenic events associated with electron and proton transfers within the cytochrome b(6)/f complex

The kinetics and amplitude of the membrane potential changes associated with electron and proton transfers within the cytochrome b(6)/f (cyt b/f) complex (phase b) are measured in vivo in Chlamydomonas reinhardtii under anaerobic conditions. Upon saturating flash excitation, fast components in the membrane potential decay superimposed on phase b lead to an underestimation of the amplitude of this phase. In the FUD50 mutant strain, which lacks the ATP synthase, the decay of the membrane potential is slowed down compared to the wild type, and the kinetics and amplitude of phase b may be accurately determined. This amplitude corresponds to the transfer of at least 1.5 charges across the membrane per positive charge transferred to photosystem I, whatever the flash energy. This value largely exceeds that predicted by a Q-cycle process. Similar conclusions are reached using the wild type strain in the presence of 9 microM dicyclohexylcarbodiimide, which specifically inhibits the ATP synthase. It is concluded that a proton pumping process is operating in parallel with the Q-cycle, with a yield of approximately 0.5 proton pumped by cyt b/f complex turnover, irrespective of the flash energy.

Kinetic effects of the electrochemical proton gradient on plastoquinone reduction at the Qi site of the cytochrome b6f complex

We have investigated the effects of the light-induced thylakoid transmembrane potential on the turnover of the b(6)f complex in cells of the unicellular green alga Chlamydomonas reinhardtii. The reduction of the potential by either decreasing the light intensity or by adding increasing concentrations of the ionophore carbonylcyanide p-(trifluoromethoxy)phenylhydrazone (FCCP) revealed a marked inhibition of the cytochrome b(6) oxidation rate (10-fold) without substantial modifications of cytochrome f oxidation kinetics. Partial recovery of this inhibition could be obtained in the presence of ionophores provided that the membrane potential was re-established by illumination with a train of actinic flashes fired at a frequency higher than its decay. Measurements of isotopic effects on the kinetics of cytochrome b(6) oxidation revealed a synergy between the effects of ionophores and the H(2)O-D(2)O exchange. We propose therefore, that protonation events influence the kinetics of cytochrome b(6) oxidation at the Qi site and that these reactions are strongly influenced by the light-dependent generation of a transmembrane potential.

A new subunit of cytochrome b6f complex undergoes reversible phosphorylation upon state transition

A 15.2-kDa polypeptide, encoded by the nuclear gene PETO, was identified as a novel cytochrome b(6)f subunit in Chlamydomonas reinhardtii. The PETO gene product is a bona fide subunit, subunit V, of the cytochrome b(6)f complex, because (i) it copurifies with the other cytochrome b(6)f subunits in the early stages of the purification procedure, (ii) it is deficient in cytochrome b(6)f mutants accumulating little of the complex, and (iii) it colocalizes with cytochrome f, which migrates between stacked and unstacked membrane regions upon state transition. Sequence analysis and biochemical characterization of subunit V shows that it has a one transmembrane alpha-helix topology with two large hydrophilic domains extending on the stromal and lumenal side of the thylakoid membranes, with a lumenal location of the N terminus. Subunit V is reversibly phosphorylated upon state transition, a unique feature that, together with its topological organization, points to the possible role of subunit V in signal transduction during redox-controlled short term and long term adaptation of the photosynthetic apparatus in eukaryotes.

Structure and function of the chloroplast cytochrome bf complex

The chloroplast cytochrome bf complex is an intrinsic multisubunit protein from the thylakoid membrane consisting of four polypeptides: cytochrome f, a two heme containing cytochrome b 6, the Rieske iron-sulfur protein, and a 17 kD polypeptide of undefined function. The complex functions in electron transfer between PSII and PSI, where most mechanisms suggest that the transfer of a single reducing equivalent from plastoquinol to plastocyanin results in the translocation of two protons across the membrane. Primary sequence analyses, dichroism studies, and functional considerations allow the construction of an approximate structural model of a monomeric complex, although some evidence exists for a dimeric structure. Resolution of the properties of the two cytochrome b 6 hemes has relied upon the availability of purified solubilized complex, while evidence in the thylakoid suggests the difference between the two hemes are not as great in situ. Such variability in the spectroscopic and electrochemical properties of the cytochrome b 6 is a major concern during the experimental use of the purified complex. There is a general consensus that the complex contains a plastoquinol oxidizing (Qz) site, although the evidence for a plastoquinone reduction (Qc) site, called for in most mechanistic hypotheses, is less substantive. Probably the most severe challenge to the so called Q-cycle mechanism comes from experimental observations made with cytochrome b 6 initially reduced, where proposed interpretations more closely resemble a b-cycle than a Q-cycle. Although functional during cyclic electron transfer, the role of the complex and its possible interaction with other proteins, has not been completely resolved.

Kinetic measurements of electron transfer in coupled chromatophores from photosynthetic bacteria. A method of correction for the electrochromic effects

A quantitative study of the kinetics of electron transfer under coupled conditions in photosynthetic bacteria has so far been prevented by overlap of the electrochromic signals of carotenoids and bacteriochlorophyll with the absorbance changes of cytochromes and reaction centers. In this paper a method is presented by which the electrochromic contribution at any wavelength can be calculated from the electrochromic signal recorded at 505 nm, using a set of empirically determined polynomial functions. The electrochromic contribution to kinetic changes at any wavelength can then be subtracted to leave the true kinetics of the redox changes. The corrected redox changes of the reaction center measured at 542 and 605 nm mutually agree, thus providing an excellent test of self-consistency of the method. The corrected traces for reaction center and of cytochrome b-566 demonstrate large effects of the membrane potential on the rate and poise of electron transfer. It will be possible to study the interrelation between proton gradient and individual electron reactions under flash or steady-state illumination.

Oxygenic photoreduction of methyl viologen and nicotinamide adenine dinucleotide phosphate without the involvement of photosystem I during plastid development

Studies on the appearance of various electron transport functions were followed during greening of etiolated cucumber cotyledons. Appearance of dichlorodimethoxy-p-benzoquinone, dimethyl quinone, tetramethyl-p-phenylenediamine, dichlorophenol indophenol and ferricyanide Hill reactions were observed after 8h of greening. However, photoreduction of methyl viologen (MV) and nicotinamide adenine dinucleotide phosphate (NADP) was observed from 2h of greening. Variable fluorescence, which is a direct indication of water-splitting function, was observed from 2h of greening in cotyledons, thylakoid membranes and photosystem II (PSII) particles. The decrease in variable fluorescence in the presence of MV (due to rapid reoxidation of Q-) observed from early stages of greening confirmed the photoreduction of MV by PSII. The early development of water-splitting function was further confirmed by the abolition of variable fluorescence in thylakoid membranes and PSII particles by heat treatment and concomittant loss of light dependent oxygen uptake in the presence of MV in heat treated chloroplasts. However, the photoreduction of MV and NADP was insensitive to intersystem electron transport inhibitors, dichlorophenyl dimethylurea or dibromomethyl isopropyl-p-benzoquinone till 8h of greening. Though the oxidation of intersystem electron carrier cytochrome f was observed from early stages of greening, the reduction of cytochrome f was not observed till 8h of greening. All these observations confirm that during early stages of greening MV and NADP are photoreduced by PSII without the involvement of intersystem electron carriers or the collaboration of PSI. Since these observations are at variance with the currently prevalent concept (Z-Scheme) of the photosynthetic generation of reducing power, which requires definite collaboration of the two photosystems, an alternate electron flow pathway is proposed.

A redox study of the electron transport pathway responsible for generation of the slow electrochromic phase in chloroplasts

The amplitude of the slow phase of the electrochromic bandshift and the dark redox state of cytochrome b6, as well as its flash-induced turnover, have been measured as a function of ambient redox potential between +200 and -200 mV. Formation of a quinol-like donor with an Em,7 = +100 +/- 10 mV is required for generation of the slow phase. 80-100% of the amplitude of this signal with a t 1/2 = 3-4 ms is observed at -200 mV where cytochrome b6 was almost fully reduced (Em,7 of dark and flash-induced photoreduction was -30 mV and -75 mV, respectively). The change in the photoreduction of cytochrome b6 above 0 mV had an Em,7 of +50 mV, about 50 mV more negative than the midpoint at this pH for the onset of the slow electrochromic change. At potentials below -140 mV the amplitude of b6 photoreduction becomes small or negligible. The nature of the cytochrome b6 photoresponse is changed at potentials below -140 mV from a net photoreduction with a t1/2 = approximately less than 1 ms to a photooxidation with a t1/2 = 15-20 ms that is substantially slower than the electrochromic band-shift with a t1/2 = 3-4 ms. It is concluded that the slow electrochromic phase probably does not arise from a mechanism involving a turnover of cytochrome b6. From consideration of the possible flash-induced electron-transfer steps and alternative mechanisms for generation of the slow phase, it is suggested that it may arise from a redox-linked H+ pump involving the high potential iron-sulfur protein.