Diversity of cytochrome bc complexes: example of the Rieske protein in green sulfur bacteria

Soluble domains of cytochrome c-556 and Rieske iron-sulfur protein from Chlorobaculum tepidum: Crystal structures and interaction analysis

In photosynthetic green sulfur bacteria, the electron transfer reaction from menaquinol:cytochrome c oxidoreductase to the P840 reaction center (RC) complex occurs directly without any involvement of soluble electron carrier protein(s). X-ray crystallography has determined the three-dimensional structures of the soluble domains of the CT0073 gene product and Rieske iron-sulfur protein (ISP). The former is a mono-heme cytochrome c with an α-absorption peak at 556 nm. The overall fold of the soluble domain of cytochrome c-556 (designated as cyt c-556_sol) consists of four α-helices and is very similar to that of water-soluble cyt c-554 that independently functions as an electron donor to the P840 RC complex. However, the latter's remarkably long and flexible loop between the α3 and α4 helices seems to make it impossible to be a substitute for the former. The structure of the soluble domain of the Rieske ISP (Rieske_sol protein) shows a typical β-sheets-dominated fold with a small cluster-binding and a large subdomain. The architecture of the Rieske_sol protein is bilobal and belongs to those of b₆f-type Rieske ISPs. Nuclear magnetic resonance (NMR) measurements revealed weak non-polar but specific interaction sites on Rieske_sol protein when mixed with cyt c-556_sol. Therefore, menaquinol:cytochrome c oxidoreductase in green sulfur bacteria features a Rieske/cytb complex tightly associated with membrane-anchored cyt c-556.

Reduction midpoint potentials of bifurcating electron transfer flavoproteins

Recently, a variety of enzymes have been found to accept electrons from NAD(P)H yet reduce lower-potential carriers such as ferredoxin and flavodoxin semiquinone, in apparent violation of thermodynamics. The reaction is favorable overall, however, because these enzymes couple the foregoing endergonic one-electron transfer to exergonic transfer of the other electron from each NAD(P)H, in a process called "flavin-based electron bifurcation." The reduction midpoint potentials (E°s) of the multiple flavins in these enzymes are critical to their mechanisms. We describe methods we have found to be useful for measuring each of the E°s of each of the flavins in bifurcating electron transfer flavoproteins.

Hyperfine Sublevel Correlation Spectroscopy Studies of Iron-Sulfur Cluster in Rieske Protein from Green Sulfur Bacterium Chlorobaculum tepidum

The magnetic properties of the Rieske protein purified from Chlorobaculum tepidum were investigated using electron paramagnetic resonance and hyperfine sublevel correlation spectroscopy (HYSCORE). The g-values of the Fe₂S₂ center were g_x = 1.81, g_y = 1.90, and g_z = 2.03. Four classes of nitrogen signals were obtained by HYSCORE. Nitrogens 1 and 2 had relatively strong magnetic hyperfine couplings and were assigned as the nitrogen directly ligated to Fe. Nitrogens 3 and 4 had relatively weak magnetic hyperfine couplings and were assigned as the other nitrogen of the His ligands and peptide nitrogen connected to the sulfur atom via hydrogen bonding, respectively. The anisotropy of nitrogen 3 reflects the different spin density distributions on the His ligands, which influences the electron transfer to quinone.

Mechanism of flavin reduction and oxidation in the redox-sensing quinone reductase Lot6p from Saccharomyces cerevisiae

Quinone reductases are flavin-containing enzymes that have been implicated in protecting organisms from redox stress and, more recently, as redox switches controlling the action of the proteasome. The reactions of the catalytic cycle of the dimeric quinone reductase Lot6p from Saccharomyces cerevisiae were studied in anaerobic stopped-flow experiments at 4 degrees C. Both NADH and NADPH reacted similarly, reducing the FMN prosthetic group rapidly at saturation but binding with very low affinity. The enzyme stereospecifically transferred the proS-hydride of NADPH with an isotope effect of 3.6, indicating that hydride transfer, and not an enzyme conformational change, is rate-determining in the reductive half-reaction. No intermediates such as charge-transfer complexes were detected. In the oxidative half-reaction, reduced enzyme reacted in a single phase with the six quinone substrates tested. The observed rate constants increased linearly with quinone concentration up to the limits allowed by solubility, indicating either a bimolecular reaction or very weak binding. The logarithm of the bimolecular rate constant increases linearly with the reduction potential of the quinone, consistent with the notion that quinone reductases strongly disfavor radical intermediates. Interestingly, both half-reactions of the catalytic cycle strongly resemble bioorganic model reactions; the reduction of Lot6p by NAD(P)H is moderately faster than nonenzymatic models, while the oxidation of Lot6p by quinones is actually slower than nonenzymatic reactions. This curious situation is consistent with the structure of Lot6p, which has a crease we propose to be the binding site for pyridine nucleotides and a space, but no obvious catalytic residues, near the flavin allowing the quinone to react. The decidedly suboptimized catalytic cycle suggests that selective pressures other than maximizing quinone consumption shaped the evolution of Lot6p. This may reflect the importance of suppressing other potentially deleterious side reactions, such as oxygen reduction, or it may indicate that the role Lot6p plays as a redox sensor in controlling the proteasome is more important than its role as a detoxifying enzyme.

Reaction Intermediates of Quinol Oxidation in a Photoactivatable System that Mimics Electron Transfer in the Cytochromebc1Complex

Current competing models for the two-electron oxidation of quinol (QH2) at the cytochrome bc1 complex and related complexes impose distinct requirements for the reaction intermediate. At present, the intermediate species of the enzymatic oxidation process have not been observed or characterized, probably due to their transient nature. Here, we use a biomimetic oxidant, excited-state Ru(bpy)2(pbim)+ (bpy=2,2'-dipyridyl, pbim=2-(2-pyridyl)benzimidazolate) in an aprotic medium to probe the oxidation of the ubiquinol analogue, 2,3-dimethoxy-5-methyl-1,4-benzoquinol (UQH2-0), and the plastoquinol analogue, trimethyl-1,4-benzoquinol (TMQH2-0), using time-resolved and steady-state spectroscopic techniques. Despite its simplicity, this system qualitatively reproduces key features observed during ubiquinol oxidation by the mitochondrial cytochrome bc1 complex. Comparison of isotope-dependent activation properties in the native and synthetic systems as well as analysis of the time-resolved direct-detection electron paramagnetic resonance signals in the synthetic system allows us to conclude that (1) the initial and rate-limiting step in quinol oxidation, both in the biological and biomimetic systems, involves electron and proton transfer, probably via a proton-coupled electron-transfer mechanism, (2) a neutral semiquinone intermediate is formed in the biomimetic system, and (3) oxidation of the QH*/QH2 couple for UQH2-0, but not TMQH2-0, exhibits an unusual and unexpected primary deuterium kinetic isotope effect on its Arrhenius activation energy (DeltaGTS), where DeltaGTS for the protiated form is larger than that for the deuterated form. The same behavior is observed during steady-state turnover of the cyt bc1 complex using ubiquinol, but not plastoquinol, as a substrate, leading to the conclusion that similar chemical pathways are involved in both systems. The synthetic system is an unambiguous n=1 electron acceptor, and it is thus inferred that sequential oxidation of ubiquinol (by two sequential n=1 processes) is more rapid than a truly concerted (n=2) oxidation in the cyt bc1 complex.

The Cytochrome bc (1) Complex and its Homologue the b (6) f Complex: Similarities and Differences

The ubihydroquinone:cytochrome c oxidoreductase (also called complex III, or bc (1) complex), is a multi subunit enzyme encountered in a very broad variety of organisms including uni- and multi-cellular eukaryotes, plants (in their mitochondria) and bacteria. Most bacteria and mitochondria harbor various forms of the bc (1) complex, while plant and algal chloroplasts as well as cyanobacteria contain a homologous protein complex called plastohydroquinone:plastocyanin oxidoreductase or b (6) f complex. Together, these enzyme complexes constitute the superfamily of the bc complexes. Depending on the physiology of the organisms, they often play critical roles in respiratory and photosynthetic electron transfer events, and always contribute to the generation of the proton motive force subsequently used by the ATP synthase. Primarily, this review is focused on comparing the 'mitochondrial-type' bc (1) complex and the 'chloroplast-type' b (6) f complex both in terms of structure and function. Specifically, subunit composition, cofactor content and assembly, inhibitor sensitivity, proton pumping, concerted electron transfer and Fe-S subunit large-scale domain movement of these complexes are discussed. This is a timely undertaking in light of the structural information that is emerging for the b (6) f complex.

Orientation of the g-Tensor Axes of the Rieske Subunit in the Cytochrome bc1 Complex

The orientation of the g-tensors of the Rieske iron-sulfur protein subunit was determined in a single crystal of the bovine mitochondrial cytochrome bc1 complex with stigmatellin in the Qo quinol binding site. The g-tensor principal axes are skewed with respect to the Fe-Fe and S-S atom direction in the 2Fe2S cluster, which is allowed by the lack of rigorous symmetry of the cluster. The asymmetric unit in the crystal is the active dimer, and the g-tensor axes have slightly different orientations relative to the iron-sulfur cluster in the two halves of the dimer. The g approximately 1.79 axis makes an average angle of 30 degrees with respect to the Fe-Fe direction and the g approximately 2.024 axis an average angle of 26 degrees with respect to the S-S direction. This assignment of the g-tensor axis directions indicates that conformations of the Rieske protein are likely the same in the cytochrome bc1 and b6f complexes and that the extent of motion of the Rieske head domain during the catalytic cycle has been highly conserved during evolution of these distantly related complexes.

The naphthoquinol oxidizing cytochrome bc1 complex of the hyperthermophilic knallgasbacterium Aquifex aeolicus: properties and phylogenetic relationships

Phylogenetic analysis of constituent proteins of Rieske/cytochrome b complexes [Schütz et al. (2000) J. Mol. Biol. 300, 663-675] indicated that the respective enzyme from the hyperthermophile Aquifex (A.) aeolicus is closely related to proteobacterial counterparts, in disagreement with positioning of its parent species on small subunit rRNA trees. An assessment of the details and possible reasons for this discrepancy necessitates a thorough understanding of the biochemical and biophysical properties of the enzyme in addition to the bioinformatic data. The cytochrome bc(1) complex from A. aeolicus, which is part of the "Knallgasreaction" pathway, was therefore studied in membranes and in detergent-solubilized, isolated complex. Hemes b(L) (E(m,7) = -190 mV; g(z)= 3.7), b(H) (E(m,7) = -60 mV; g(z )= 3.45), and c(1) (E(m,7) = +160 mV; g(z )= 3.55) were identified by EPR and optical spectroscopy in combination with electrochemical methods. Two electrochemically distinct (E(m,7) = +95 mV; E(m,7) = +210 mV) Rieske centers were detected in membranes, and the +210 mV species was shown to correspond to the Rieske center of the cyt bc(1) complex. The gene coding for this latter Rieske protein was heterologously expressed in Escherichia coli, and the resulting protein was characterized in detail. The pool quinone of A. aeolicus was determined to be naphthoquinone. The redox poises of the individual electron-transfer steps are compared to those of other Rieske/cyt b complexes. The Aquifex enzyme was found to represent the only extant naphthoquinol oxidizing true cyt bc(1) complex described so far. An improved scenario for the phylogenetic positioning of the Aquifex cyt bc(1) complex is proposed.

The bc(1) complex of the iron-grown acidophilic chemolithotrophic bacterium Acidithiobacillus ferrooxidans functions in the reverse but not in the forward direction. Is there a second bc(1) complex?

Acidithiobacillus ferrooxidans is an acidophilic chemolithotrophic bacterium that can grow in the presence of either a weak reductant, Fe(2+), or reducing sulfur compounds that provide more energy for growth than Fe(2+). Here we first review the latest findings about the uphill electron transfer pathway established in iron-grown A. ferrooxidans, which has been found to involve a bc(1) complex. We then provide evidence that this bc(1) complex cannot function in the forward direction (exergonic reaction), even with an appropriate substrate. A search for the sequence of the three redox subunits of the A. ferrooxidans bc(1) complex (strain ATCC 19859) in the complete genome sequence of the A. ferrooxidans ATCC 23270 strain showed the existence of two different bc(1) complexes in A. ferrooxidans. Cytochrome b and Rieske protein sequence comparisons allowed us to point out some sequence particularities of these proteins in A. ferrooxidans. Lastly, we discuss the possible reasons for the existence of two different "classical" bc(1) complexes and put forward some suggestions as to what role these putative complexes may play in this acidophilic chemolithotrophic bacterium.

Cytochromes c555 from the hyperthermophilic bacterium Aquifex aeolicus (VF5). 1. Characterization of two highly homologous, soluble and membranous, cytochromes c555

Two distinct class I (monoheme) c-type cytochromes from the hyperthermophilic bacterium Aquifex aeolicus were studied by biochemical and biophysical methods (i.e., optical and EPR spectroscopy, electrochemistry). The sequences of these two heme proteins (encoded by the cycB1 and cycB2 genes) are close to identical (85% identity in the common part of the protein) apart from the presence of an N-terminal stretch of 62 amino acid residues present only in the cycB1 gene. A soluble cytochrome was purified and identified by N-terminal sequencing as the cycB2 gene product. It showed an alpha-peak at 555 nm, an E(m) value of +220 mV, and electron paramagnetic resonance parameters of gz = 2.89, gy = 2.287, and gx = 1.52. A firmly membrane-bound cytochrome characterized by nearly identical properties was detected and attributed to the cycB1 gene product. The very high degree of homology of its N-terminal part to cytochrome c553 from Heliobacterium gestii strongly suggests it to be anchored to the membrane via N-terminally attached lipid molecules. The two heme proteins were named cytochrome c555s (soluble) and cytochrome c555m (membranous). Electron paramagnetic resonance on partially ordered membrane multilayers suggests that the solvent-exposed heme domain of cytochrome c555m is flexible with respect to the membrane plane. Possible functional roles for both cytochromes are discussed.

Structure and function of cytochrome bc complexes

The cytochrome bc complexes represent a phylogenetically diverse group of complexes of electron-transferring membrane proteins, most familiarly represented by the mitochondrial and bacterial bc1 complexes and the chloroplast and cyanobacterial b6f complex. All these complexes couple electron transfer to proton translocation across a closed lipid bilayer membrane, conserving the free energy released by the oxidation-reduction process in the form of an electrochemical proton gradient across the membrane. Recent exciting developments include the application of site-directed mutagenesis to define the role of conserved residues, and the emergence over the past five years of X-ray structures for several mitochondrial complexes, and for two important domains of the b6f complex.

The membrane-extrinsic domain of cytochrome b(558/566) from the archaeon Sulfolobus acidocaldarius performs pivoting movements with respect to the membrane surface

The orientation of the membrane-attached cytochrome b(558/566)-haem with respect to the membrane was determined by electron paramagnetic resonance spectroscopy on two-dimensionally ordered oxidised membrane fragments from Sulfolobus acidocaldarius. Unlike the other redox centres in the membrane, the cytochrome b(558/566)-haem was found to cover a range of orientations between 25 degrees and 90 degrees. The described results are reminiscent of those obtained on the Rieske cluster of bc complexes and indicate that the membrane-extrinsic domain of cytochrome b(558/566) can perform pivoting motion between two extreme positions. Such a conformational flexibility is likely to play a role in electron transfer with its redox partners.

A cluster exposed: structure of the Rieske ferredoxin from biphenyl dioxygenase and the redox properties of Rieske Fe-S proteins

BACKGROUND

Ring-hydroxylating dioxygenases are multicomponent systems that initiate biodegradation of aromatic compounds. Many dioxygenase systems include Rieske-type ferredoxins with amino acid sequences and redox properties remarkably different from the Rieske proteins of proton-translocating respiratory and photosynthetic complexes. In the latter, the [Fe2S2] clusters lie near the protein surface, operate at potentials above +300 mV at pH 7, and express pH- and ionic strength-dependent redox behavior. The reduction potentials of the dioxygenase ferredoxins are approximately 150 mV and are pH-independent. These distinctions were predicted to arise from differences in the exposure of the cluster and/or interactions of the histidine ligands.

RESULTS

The crystal structure of BphF, the Rieske-type ferredoxin associated with biphenyl dioxygenase, was determined by multiwavelength anomalous diffraction and refined at 1.6 A resolution. The structure of BphF was compared with other Rieske proteins at several levels. BphF has the same two-domain fold as other Rieske proteins, but it lacks all insertions that give the others unique structural features. The BphF Fe-S cluster and its histidine ligands are exposed. However, the cluster has a significantly different environment in that five fewer polar groups interact strongly with the cluster sulfide or the cysteinyl ligands.

CONCLUSIONS

BphF has structural features consistent with a minimal and perhaps archetypical Rieske protein. Variations in redox potentials among Rieske clusters appear to be largely the result of local electrostatic interactions with protein partial charges. Moreover, it appears that the redox-linked ionizations of the Rieske proteins from proton-translocating complexes are also promoted by these electrostatic interactions.

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.

Early evolution of cytochrome bc complexes

Primary structures, functional characteristics and phylogenetic relationships of subunits of cytochrome bc complexes from phylogenetically diverse bacterial and archaeal species were analysed. A single case of lateral gene transfer, i.e. the import of an epsilon-proteobacterial cytochrome bc(1) complex into Aquificales, was identified. For the enzyme in the remainder of the species studied, the obtained phylogenies were globally in line with small subunit rRNA trees. The distribution of a few key phylogenetic markers, such as contiguousness of cytochrome b, nature of the c-type subunit or spacing between b-heme ligands, are discussed. A localised modification of previous tree topologies is proposed on the basis of the obtained data. The comparison of extant enzymes furthermore allowed us to define the minimal functional and evolutionary core of the enzyme. The data furthermore suggest that the ancestral enzyme was put together from subunits that previously had played a role in other electron transfer chains.

Specific mutagenesis of the rieske iron-sulfur protein in Rhodobacter sphaeroides shows that both the thermodynamic gradient and the pK of the oxidized form determine the rate of quinol oxidation by the bc(1) complex

In the Rieske iron-sulfur protein (ISP) of the ubiquinol:cytochrome c(2) oxidoreductase (bc(1) complex) of Rhodobacter sphaeroides, residue Tyr 156 is located close to the iron-sulfur cluster. Previous studies of the equivalent residue in both Saccharomyces cerevisiae [Denke, E., Merbitz-Zahradnik, T., Hatzfeld, O. M., Snyder, C. H., Link, T. A., and Trumpower, B. L. (1998) J. Biol. Chem. 273, 9085-9093] and Paracoccus denitrificans [Schroter, T., Hatzfeld, O. M., Gemeinhardt, S., Korn, M., Friedrich, T., Ludwig, B. , and Link, T. A. (1998) Eur. J. Biochem. 255, 100-106] have indicated that mutations at this site can lead to modifications in the redox potential of the ISP. To study the effect of similar modifications on the thermodynamic behavior and kinetics of partial reactions of the bc(1) complex upon flash activation, we have constructed four mutant strains of Rb. sphaeroides where Tyr 156 was mutated to His, Leu, Phe, or Trp. The bc(1) complex was assembled and able to support photosynthetic growth in all mutants. Three substitutions (Leu, Phe, Trp) led to alteration of the midpoint potential (E(m)) of the ISP and a slowing in rate of quinol oxidation, suggesting that electron transfer from quinol to the oxidized ISP controls the overall rate and that this step includes the high activation barrier. The Trp mutation led to an increase of approximately 1 pH unit in the pK value of the oxidized ISP. The pH dependence of the rate of quinol oxidation in this mutant was also shifted up by approximately 1 pH unit, showing the importance of the protonation state of the ISP for quinol oxidation. This provides support for a model in which the dissociated form of the oxidized ISP is required for formation of the enzyme-substrate complex [Ugulava, N., and Crofts, A. R. (1998) FEBS Lett. 440, 409-413].

A spectroscopic method for observing the domain movement of the Rieske iron-sulfur protein

The g-tensor orientation of the chemically reduced Rieske cluster in cytochrome bc(1) complex from Rhodovulum sulfidophilum with respect to the membrane was determined in the presence and absence of inhibitors and in the presence of oxidized and reduced quinone in the quinol-oxidizing-site (Q(o)-site) by EPR on two-dimensionally ordered samples. Almost identical orientations were observed when oxidized or reduced quinone, stigmatellin, or 5-(n-undecyl)-6-hydroxy-4,7-dioxobenzothiazole was present. Occupancy of the Q(o)-site by myxothiazole induced appearance of a minority population with a substantially differing conformation and presence of E-beta-methoxyacrylate-stilbene significantly reduced the contribution of the major conformation observed in the other cases. Furthermore, when the oxidized iron-sulfur cluster was reduced at cryogenic temperatures by the products of radiolysis, the orientation of its magnetic axes was found to differ significantly from that of the chemically reduced center. The "irradiation-induced" conformation converts to that of the chemically reduced center after thawing of the sample. These results confirm the effects of Q(o)-site inhibitors on the equilibrium conformation of the Rieske iron-sulfur protein and provide evidence for a reversible redox-influenced interconversion between conformational states. Moreover, the data obtained with the iron-sulfur protein demonstrate that the conformation of "EPR-inaccessible" reduction states of redox centers can be studied by inducing changes of redox state at cryogenic temperatures. This technique appears applicable to a wide range of comparable electron transfer systems performing redox-induced conformational changes.

On the spatial organization of hemes and chlorophyll in cytochrome b(6)f. A linear and circular dichroism study

The organization of chromophores in the cytochrome b(6) f from Chlamydomonas reinhardtii has been studied spectroscopically. Linear dichroism (LD) measurements, performed on the complex co-reconstituted into vesicles with photosynthetic reaction centers as an internal standard, allow the determination of the orientations of the chromophore with respect to the membrane plane. The orientations of the b(H)- and b(L)-hemes are comparable to those determined crystallographically on the cytochrome bc(1). The excitonic CD signal, resulting from the interaction between b-hemes, is similar to that reported for the cytochrome bc(1). LD and CD data are consistent with the differences between the b(6) f and bc(1) leaving the orientation of the b-hemes unaffected. By contrast, the LD data yield a different orientation for the heme f as compared either to the heme c(1) in the crystallographic structures or to the heme f as studied by electron paramagnetic resonance. This difference could either result from incorrect assumptions regarding the orientations of the electronic transitions of the f-heme or may point to the possibility of a redox-dependent movement of cytochrome f. The chlorophyll a was observed in a well defined orientation, further corroborating a specific binding site for it in the b(6) f complex.

Relative orientation of the hemes of the cytochrome bc(1) complexes from Rhodobacter sphaeroides, Rhodospirillum rubrum, and beef heart mitochondria. A linear dichroism study

The orientation of the chromophores in the cytochrome bc(1) of Rhodospirillum rubrum, Rhodobacter sphaeroides, and beef heart mitochondria is reported. The combination of redox-resolved absorption spectrophotometry and linear dichroism experiments at low temperature allows the determination of the orientation of the three hemes with respect to the membrane plane. The orientations of the b(H)-and b(L)-hemes of the R. sphaeroides and beef heart mitochondrial complexes are similar to those determined by crystallographic studies of the mitochondrial cytochrome bc(1). On the other hand the orientations of the b-hemes of the R. rubrum complex lead to the conclusion that the b(H)-heme is more perpendicular to the membrane plane than the b(L)-heme. This could be explained by a specific organization of the b-hemes due to subunit composition of the complex or, alternatively, to a different spatial position of the heme transitions with respect to the porphyrin macrocycle compared with the other complexes. Moreover, our results demonstrate a different orientation of the heme c(1) of the three studied complexes in comparison to crystallographic studies. This difference may arise from the above hypothesis on the transitions of the heme or from flexibility of this subunit in function of its redox state.

Effects of inhibitors on the activity of the cytochrome b(6)f complex: evidence for the existence of two binding pockets in the lumenal site

The effects of two inhibitors of electron transfer in the cytochrome b(6)f complex have been studied in whole cells of Chlamydomonas reinhardtii. DNP-INT affected equally the two steps of the concerted oxidation of plastoquinol at the Q(o) site; it decreased the rates of both cytochrome f reduction and cytochrome b(6) turnover, without affecting the amplitude of their redox signals. On the contrary, DBMIB inhibited only the rate of cytochrome f reduction while reducing, at the same time, the amplitude of cytochrome b(6) signals. The accessibility of DNP-INT to the Q(o) site was unaffected by preillumination, while that of DBMIB was greatly enhanced, even after a single turnover of the cytochrome b(6)f complex. Similar results were obtained with a mutant strain (FUD2) where the Q(o) site has an affinity for plastoquinol that is diminished by a factor of approximately 50 [Finazzi, G., et al. (1997) Biochemistry 36, 2867-2874]. However, the binding of the two inhibitors was differentially influenced by the mutation: a factor of approximately 250 was calculated for DNP-INT and a factor of only approximately 5 for DBMIB. This suggests that they bind within the Q(o) site in two distinct pockets, which are differentially involved in the process of quinol oxidation, in agreement with a recent model where two distinct positions for the reduced and semireduced quinones are considered [Crofts, A. R., and Berry, E. A. (1998) Curr. Opin. Struct. Biol. 8, 501-509].

Redox components of cytochrome bc-type enzymes in acidophilic prokaryotes. II. The Rieske protein of phylogenetically distant acidophilic organisms

The Rieske proteins of two phylogenetically distant acidophilic organisms, i.e. the proteobacterium Thiobacillus ferrooxidans and the crenarchaeon Sulfolobus acidocaldarius, were studied by EPR. Redox titrations at a range of pH values showed that the Rieske centers of both organisms are characterized by redox midpoint potential-versus-pH curves featuring a common pK value of 6.2. This pK value is significantly more acidic (by almost 2 pH units) than that of Rieske proteins in neutrophilic species. The orientations of the Rieske center's g tensors with respect to the plane of the membrane were studied between pH 4 and 8 using partially ordered samples. At pH 4, the Sulfolobus Rieske cluster was found in the "typical" orientation of chemically reduced Rieske centers, whereas this orientation changed significantly on going toward high pH values. The Thiobacillus protein, by contrast, appeared to be in the "standard" orientation at both low and high pH values. The results are discussed with respect to the molecular parameters conveying acid resistance and in light of the recently demonstrated long-range conformational movement of the Rieske protein during enzyme turnover in cytochrome bc1 complexes.

The Qo-site inhibitor DBMIB favours the proximal position of the chloroplast Rieske protein and induces a pK-shift of the redox-linked proton

The interaction of the inhibitor 2,5-dibromo-3-methyl-6-isopropylbenzoquinone (DBMIB) with the Rieske protein of the chloroplast b6f complex has been studied by EPR. All three redox states of DBMIB were found to interact with the iron-sulphur cluster. The presence of the oxidised form of DBMIB altered the equilibrium distribution of the Rieske protein's conformational substates, strongly favouring the proximal position close to heme bL. In addition to this conformational effect, DBMIB shifted the pK-value of the redox-linked proton involved in the iron-sulphur cluster's redox transition by about 1.5 pH units towards more acidic values. The implications of these results with respect to the interaction of the native quinone substrate and the Rieske cluster in cytochrome bc complexes are discussed.