Interactions between lipids and bacterial reaction centers determined by protein crystallography

Intrinsic curvature properties of photosynthetic proteins in chromatophores

In purple bacteria, photosynthesis is carried out on large indentations of the bacterial plasma membrane termed chromatophores. Acting as primitive organelles, chromatophores are densely packed with the membrane proteins necessary for photosynthesis, including light harvesting complexes LH1 and LH2, reaction center (RC), and cytochrome bc(1). The shape of chromatophores is primarily dependent on species, and is typically spherical or flat. How these shapes arise from the protein-protein and protein-membrane interactions is still unknown. Now, using molecular dynamics simulations, we have observed the dynamic curvature of membranes caused by proteins in the chromatophore. A membrane-embedded array of LH2s was found to relax to a curved state, both for LH2 from Rps. acidophila and a homology-modeled LH2 from Rb. sphaeroides. A modeled LH1-RC-PufX dimer was found to develop a bend at the dimerizing interface resulting in a curved shape as well. In contrast, the bc(1) complex, which has not been imaged yet in native chromatophores, did not induce a preferred membrane curvature in simulation. Based on these results, a model for how the different photosynthetic proteins influence chromatophore shape is presented.

Atomic-level structural and functional model of a bacterial photosynthetic membrane vesicle

The photosynthetic unit (PSU) of purple photosynthetic bacteria consists of a network of bacteriochlorophyll-protein complexes that absorb solar energy for eventual conversion to ATP. Because of its remarkable simplicity, the PSU can serve as a prototype for studies of cellular organelles. In the purple bacterium Rhodobacter sphaeroides the PSU forms spherical invaginations of the inner membrane, approximately 70 nm in diameter, composed mostly of light-harvesting complexes, LH1 and LH2, and reaction centers (RCs). Atomic force microscopy studies of the intracytoplasmic membrane have revealed the overall spatial organization of the PSU. In the present study these atomic force microscopy data were used to construct three-dimensional models of an entire membrane vesicle at the atomic level by using the known structure of the LH2 complex and a structural model of the dimeric RC-LH1 complex. Two models depict vesicles consisting of 9 or 18 dimeric RC-LH1 complexes and 144 or 101 LH2 complexes, representing a total of 3,879 or 4,464 bacteriochlorophylls, respectively. The in silico reconstructions permit a detailed description of light absorption and electronic excitation migration, including computation of a 50-ps excitation lifetime and a 95% quantum efficiency for one of the model membranes, and demonstration of excitation sharing within the closely packed RC-LH1 dimer arrays.

Structure and function of glycoglycerolipids in plants and bacteria

Phosphoglycerolipids are abundant membrane constituents in prokaryotic and eukaryotic cells. However, glycoglycerolipids are the predominant lipids in chloroplasts of plants and eukaryotic algae and in cyanobacteria. Membrane composition in chloroplasts and cyanobacteria is highly conserved, with monogalactosyldiacylglycerol (MGD) and digalactosyldiacylglycerol (DGD) representing the most abundant lipids. The genes encoding enzymes of galactolipid biosynthesis have been isolated from Arabidopsis. Galactolipids are crucial for growth under normal and phosphate limiting conditions. Furthermore, they are indispensable for maximal efficiency of photosynthesis. A wide variety of glycoglycerolipids is found in different bacteria. These lipids contain glucose or galactose, in some cases also mannose or other sugars with different glycosidic linkages in their head group. Some bacterial species produce unusual glycoglycerolipids, such as glycophospholipids or glycoglycerolipids carrying sugar head groups esterified with acyl residues. A number of genes coding for bacterial glycoglycerolipid synthases have been cloned and the enzymes characterized. In contrast to the breadth of information available on their structural diversity, much less is known about functional aspects of bacterial glycoglycerolipids. In some bacteria, glycoglycerolipids are required for membrane bilayer stability, they serve as precursors for the formation of complex membrane components, or they are crucial to support anoxygenic photosynthesis or growth during phosphate deficiency.

The thermotropic phase behaviour and phase structure of a homologous series of racemic β-d-galactosyl dialkylglycerols studied by differential scanning calorimetry and X-ray diffraction

The thermotropic phase behaviour of aqueous dispersions of some synthetic 1,2-di-O-alkyl-3-O-(beta-D-galactosyl)-rac-glycerols (rac-beta-D-GalDAGs) with both odd and even hydrocarbon chain lengths was studied by differential scanning calorimetry (DSC), small-angle (SAXS) and wide-angle (WAXS) X-ray diffraction. DSC heating curves show a complex pattern of lamellar (L) and nonlamellar (NL) phase polymorphism dependent on the sample's thermal history. On cooling from 95 degrees C and immediate reheating, rac-beta-D-GalDAGs typically show a single, strongly energetic phase transition, corresponding to either a lamellar gel/liquid-crystalline (L(beta)/L(alpha)) phase transition (N< or =15 carbon atoms) or a lamellar gel/inverted hexagonal (L(beta)/H(II)) phase transition (N> or =16). At higher temperatures, some shorter chain compounds (N=10-13) exhibit additional endothermic phase transitions, identified as L/NL phase transitions using SAXS/WAXS. The NL morphology and the number of associated intermediate transitions vary with hydrocarbon chain length. Typically, at temperatures just above the L(alpha) phase boundary, a region of phase coexistence consisting of two inverted cubic (Q(II)) phases are observed. The space group of the cubic phase seen on initial heating has not been determined; however, on further heating, this Q(II) phase disappears, enabling the identification of the second Q(II) phase as Pn3 m (space group Q(224)). Only the Pn3 m phase is seen on cooling. Under suitable annealing conditions, rac-beta-D-GalDAGs rapidly form highly ordered lamellar-crystalline (L(c)) phases at temperatures above (N< or =15) or below (N=16-18) the L(beta)/L(alpha) phase transition temperature (T(m)). In the N< or =15 chain length lipids, DSC heating curves show two overlapping, highly energetic, endothermic peaks on heating above T(m); corresponding changes in the first-order spacings are observed by SAXS, accompanied by two different, complex patterns of reflections in the WAXS region. The WAXS data show that there is a difference in hydrocarbon chain packing, but no difference in bilayer dimensions or hydrocarbon chain tilt for these two L(c) phases (termed L(c1) and L(c2), respectively). Continued heating of suitably annealed, shorter chain rac-beta-D-GalDAGs from the L(c2) phase results in a phase transition to an L(alpha) phase and, on further heating, to the same Q(II) or H(II) phases observed on first heating. On reheating annealed samples with longer chain lengths, a subgel phase is formed. This is characterized by a single, poorly energetic endotherm visible below the T(m). SAXS/WAXS identifies this event as an L(c)/L(beta) phase transition. However, the WAXS reflections in the di-16:0 lipid do not entirely correspond to the reflections seen for either the L(c1) or L(c2) phases present in the shorter chain rac-beta-D-GalDAGs; rather these consist of a combination of L(c1), L(c2) and L(beta) reflections, consistent with DSC data where all three phase transitions occur within a span of 5 degrees C. At very long chain lengths (N> or =19), the L(beta)/L(c) conversion process is so slow that no L(c) phases are formed over the time scale of our experiments. The L(beta)/L(c) phase conversion process is significantly faster than that seen in the corresponding rac-beta-D-GlcDAGs, but is slower than in the 1,2-sn-beta-D-GalDAGs already studied. The L(alpha)/NL phase transition temperatures are also higher in the rac-beta-D-GalDAGs than in the corresponding rac-beta-D-GlcDAGs, suggesting that the orientation of the hydroxyl at position 4 and the chirality of the glycerol molecule in the lipid/water interface influence both the L(c) and NL phase properties of these lipids, probably by controlling the relative positions of hydrogen bond donors and acceptors in the polar region of the membrane.

Isolation and characterization of lipids strictly associated to PSII complexes: Focus on cardiolipin structural and functional role

In this work, lipid extracts from spinach membrane fragments enriched in Photosystem II (PSII) and from spinach PSII dimers were analyzed, by means of Thin Layer Chromatography (TLC) and Electro-Spray Ionization Mass Spectrometry. Cardiolipin found in association with PSII was isolated and purified by preparative TLC, then characterized by mass and mass-mass analyses. Cardiolipin structures with four unsaturated C18 acyl chains and variable saturation degrees were evidenced. Structural and functional effects of different phospholipids on PSII complexes were investigated by Fluorescence, Resonance Light Scattering and Oxygen Evolution Rate measurements. An increment of PSII thermal stability was observed in the presence of cardiolipin and phosphatidylglycerol.

Stabilization of charge separation and cardiolipin confinement in antenna-reaction center complexes purified from Rhodobacter sphaeroides

The reaction center-light harvesting complex 1 (RC-LH1) purified from the photosynthetic bacterium Rhodobacter sphaeroides has been studied with respect to the kinetics of charge recombination and to the phospholipid and ubiquinone (UQ) complements tightly associated with it. In the antenna-RC complexes, at 6.5<pH<9.0, P(+)Q(B)(-) recombines with a pH independent average rate constant <k> more than three times smaller than that measured in LH1-deprived RCs. At increasing pH values, for which <k> increases, the deceleration observed in RC-LH1 complexes is reduced, vanishing at pH >11.0. In both systems kinetics are described by a continuous rate distribution, which broadens at pH >9.5, revealing a strong kinetic heterogeneity, more pronounced in the RC-LH1 complex. In the presence of the antenna the Q(A)Q(B)(-) state is stabilized by about 40 meV at 6.5<pH<9.0, while it is destabilized at pH >11. The phospholipid/RC and UQ/RC ratios have been compared in chromatophore membranes, in RC-LH1 complexes and in the isolated peripheral antenna (LH2). The UQ concentration in the lipid phase of the RC-LH1 complexes is about one order of magnitude larger than the average concentration in chromatophores and in LH2 complexes. Following detergent washing RC-LH1 complexes retain 80-90 phospholipid and 10-15 ubiquinone molecules per monomer. The fractional composition of the lipid domain tightly bound to the RC-LH1 (determined by TLC and (31)P-NMR) differs markedly from that of chromatophores and of the peripheral antenna. The content of cardiolipin, close to 10% weight in chromatophores and LH2 complexes, becomes dominant in the RC-LH1 complexes. We propose that the quinone and cardiolipin confinement observed in core complexes reflects the in vivo heterogeneous distributions of these components. Stabilization of the charge separated state in the RC-LH1 complexes is tentatively ascribed to local electrostatic perturbations due to cardiolipin.

pH modulates the quinone position in the photosynthetic reaction center from Rhodobacter sphaeroides in the neutral and charge separated states

The structure of the photosynthetic reaction-center from Rhodobacter sphaeroides has been determined at four different pH values (6.5, 8.0, 9.0, 10.0) in the neutral and in charge separated states. At pH 8.0, in the neutral state, we obtain a resolution of 1.87 A, which is the best ever reported for the bacterial reaction center protein. Our crystallographic data confirm the existence of two different binding positions of the secondary quinone (QB). We observe a new orientation of QB in its distal position, which shows no ring-flip compared to the orientation in the proximal position. Datasets collected for the different pH values show a pH-dependence of the population of the proximal position. The new orientation of QB in the distal position and the pH-dependence could be confirmed by continuum electrostatics calculations. Our calculations are in agreement with the experimentally observed proton uptake upon charge separation. The high resolution of our crystallographic data allows us to identify new water molecules and external residues being involved in two previously described hydrogen bond proton channels. These extended proton-transfer pathways, ending at either of the two oxo-groups of QB in its proximal position, provide additional evidence that ring-flipping is not required for complete protonation of QB upon reduction.

Structures and physiological roles of 13 integral lipids of bovine heart cytochrome c oxidase

All 13 lipids, including two cardiolipins, one phosphatidylcholine, three phosphatidylethanolamines, four phosphatidylglycerols and three triglycerides, were identified in a crystalline bovine heart cytochrome c oxidase (CcO) preparation. The chain lengths and unsaturated bond positions of the fatty acid moieties determined by mass spectrometry suggest that each lipid head group identifies its specific binding site within CcOs. The X-ray structure demonstrates that the flexibility of the fatty acid tails facilitates their effective space-filling functions and that the four phospholipids stabilize the CcO dimer. Binding of dicyclohexylcarbodiimide to the O(2) transfer pathway of CcO causes two palmitate tails of phosphatidylglycerols to block the pathway, suggesting that the palmitates control the O(2) transfer process.The phosphatidylglycerol with vaccenate (cis-Delta(11)-octadecenoate) was found in CcOs of bovine and Paracoccus denitrificans, the ancestor of mitochondrion, indicating that the vaccenate is conserved in bovine CcO in spite of the abundance of oleate (cis-Delta(9)-octadecenoate). The X-ray structure indicates that the protein moiety selects cis-vaccenate near the O(2) transfer pathway against trans-vaccenate. These results suggest that vaccenate plays a critical role in the O(2) transfer mechanism.

Stabilization effect of single-walled carbon nanotubes on the functioning of photosynthetic reaction centers

The interaction between single-walled carbon nanotubes and photosynthetic reaction centers purified from purple bacterium Rhodobacter sphaeroides R-26 has been investigated. Atomic force microscopy studies provide evidence that reaction center protein can be attached effectively to the nanotubes. The typical diameter of the nanotube is 1-4 nm and 15 +/- 2 nm without and with the reaction centers, respectively. Light-induced absorption change measurements indicate the stabilization of the P+(Q(A)Q(B))- charge pair, which is formed after single saturating light excitation after the attachment to nanotubes. The separation of light-induced charges is followed by slow reorganization of the protein structure. The stabilization effect of light-initiated charges by the carbon nanotubes opens a possible direction of several applications, the most promising being in energy conversion and storage devices.

Hypothesis on chlorosome biogenesis in green photosynthetic bacteria

Chlorosomes are specialized compartments that constitute the main light harvesting system of green sulfur bacteria (GSB) and some filamentous anoxygenic phototrophs (FAP). Chlorosome biogenesis promises to be a complex process requiring the generation of a unilayer membrane and the targeting of bacteriochlorophyll, carotenoids, quinones, and proteins to the chlorosome. The biogenesis of chlorosomes as well as their presence in two distinct bacterial groups, GSB and FAP, remains enigmatic. The photosynthetic machinery and overall metabolic characteristics of these two bacterial groups are very different, and horizontal gene transfer has been proposed to explain chlorosome distribution. Chlorosomes have been considered to be unique structures that require a specific assembly machinery. We propose that no special machinery is required for chlorosome assembly. Instead, it is suggested that chlorosomes are a special form of lipid body. We present a model for chlorosome biogenesis that combines aspects of lipid body biogenesis with established chlorosome characteristics and may help explain the presence of chlorosomes in two metabolically diverse organism groups.

Enthalpy/entropy driven activation of the first interquinone electron transfer in bacterial photosynthetic reaction centers embedded in vesicles of physiologically important phospholipids

The thermodynamics and kinetics of light-induced electron transfer in bacterial photosynthetic RCs are sensitive to physiologically important lipids (phosphatidylcholine, cardiolipin and phosphatidylglycerol) in the environment. The analysis of the temperature-dependence of the rate of the P(+)Q(A)(-)Q(B)-->P(+)Q(A)Q(B)(-) interquinone electron transfer revealed high enthalpy change of activation in zwitterionic or neutral micelles and vesicles and low enthalpy change of activation in vesicles constituted of negatively charged phospholipids. The entropy change of activation was compensated by the changes of enthalpy, thus the free energy change of activation ( approximately 500 meV) did not show large variation in vesicles of different lipids.

Lipids in photosynthetic reaction centres: structural roles and functional holes

Photosynthetic proteins power the biosphere. Reaction centres, light harvesting antenna proteins and cytochrome b(6)f (or bc(1)) complexes are expressed at high levels, have been subjected to an intensive spectroscopic, biochemical and mutagenic analysis, and several have been characterised to an informatively high resolution by X-ray crystallography. In addition to revealing the structural basis for the transduction of light energy, X-ray crystallography has brought molecular insights into the relationships between these multicomponent membrane proteins and their lipid environment. Lipids resolved in the X-ray crystal structures of photosynthetic proteins bind light harvesting cofactors, fill intra-protein cavities through which quinones can diffuse, form an important part of the monomer-monomer interface in multimeric structures and may facilitate structural flexibility in complexes that undergo partial disassembly and repair. It has been proposed that individual lipids influence the biophysical properties of reaction centre cofactors, and so affect the rate of electron transfer through the complex. Lipids have also been shown to be important for successful crystallisation of photosynthetic proteins. Comparison of the three types of reaction centre that have been structurally characterised reveals interesting similarities in the position of bound lipids that may point towards a generic requirement to reinforce the structure of the core electron transfer domain. The crystallographic data are also providing new opportunities to find molecular explanations for observed effects of different types of lipid on the structure, mechanism and organisation of reaction centres and other photosynthetic proteins.

Photosynthetic reaction center functionalized nano-composite films: effective strategies for probing and exploiting the photo-induced electron transfer of photosensitive membrane protein

Photosynthetic reaction center (RC), a robust transmembrane pigment-protein complex, works as the crucial component participating the primary event of the photo-electrochemical conversion in bacteria. Sparked by the high photo-induced charge separation yield (ca. 100%) of RC, great interests have been aroused to fabricate versatile RC-functionalized nano-composite films for exploring the initial photosynthetic electron transfer (ET) of RC, and thus exploiting well-designed bio-photoelectric converters. In this review, we classify and summarize the current status about the concepts and methods of constructing RC-immobilized nano-composite films or devices for probing the photo-induced ET, and applying to novel bioelectronics if it is possible.

Problems in obtaining diffraction-quality crystals of hetero-oligomeric integral membrane proteins

The major barrier responsible for the slow pace of structure determination of integral membrane proteins is the difficulty of crystallizing detergent-solubilized hydrophobic proteins, particularly hetero-oligomeric integral membrane proteins. For the latter class of multi-subunit proteins, we have encountered the following problems in addition to the ubiquitous problem of detergent compatibility: (i) instability caused by over-purification that results in delipidation; (ii) protease activity degrading exposed loops and termini of subunits of the complex that could not be inhibited; (iii) poor protein-protein contacts presumably arising from masking by the detergent micelle. Problem (i) could be ameliorated in crystallization of the cytochrome b(6)f complex by augmenting the delipidated complex with synthetic lipid. Problem (ii) has not been solved. Problem (iii) has been solved in other systems by the use of monoclonal antibodies (or other protein ligands) to increase the probability of protein-protein contacts. In the case of the complex formed by the cobalamin and colicin receptor, BtuB, and the receptor binding domain of colicin E3, the latter served as a ligand for protein-protein contacts that facilitated crystallization.

Lipid conformation in crystalline bilayers and in crystals of transmembrane proteins

Dihedral torsion angles evaluated for the phospholipid molecules resolved in the X-ray structures of transmembrane proteins in crystals are compared with those of phospholipids in bilayer crystals, and with the phospholipid conformations in fluid membranes. Conformations of the lipid glycerol backbone in protein crystals are not restricted to the gauche C1-C2 rotamers found invariably in phospholipid bilayer crystals. Lipid headgroup conformations in protein crystals also do not conform solely to the bent-down conformation, with gauche-gauche configuration of the phospho-diester, that is characteristic of phospholipid bilayer membranes. This suggests that the lipids that are resolved in crystals of membrane proteins are not representative of the entire lipid-protein interface. Much of the chain configurational disorder of the membrane-bound lipids in crystals arises from energetically disallowed skew conformations. This indicates a configurational heterogeneity in the lipids at a single binding site: eclipsed conformations occur also in some glycerol backbone torsion angles and C-C torsion angles in the lipid headgroups. Stereochemical violations in the protein-bound lipids are evidenced by one-third of the ester carboxyl groups in non-planar configurations, and certain of the carboxyls in the cis configuration. Some of the lipid structures in protein crystals have the incorrect enantiomeric configuration of the glycerol backbone, and many of the branched methyl groups in structures of the phytanyl chains associated with bacteriorhodopsin crystals are in the incorrect S-configuration.

On the role of basic residues in adapting the reaction centre-LH1 complex for growth at elevated temperatures in purple bacteria

The purple photosynthetic bacterium Thermochromatium tepidum is a moderate thermophile, with a growth optimum of 48-50 degrees C. The X-ray crystal structure of the reaction centre from this organism has been determined, and compared with that from mesophilic bacteria such as Blastochloris viridis and Rhodobacter sphaeroides (Nogi T et al. (2000) Proc Natl Acad Sci USA 97: 13561-13566). Structural features that could contribute to the enhanced thermal stability of the Thermochromatium tepidum reaction centre were discussed, including three arginine residues exposed at the periplasmic side of the membrane that are not present in reaction centres from mesophilic organisms, and potentially could increase the affinity of the complex for the surrounding membrane. In the present report these arginine residues, plus a histidine identified from an extensive sequence alignment, were engineered into structurally homologous positions in the Rhodobacter sphaeroides reaction centre, and the effect on the thermal stability of the Rhodobacter sphaeroides complex was examined. We find that these residues do not enhance the thermal stability of the reaction centre, as assessed by absorbance spectroscopy of the bacteriochlorin cofactors in membrane-bound reaction centres. Possible roles of these residues in the Thermochromatium tepidum reaction centre are discussed, and it is proposed that they facilitate stronger binding of the reaction centre to the encircling LH1 antenna complex, through ionic interactions with acidic residues at the C-terminal end of the LH1 alpha-polypeptide. Such an interaction could enhance the stability of the so-called 'RC-LH1 core' complex that is formed between the reaction centre and the LH1 antenna, and which represents the minimal functional photosynthetic unit in all known purple photosynthetic bacteria. Stronger bonding interactions between the two complexes could also contribute to an increase in the rigidity of the photosynthetic membrane in Thermochromatium tepidum, in accord with the general finding that the cytoplasmic membrane from thermophilic eubacteria is less fluid than its counterpart in mesophilic bacteria.

Glycoengineering of cyanobacterial thylakoid membranes for future studies on the role of glycolipids in photosynthesis

The lipid composition of thylakoid membranes is conserved from cyanobacteria to angiosperms. The predominating components are monogalactosyl- and digalactosyldiacylglycerol. In cyanobacteria, thylakoid membrane biosynthesis starts with the formation of monoglucosyldiacylglycerol which is C4-epimerized to the corresponding galactolipid, whereas in plastids monogalactosyldiacylglycerol is formed at the beginning. This suggests that galactolipids have specific functions in thylakoids. We wanted to investigate whether galactolipids can be replaced by glycosyldiacylglycerols with headgroups differing in their epimeric and anomeric details as well as the attachment point of the terminal hexose in diglycosyldiacylglycerols. For this purpose putative glycosyltransferase sequences were identified in databases to be used for functional expression in various host organisms. From 18 newly identified sequences, four turned out to encode glycosyltransferases catalyzing final steps in glycolipid biosynthesis: two alpha-glucosyltransferases, one beta-galactosyltransferase and one beta-glucosyltransferase. Their functional annotation was based on detailed structural characterization of the new glycolipids formed in the transformant hosts as well as on in vitro enzymatic assays. The expression of alpha-glucosyltransferases in the cyanobacterium Synechococcus resulted in the accumulation of the new alpha-galactosyldiacylglycerol which is ascribed to epimerization of the corresponding glucolipid. The expression of the beta-glucosyltransferase led to a high proportion of new beta-glucosyl-(1-->6)-beta-galactosyldiacylglycerol almost entirely replacing the native digalactosyldiacylglycerol. These results demonstrate that modifications of the glycolipid pattern in thylakoids are possible.

Lipids in and around photosynthetic reaction centres

Reaction centres are membrane-embedded pigment–protein complexes that transduce the energy of sunlight into a biologically useful form. The most heavily studied reaction centres are the PS-I (Photosystem I) and PS-II complexes from oxygenic phototrophs, and the reaction centre from purple photosynthetic bacteria. A great deal is known about the compositions and structures of these reaction centres, and the mechanism of light-activated transmembrane electron transfer, but less is known about how they interact with other components of the photosynthetic membrane, including the membrane lipids. X-ray crystallography has provided high-resolution structures for PS-I and the purple bacterial reaction centre, and revealed binding sites for a number of lipids, either embedded in the protein interior or attached to the protein surface. These lipids play a variety of roles, including the binding of cofactors and the provision of structural support. The challenges of modelling surface-associated electron density features such as lipids, detergents, small amphiphiles and ions are discussed.

Influence of Cardiolipin on the Functionality of the QA Site of the Photosynthetic Bacterial Reaction Center

The effect of cardiolipin on the functionality of the Q(A) site of a photosynthetic reaction center (RC) was studied in RCs from the purple non-sulfur bacterium Rhodobacter sphaeroides by means of time-resolved absorbance measurements. The binding of the ubiquinone-10 to the Q(A) site of the RC embedded in cardiolipin or lecithin liposomes has been followed at different temperatures and phospholipid loading. A global fit of the experimental data allowed us to get quite reliable values of the thermodynamic parameters joined to the binding process. The presence of cardiolipin does not affect the affinity of the Q(A) site for ubiquinone but has a marked influence on the rate of P+QA(-) --> PQA electron transfer. The P+QA(-) charge recombination kinetics has been examined in liposomes made of cardiolipin/lecithin mixtures and in detergent (DDAO) micelles doped with cardiolipin. The electron-transfer rate constant increases upon cardiolipin loading. It appears that the main effect of cardiolipin on the electron transfer can be ascribed to a destabilization of the charge-separated state. Results obtained in micelles and vesicles follow the same titration curve when cardiolipin concentration evaluated with respect to the apolar phase is used as a relevant variable. The dependence of the P+QA(-) recombination rate on cardiolipin loading suggests two classes of binding sites. In addition to a high-affinity site (compatible with previous crystallographic studies), a cooperative binding, involving about four cardiolipin molecules, takes place at high cardiolipin loading.

New tetragonal form of reaction centers from Rhodobacter sphaeroides and the involvement of a manganese ion at a crystal contact point

Crystals have been obtained of wild-type reaction centers from Rhodobacter sphaeroides using manganese chloride as a precipitating agent. The crystals belong to the tetragonal space group P4(2)22, with unit-cell parameters a = b = 207.8, c = 107.5 A. The crystal structure has been determined to a resolution limit of 4.6 A using a previously determined structure of the reaction center as a molecular-replacement model. The calculated electron-density maps show the presence of a manganese ion at one of the crystal contact points bridging two symmetry-related histidine residues, suggesting that the metal plays a key role in facilitating the crystallization of the protein in this form.

Strong Effects of an Individual Water Molecule on the Rate of Light-driven Charge Separation in the Rhodobacter sphaeroides Reaction Center

The role of a water molecule (water A) located between the primary electron donor (P) and first electron acceptor bacteriochlorophyll (B(A)) in the purple bacterial reaction center was investigated by mutation of glycine M203 to leucine (GM203L). The x-ray crystal structure of the GM203L reaction center shows that the new leucine residue packs in such a way that water A is sterically excluded from the complex, but the structure of the protein-cofactor system around the mutation site is largely undisturbed. The results of absorbance and resonance Raman spectroscopy were consistent with either the removal of a hydrogen bond interaction between water A and the keto carbonyl group of B(A) or a change in the local electrostatic environment of this carbonyl group. Similarities in the spectroscopic properties and x-ray crystal structures of reaction centers with leucine and aspartic acid mutations at the M203 position suggested that the effects of a glycine to aspartic acid substitution at the M203 position can also be explained by steric exclusion of water A. In the GM203L mutant, loss of water A was accompanied by an approximately 8-fold slowing of the rate of decay of the primary donor excited state, indicating that the presence of water A is important for optimization of the rate of primary electron transfer. Possible functions of this water molecule are discussed, including a switching role in which the redox potential of the B(A) acceptor is rapidly modulated in response to oxidation of the primary electron donor.

Cyanobacterial photosystem II at 3.2 A resolution - the plastoquinone binding pockets

Photosystem II from thylakoid membranes of the thermophilic cyanobacterium Thermosynechococcus elongatus was solubilized with n-beta-dodecylmaltoside and purified using anion exchange chromatography. Molecular weight, pigment stoichiometry and subunit composition were assayed using various techniques. The holocomplex is dimeric with a molecular mass of 756 +/- 18 kDa and functionally fully active. Crystals obtained from these samples showed significantly improved quality leading to a 3D structure at 3.2 A resolution. Several loop regions of the principal protein subunits are now defined that were not interpretable at lower (3.8 A) resolution, thus resulting in a more complete model. The head groups of the cofactors of the electron transfer chain and of the antennae have been modeled, coordinating and hydrogen bonding amino acids identified and the nature of the binding pockets derived. The orientations of these cofactors resemble those of the reaction centre from anoxygenic purple bacteria. For the two plastoquinones, electron density was only found for the head group of QA and none for QB indicating low or even no occupancy of this site in the crystal structure. Both binding pockets and problems related to the QB site are discussed here and compared to the situation in the purple bacterial reaction centre.

Role of mitochondrial cardiolipin peroxidation in apoptotic photokilling of 5-aminolevulinate-treated tumor cells

In 5-aminolevulinic acid (ALA)-based photodynamic therapy (PDT), ALA taken up by tumor cells is metabolized to protoporphyrin IX (PpIX), which sensitizes photodamage leading to apoptotic or necrotic cell death. Since lipophilic PpIX originates in mitochondria, we postulated that photoperoxidation of highly unsaturated cardiolipin (CL), which anchors cytochrome c (cyt c) to the inner membrane, is an early proapoptotic event. As initial evidence, PpIX-sensitized photooxidation of liposomal CL to hydroperoxide (CLOOH) species precluded cyt c binding, but this could be reinstated by GSH/selenoperoxidase (GPX4) treatment. Further support derived from site-specific effects observed using (i) a mitochondrial GPX4-overexpressing clone (7G4) of COH-BR1 tumor cells, and (ii) an ALA treatment protocol in which most cellular PpIX is either inside (Pr-1) or outside (Pr-2) mitochondria. Sensitized cells were exposed to a lethal light dose, and then analyzed for death mechanism and lipid hydroperoxide (LOOH) levels. Irradiated Pr-1 vector control (VC) cells died apoptotically following cyt c release and caspase-3 activation, whereas 7G4 cells were highly resistant. Irradiated Pr-2 VC and 7G4 cells showed negligible cyt c release or caspase-3 activation, and both types died via necrosis. CLOOH (detected long before cyt c release) accumulated approximately 70% slower in Pr-1 7G4 cells than in Pr-1 VC, and this slowdown exceeded that of all other LOOHs. These and related findings support the hypothesis that CL is a key upstream target in mitochondria-dependent ALA-PDT-induced apoptosis.

A vibrational spectral maker for probing the hydrogen-bonding status of protonated Asp and Glu residues

Hydrogen bonding is a fundamental element in protein structure and function. Breaking a single hydrogen bond may impair the stability of a protein. We report an infrared vibrational spectral marker for probing the hydrogen-bond number for buried, protonated Asp or Glu residues in proteins. Ab initio computational studies were performed on hydrogen-bonding interactions of a COOH group with a variety of side-chain model compounds of polar and charged amino acids in vacuum using density function theory. For hydrogen-bonding interactions with polar side-chain groups, our results show a strong correlation between the C=O stretching frequency and the hydrogen bond number of a COOH group: approximately 1759-1776 cm(-1) for zero, approximately 1733-1749 cm(-1) for one, and 1703-1710 cm(-1) for two hydrogen bonds. Experimental evidence for this correlation will be discussed. In addition, we show an approximate linear correlation between the C=O stretching frequency and the hydrogen-bond strength. We propose that a two-dimensional infrared spectroscopy, C=O stretching versus O-H stretching, may be employed to identify the specific type of hydrogen-bonding interaction. This vibrational spectral marker for hydrogen-bonding interaction is expected to enhance the power of time-resolved Fourier transform infrared spectroscopy for structural characterization of functionally important intermediates of proteins.

Trapping of a long-living charge separated state of photosynthetic reaction centers in proteoliposomes of negatively charged phospholipids

Reaction centers from the purple bacterium Rhodobacter sphaeroides strain R-26.1 were purified and reconstituted in proteoliposomes formed by the anionic phospholipids phosphatidylglycerol, phosphatidylserine and phosphatidylinositol and by the zwitterionic phospholipid phosphatidylcholine by size-exclusion chromatography in the dark and under illumination. We report the large stabilizing effect induced by anionic phospholipids on the protein charge separated state which results trapped in a long-living (up to tens of minutes) state with a yield up to 80%. This fully reversible state is formed in oxygenic conditions regardless the presence of the secondary quinone QB and its lifetime and relative yield increase at low pH. In proteoliposomes formed with QA-depleted reaction centers (RCs) the resulting protein is very light-sensitive and the long living charge separated state is not observed. The data collected in negatively charged proteoliposomes are discussed in terms of the electrostatic effect on the primary quinone acceptor and compared with similar long living species reported in literature and obtained in anionic, zwitterionic, and non-ionic detergents.

Peptide–lipid interactions: insights and perspectives

As the number of membrane proteins in the Protein Data Bank increases, efforts to understand how they interact with their natural environment are increasing in importance. A number of membrane proteins crystallise with lipid molecules implicitly bound at discrete locations that are consistent with the transmembrane regions of the protein. Bioinformatics studies also point to the specific interactions of some amino acids with membrane lipids. The results of experiments using model systems are revealing how these interactions contribute to the stability of both the protein and the membrane in which it is embedded. From a different perspective, the processes involved in the binding of peptides to membrane surfaces to produce a variety of effects are being understood in ever-increasing detail. This review describes current research efforts and thinking in this area.

Purification, characterisation and crystallisation of photosystem II from Thermosynechococcus elongatus cultivated in a new type of photobioreactor

The thermophilic cyanobacterium Thermosynechococcus elongatus was cultivated under controlled growth conditions using a new type of photobioreactor, allowing us to optimise growth conditions and the biomass yield. A fast large-scale purification method for monomeric and dimeric photosystem II (PSII) solubilized from thylakoid membranes of this cyanobacterium was developed using fast protein liquid chromatography (FPLC). The obtained PSII core complexes (PSIIcc) were analysed for their pigment stoichiometry, photochemical and oxygen evolution activities, as well as lipid and detergent composition. Thirty-six chlorophyll a (Chla), 2 pheophytin a (Pheoa), 9+/- 1 beta-carotene (Car), 2.9+/-0.8 plastoquinone 9 (PQ9) and 3.8+/-0.5 Mn were found per active centre. For the monomeric and dimeric PSIIcc, 18 and 20 lipid as well as 145 and 220 detergent molecules were found in the detergent shell, respectively. The monomeric and dimeric complexes showed high oxygen evolution activity with 1/4 O(2) released per 37-38 Chla and flash in the best cases. Crystals were obtained from dimeric PSIIcc by a micro-batch method. They diffract synchrotron X-rays to a maximum resolution of 2.9-A, resulting in complete data sets of 3.2 A resolution.

Enrichment of cardiolipin content throughout the purification procedure of photosystem II

Photosystem II is a multisubunit membrane complex which performs the water oxidation process in the higher plants. Core dimers and monomers of photosystem II have been isolated from thylakoid membranes by sucrose density gradient centrifugation. Lipids extracted from different photosystem II-enriched fractions obtained from spinach thylakoids have been analysed by thin layer chromatography. Cardiolipin is enriched throughout the purification of photosystem II complexes; in particular dimers contained two times more cardiolipin than their monomeric counterparts.

The protein–lipid interface: perspectives from magnetic resonance and crystal structures

Lipid-protein interactions in membranes are dynamic, and consequently are well studied by magnetic resonance spectroscopy. More recently, lipids associated with integral membrane proteins have been resolved in crystals by X-ray diffraction, mostly at cryogenic temperatures. The conformation and chain ordering of lipids in crystals of integral proteins are reviewed here and are compared and contrasted with results from magnetic resonance and with the crystal structures of phospholipid bilayers. Various aspects of spin-label magnetic resonance studies on lipid interactions with single integral proteins are also reviewed: specificity for phosphatidylcholine, competition with local anaesthetics, oligomer formation of single transmembrane helices, and protein-linked lipid chains. Finally, the interactions between integral proteins and peripheral or lipid-linked proteins, as reflected by the lipid-protein interactions in double reconstitutions, are considered.

Lipids in membrane protein structures

This review describes the recent knowledge about tightly bound lipids in membrane protein structures and deduces general principles of the binding interactions. Bound lipids are grouped in annular, nonannular, and integral protein lipids. The importance of lipid binding for vertical positioning and tight integration of proteins in the membrane, for assembly and stabilization of oligomeric and multisubunit complexes, for supercomplexes, as well as their functional roles are pointed out. Lipid binding is stabilized by multiple noncovalent interactions from protein residues to lipid head groups and hydrophobic tails. Based on analysis of lipids with refined head groups in membrane protein structures, distinct motifs were identified for stabilizing interactions between the phosphodiester moieties and side chains of amino acid residues. Differences between binding at the electropositive and electronegative membrane side, as well as a preferential binding to the latter, are observed. A first attempt to identify lipid head group specific binding motifs is made. A newly identified cardiolipin binding site in the yeast cytochrome bc(1) complex is described. Assignment of unsaturated lipid chains and evolutionary aspects of lipid binding are discussed.

Watching the photosynthetic apparatus in native membranes

Over the last 9 years, the structures of the various components of the bacterial photosynthetic apparatus or their homologues have been determined by x-ray crystallography to at least 4.8-A resolution. Despite this wealth of structural information on the individual proteins, there remains an urgent need to examine the architecture of the photosynthetic apparatus in intact photosynthetic membranes. Information on the arrangement of the different complexes in a native system will help us to understand the processes that ensure the remarkably high quantum efficiency of the system. In this work we report images obtained with an atomic force microscope of native photosynthetic membranes from the bacterium Rhodospirillum photometricum. Several proteins can be seen and identified at molecular resolution, allowing the analysis and modeling of the lateral organization of multiple components of the photosynthetic apparatus within a native membrane. Analysis of the distribution of the complexes shows that their arrangement is far from random, with significant clustering both of antenna complexes and core complexes. The functional significance of the observed distribution is discussed.

Interactions of anionic phospholipids and phosphatidylethanolamine with the potassium channel KcsA

Fluorescence quenching methods have been used to study interactions of anionic phospholipids with the potassium channel KcsA from Streptomyces lividans. Quenching of the Trp fluorescence of KcsA reconstituted into mixtures of dioleoylphosphatidylcholine (DOPC) and an anionic phospholipid with dibromostearoyl chains is more marked at low mole fractions of the brominated anionic phospholipid than is quenching in mixtures of dibromostearoylphosphatidylcholine and nonbrominated anionic lipid. The quenching data are consistent with two classes of binding site for lipid on KcsA, one set corresponding to annular binding sites around KcsA to which DOPC and two-chain anionic phospholipids bind with similar affinities, the other set (non-annular sites) corresponding to sites at which anionic phospholipids can bind but from which DOPC is either excluded or binds with very low affinity. The binding constant for tetraoleoylcardiolipin at the annular sites is significantly less than that for DOPC, being comparable to that for dioleoylphosphatidylethanolamine. Tetraoleoylcardiolipin binds with highest affinity to the non-annular sites, the affinity for dioleoylphosphatidylglycerol being the lowest. The affinity for dioleoylphosphatidylserine decreases at high ionic strength, suggesting that electrostatic interactions between the anionic phospholipid headgroup and positively charged residues on KcsA are important for binding at the non-annular site. The effect of ionic strength on the binding of phosphatidic acid is less marked than on phosphatidylserine. The value of the binding constant for the non-annular site depends on the extent of Trp fluorescence quenching following from binding at the non-annular site. It is suggested that the non-annular site to which binding is detected in the fluorescence quenching experiments corresponds to the binding site for phosphatidylglycerol detected at monomer-monomer interfaces in x-ray diffraction studies.

Modulation of the free energy of the primary quinone acceptor (QA) in reaction centers from Rhodobacter sphaeroides: contributions from the protein and protein-lipid(cardiolipin) interactions

The redox midpoint potential (E(m)) of Q(A), the primary quinone of bacterial reaction centers, is substantially modulated by the protein environment. Quite subtle mutations in the Q(A) binding site, e.g., at residues M218, M252 and M265, cause significant increases in the equilibrium constant for electron transfer to Q(B), which indicate relative lowering of the E(m) of Q(A). However, reports of functional linkage between the Q(A) and Q(B) sites make it difficult to partition such effects between Q(A) and Q(B) from purely relative changes. We report here measurements on the yield of delayed fluorescence emission from the primary donor (P) accompanying the thermally activated charge recombination of P(+)Q(A)(-) to form the excited singlet state of the primary donor, P*. The results show that for mutations of the Q(A) site residues, Met(M218) and Ile(M265), essentially all the substantial thermodynamic effect is localized at Q(A), with no evidence for a significant effect of these residues on the properties of Q(B) or the mutual influence (linkage) of Q(A) and Q(B). We also report a significant lowering of the E(m) of Q(A) by the native lipid, cardiolipin, which brings the E(m) in isolated reaction centers more in line with that seen in native membrane vesicles (chromatophores). Possible origins of this effect are discussed in the context of the Q(A) binding site structure.

Comparative analyses of three-dimensional models of bacterial reaction centers

Despite the fact that the three-dimensional structure of an integral membrane protein was first determined 20 years ago, structures have been solved for very few membrane proteins. The reaction center is an exception with many mutant and modified structures available from 3 different bacterial species. In order to relate these structures to the function of the reaction center, an accurate assessment of the reliability of the structural models is required. Here we describe the quality of the structures of the bacterial reaction center based upon different criteria, such as evaluation of the geometry of the models and comparison of different models. Overall, the structures are found to be most accurate in the membrane-embedded regions with the periplasmic and cytoplasmic exposed regions having more disorder and differences among the structural models. In general, the cofactors and the surrounding protein regions are among the most accurately determined regions of the protein, except for the secondary quinone and its binding pocket that shows a large variation among structures. The limited accuracy of the secondary quinone is due to its partial occupancy as a consequence of its functional role and to the presence of surface features, including lipids and detergent molecules.

Mass spectrometric detection of protein, lipid and heme components of cytochrome c oxidase from R. sphaeroides and the stabilization of non-covalent complexes from the enzyme

The cytochrome c oxidase enzyme from the Rhodobacter sphaeroides bacteria exists as a complex of four peptide subunits, two hemes, and a variety of lipids and metal ions held together by non-covalent forces. While the native enzyme functions as an associated unit, this complex usually dissociates during MALDI- TOF analysis. Through the use of matrix additives such as sucrose, the complete complex and partial complexes can be stabilized in the MALDI-TOF experiment. The dissociation of the complex allows for the detection of the components of the enzyme. The direct detection of associated lipids from an aqueous solution of the intact enzyme may eliminate the need for enzyme disruption and lipid extraction. The partial dissociation of multisubunit enzymes in such experiments may allow for the determination of subunit-subunit and subunit-lipid interactions

Disruption of a specific molecular interaction with a bound lipid affects the thermal stability of the purple bacterial reaction centre

Relatively little is known about the functions of specific molecular interactions between membrane proteins and membrane lipids. The structural and functional consequences of disrupting a previously identified interaction between a molecule of the diacidic lipid cardiolipin and the purple bacterial reaction centre were examined. Mutagenesis of a highly conserved arginine (M267) that is responsible for binding the head-group of the cardiolipin (to leucine) did not affect the rate of photosynthetic growth, the functional properties of the reaction centre, or the X-ray crystal structure of the complex (determined to a resolution of 2.8 A). However, the thermal stability of the protein was compromised by this mutation, part of the reaction centre population showing an approximately 5 degrees C decrease in melting temperature in response to the arginine to leucine mutation. The crystallised mutant reaction centre also no longer bound detectable amounts of cardiolipin at this site. Taken together, these observations suggest that this particular protein-lipid interaction contributes to the thermal stability of the complex, at least when in detergent micelles. These findings are discussed in the light of proposals concerning the unfolding processes that occur when membrane proteins are heated, and we propose that one function of the cardiolipin is to stabilise the interaction between adjacent membrane-spanning alpha-helices in a region where there are no direct protein-protein interactions.

Does different orientation of the methoxy groups of ubiquinone-10 in the reaction centre of Rhodobacter sphaeroides cause different binding at QA and QB?

The different roles of ubiquinone-10 (UQ10) at the primary and secondary quinone (QA and QB) binding sites of Rhodobacter sphaeroides R26 reaction centres are governed by the protein microenvironment. The 4C=O carbonyl group of QA is unusually strongly hydrogen-bonded, in contrast to QB. This asymmetric binding seems to determine their different functions. The asymmetric hydrogen-bonding at QA can be caused intrinsically by distortion of the methoxy groups or extrinsically by binding to specific amino-acid side groups. Different X-ray-based structural models show contradictory orientations of the methoxy groups and do not provide a clear picture. To elucidate if distortion of the methoxy groups induces this hydrogen-bonding, their (ring-)C-O vibrations were assigned by use of site-specifically labelled [5-13C]UQ10 and [6-13C]UQ10 reconstituted at either the QA or the QB binding site. Two infrared bands at 1288 cm(-1) and 1264 cm(-1) were assigned to the methoxy vibrations. They did not shift in frequency at either the QA or QB binding sites, as compared with unbound UQ10. As the frequencies of these vibrations and their coupling are sensitive to the conformations of the methoxy groups, different conformations of the C(5) and C(6) methoxy groups at the QA and QB binding sites can now be excluded. Both methoxy groups are oriented out of plane at QA and QB. Therefore, hydrogen-bonding to His M219 combined with electrostatic interactions with the Fe2+ ion seems to determine the strong asymmetric binding of QA.

Lipidic cubic phase crystal structure of the photosynthetic reaction centre from Rhodobacter sphaeroides at 2.35A resolution

Well-ordered crystals of the bacterial photosynthetic reaction centre from Rhodobacter sphaeroides were grown from a lipidic cubic phase. Here, we report the type I crystal packing that results from this crystallisation medium, for which 3D crystals grow as stacked 2D crystals, and the reaction centre X-ray structure is refined to 2.35A resolution. In this crystal form, the location of the membrane bilayer could be assigned with confidence. A cardiolipin-binding site is found at the protein-protein interface within the membrane-spanning region, shedding light on the formation of crystal contacts within the membrane. A chloride-binding site was identified in the membrane-spanning region, which suggests a putative site for interaction with the light-harvesting complex I, the cytochrome bc(1) complex or PufX. Comparisons with the X-ray structures of this reaction centre deriving from detergent-based crystals are drawn, indicating that a slight compression occurs in this lipid-rich environment.

Lipid-protein interactions in biological membranes: a structural perspective

Lipid molecules bound to membrane proteins are resolved in some high-resolution structures of membrane proteins. An analysis of these structures provides a framework within which to analyse the nature of lipid-protein interactions within membranes. Membrane proteins are surrounded by a shell or annulus of lipid molecules, equivalent to the solvent layer surrounding a water-soluble protein. The lipid bilayer extends right up to the membrane protein, with a uniform thickness around the protein. The surface of a membrane protein contains many shallow grooves and protrusions to which the fatty acyl chains of the surrounding lipids conform to provide tight packing into the membrane. An individual lipid molecule will remain in the annular shell around a protein for only a short period of time. Binding to the annular shell shows relatively little structural specificity. As well as the annular lipid, there is evidence for other lipid molecules bound between the transmembrane alpha-helices of the protein; these lipids are referred to as non-annular lipids. The average thickness of the hydrophobic domain of a membrane protein is about 29 A, with a few proteins having significantly smaller or greater thicknesses than the average. Hydrophobic mismatch between a membrane protein and the surrounding lipid bilayer generally leads to only small changes in membrane thickness. Possible adaptations in the protein to minimise mismatch include tilting of the helices and rotation of side chains at the ends of the helices. Packing of transmembrane alpha-helices is dependent on the chain length of the surrounding phospholipids. The function of membrane proteins is dependent on the thickness of the surrounding lipid bilayer, sometimes on the presence of specific, usually anionic, phospholipids, and sometimes on the phase of the phospholipid.

A defined protein-detergent-lipid complex for crystallization of integral membrane proteins: The cytochrome b6f complex of oxygenic photosynthesis

The paucity of integral membrane protein structures creates a major bioinformatics gap, whose origin is the difficulty of crystallizing these detergent-solubilized proteins. The problem is particularly formidable for hetero-oligomeric integral membrane proteins, where crystallization is impeded by the heterogeneity and instability of the protein subunits and the small lateral pressure imposed by the detergent micelle envelope that surrounds the hydrophobic domain. In studies of the hetero (eight subunit)-dimeric 220,000 molecular weight cytochrome b(6)f complex, derived from the thermophilic cyanobacterium, Mastigocladus laminosus, crystals of the complex in an intact state could not be obtained from highly purified delipidated complex despite exhaustive screening. Crystals of proteolyzed complex could be obtained that grew very slowly and diffracted poorly. Addition to the purified lipid-depleted complex of a small amount of synthetic nonnative lipid, dioleolyl-phosphatidylcholine, resulted in a dramatic improvement in crystallization efficiency. Large crystals of the intact complex grew overnight, whose diffraction parameters are as follows: 94% complete at 3.40 A spacing; R(merge) = 8.8% (38.5%), space group, P6(1)22; and unit cell parameters, a = b = 156.3 A, c = 364.0 A, alpha = beta = 90 degrees, gamma = 120 degrees. It is proposed that the methodology of augmentation of a well-defined lipid-depleted integral membrane protein complex with synthetic nonnative lipid, which can provide conformational stability to the protein complex, may be of general use in the crystallization of integral membrane proteins.

Protein-lipid interactions in the purple bacterial reaction centre

The purple bacterial reaction centre uses the energy of sunlight to power energy-requiring reactions such as the synthesis of ATP. During the last 20 years, a combination of X-ray crystallography, spectroscopy and mutagenesis has provided a detailed insight into the mechanism of light energy transduction in the bacterial reaction centre. In recent years, structural techniques including X-ray crystallography and neutron scattering have also been used to examine the environment of the reaction centre. This mini-review focuses on recent studies of the surface of the reaction centre, and briefly discusses the importance of the specific protein-lipid interactions that have been resolved for integral membrane proteins.