Protein/Lipid Interaction in the Bacterial Photosynthetic Reaction Center: Phosphatidylcholine and Phosphatidylglycerol Modify the Free Energy Levels of the Quinones

Supramolecular Biohybrid Construct for Photoconversion Based on a Bacterial Reaction Center Covalently Bound to Cytochrome c by an Organic Light Harvesting Bridge

A supramolecular construct for solar energy conversion is developed by covalently bridging the reaction center (RC) from the photosynthetic bacterium Rhodobacter sphaeroides and cytochrome c (Cyt c) proteins with a tailored organic light harvesting antenna (hCy2). The RC-hCy2-Cyt c biohybrid mimics the working mechanism of biological assemblies located in the bacterial cell membrane to convert sunlight into metabolic energy. hCy2 collects visible light and transfers energy to the RC, increasing the rate of photocycle between a RC and Cyt c that are linked in such a way that enhances proximity without preventing protein mobility. The biohybrid obtained with average 1 RC/10 hCy2/1.5 Cyt c molar ratio features an almost doubled photoactivity versus the pristine RC upon illumination at 660 nm, and ∼10 times higher photocurrent versus an equimolar mixture of the unbound proteins. Our results represent an interesting insight into photoenzyme chemical manipulation, opening the way to new eco-sustainable systems for biophotovoltaics.

Light-induced reversible reorganizations in closed Type II reaction centre complexes: physiological roles and physical mechanisms

The purpose of this review is to outline our understanding of the nature, mechanism and physiological significance of light-induced reversible reorganizations in closed Type II reaction centre (RC) complexes. In the so-called 'closed' state, purple bacterial RC (bRC) and photosystem II (PSII) RC complexes are incapable of generating additional stable charge separation. Yet, upon continued excitation they display well-discernible changes in their photophysical and photochemical parameters. Substantial stabilization of their charge-separated states has been thoroughly documented-uncovering light-induced reorganizations in closed RCs and revealing their physiological importance in gradually optimizing the operation of the photosynthetic machinery during the dark-to-light transition. A range of subtle light-induced conformational changes has indeed been detected experimentally in different laboratories using different bRC and PSII-containing preparations. In general, the presently available data strongly suggest similar structural dynamics of closed bRC and PSII RC complexes, and similar physical mechanisms, in which dielectric relaxation processes and structural memory effects of proteins are proposed to play important roles.

Charge Recombination Kinetics of Bacterial Photosynthetic Reaction Centres Reconstituted in Liposomes: Deterministic Versus Stochastic Approach

In this theoretical work, we analyse the kinetics of charge recombination reaction after a light excitation of the Reaction Centres extracted from the photosynthetic bacterium Rhodobacter sphaeroides and reconstituted in small unilamellar phospholipid vesicles. Due to the compartmentalized nature of liposomes, vesicles may exhibit a random distribution of both ubiquinone molecules and the Reaction Centre protein complexes that can produce significant differences on the local concentrations from the average expected values. Moreover, since the amount of reacting species is very low in compartmentalized lipid systems the stochastic approach is more suitable to unveil deviations of the average time behaviour of vesicles from the deterministic time evolution.

Investigating the interactions of the first 17 amino acid residues of Huntingtin with lipid vesicles using mass spectrometry and molecular dynamics

The first 17 amino acid residues of Huntingtin protein (Nt17 of htt) are thought to play an important role in the protein's function; Nt17 is one of two membrane binding domains in htt. In this study the binding ability of Nt17 peptide with vesicles comprised of two subclasses of phospholipids is studied using electrospray ionization - mass spectrometry (ESI-MS) and molecular dynamics (MD) simulations. Overall, the peptide is shown to have a greater propensity to interact with vesicles of phosphatidylcholine (PC) rather than phosphatidylethanolamine (PE) lipids. Mass spectra show an increase in lipid-bound peptide adducts where the ordering of the number of such specie is 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) > 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) > 1-palmitoyl-2-oleoyl-sn-glycero-3 phosphoethanolamine (POPE). MD simulations suggest that the compactness of the bilayer plays a role in governing peptide interactions. The peptide shows greater disruption of the DOPC bilayer order at the surface and interacts with the hydrophobic tails of lipid molecules via hydrophobic residues. Conversely, the POPE vesicle remains ordered and lipids display transient interactions with the peptide through the formation of hydrogen bonds with hydrophilic residues. The POPC system displays intermediate behavior with regard to the degree of peptide-membrane interaction. Finally, the simulations suggest a helix stabilizing effect resulting from the interactions between hydrophobic residues and the lipid tails of the DOPC bilayer.

Light-Driven Biocatalysis in Liposomes and Polymersomes: Where Are We Now?

The utilization of light energy to power organic-chemical transformations is a fundamental strategy of the terrestrial energy cycle. Inspired by the elegance of natural photosynthesis, much interdisciplinary research effort has been devoted to the construction of simplified cell mimics based on artificial vesicles to provide a novel tool for biocatalytic cascade reactions with energy-demanding steps. By inserting natural or even artificial photosynthetic systems into liposomes or polymersomes, the light-driven proton translocation and the resulting formation of electrochemical gradients have become possible. This is the basis for the conversion of photonic into chemical energy in form of energy-rich molecules such as adenosine triphosphate (ATP), which can be further utilized by energy-dependent biocatalytic reactions, e.g. carbon fixation. This review compares liposomes and polymersomes as artificial compartments and summarizes the types of light-driven proton pumps that have been employed in artificial photosynthesis so far. We give an overview over the methods affecting the orientation of the photosystems within the membranes to ensure a unidirectional transport of molecules and highlight recent examples of light-driven biocatalysis in artificial vesicles. Finally, we summarize the current achievements and discuss the next steps needed for the transition of this technology from the proof-of-concept status to preparative applications.

Genes essential for phototrophic growth by a purple alphaproteobacterium

Tn-seq was used to identify genes essential for phototrophic growth by the purple bacterium Rhodopseudomonas palustris. About 167 genes required for anaerobic growth on acetate in light were identified, 35 of which are annotated as photosynthesis genes. The essentiality of many of these genes by analysing the phenotypes of independently generated mutants that had altered pigmentation was verified. Three genes were identified, two possibly involved in biogenesis of the membrane-bound photosynthetic apparatus and one for phosphatidylcholine biosynthesis, that were not known to be essential for phototrophic growth. Site-directed mutagenesis was used to show that the NADH:quinone oxidoreductase complex I_E was essential for phototrophic growth under strictly anaerobic conditions and appeared to play a role in reverse electron transport to generate NADH. A homologous NADH:quinone oxidoreductase complex I_A likely operates in the opposite direction to oxidize NADH. The operation of the two enzymes in opposition would allow R. palustris to maintain redox balance. As a complement to the genetic data, proteomics experiments were carried out in which it was found that 408 proteins were present in significantly higher amounts in cells grown anaerobically in light compared with aerobically. Among these were proteins encoded by subset of the phototrophic growth-essential genes.

Sensing photosynthetic herbicides in an electrochemical flow cell

Specific inhibitory reactions of herbicides with photosynthetic reaction centers bound to working electrodes were monitored in a conventional electrochemical cell and a newly designed microfluidic electrochemical flow cell. In both cases, the bacterial reaction centers were bound to a transparent conductive metal oxide, indium-tin-oxide, electrode through carbon nanotubes. In the conventional cell, photocurrent densities of up to a few μA/cm² could be measured routinely. The photocurrent could be blocked by the photosynthetic inhibitor terbutryn (I ₅₀ = 0.38 ± 0.14 μM) and o-phenanthroline (I ₅₀ = 63.9 ± 12.2 μM). The microfluidic flow cell device enabled us to reduce the sample volume and to simplify the electrode arrangement. The useful area of the electrodes remained the same (ca. 2 cm²), similar to the classical electrochemical cell; however, the size of the cell was reduced considerably. The microfluidic flow control enabled us monitoring in real time the binding/unbinding of the inhibitor and cofactor molecules at the secondary quinone site.

Highly oriented photosynthetic reaction centers generate a proton gradient in synthetic protocells

Photosynthesis is responsible for the photochemical conversion of light into the chemical energy that fuels the planet Earth. The photochemical core of this process in all photosynthetic organisms is a transmembrane protein called the reaction center. In purple photosynthetic bacteria a simple version of this photoenzyme catalyzes the reduction of a quinone molecule, accompanied by the uptake of two protons from the cytoplasm. This results in the establishment of a proton concentration gradient across the lipid membrane, which can be ultimately harnessed to synthesize ATP. Herein we show that synthetic protocells, based on giant lipid vesicles embedding an oriented population of reaction centers, are capable of generating a photoinduced proton gradient across the membrane. Under continuous illumination, the protocells generate a gradient of 0.061 pH units per min, equivalent to a proton motive force of 3.6 mV⋅min^-1 Remarkably, the facile reconstitution of the photosynthetic reaction center in the artificial lipid membrane, obtained by the droplet transfer method, paves the way for the construction of novel and more functional protocells for synthetic biology.

Structural and Functional Hierarchy in Photosynthetic Energy Conversion-from Molecules to Nanostructures

Basic principles of structural and functional requirements of photosynthetic energy conversion in hierarchically organized machineries are reviewed. Blueprints of photosynthesis, the energetic basis of virtually all life on Earth, can serve the basis for constructing artificial light energy-converting molecular devices. In photosynthetic organisms, the conversion of light energy into chemical energy takes places in highly organized fine-tunable systems with structural and functional hierarchy. The incident photons are absorbed by light-harvesting complexes, which funnel the excitation energy into reaction centre (RC) protein complexes containing redox-active chlorophyll molecules; the primary charge separations in the RCs are followed by vectorial transport of charges (electrons and protons) in the photosynthetic membrane. RCs possess properties that make their use in solar energy-converting and integrated optoelectronic systems feasible. Therefore, there is a large interest in many laboratories and in the industry toward their use in molecular devices. RCs have been bound to different carrier matrices, with their photophysical and photochemical activities largely retained in the nano-systems and with electronic connection to conducting surfaces. We show examples of RCs bound to carbon-based materials (functionalized and non-functionalized single- and multiwalled carbon nanotubes), transitional metal oxides (ITO) and conducting polymers and porous silicon and characterize their photochemical activities. Recently, we adapted several physical and chemical methods for binding RCs to different nanomaterials. It is generally found that the P(+)(QAQB)(-) charge pair, which is formed after single saturating light excitation is stabilized after the attachment of the RCs to the nanostructures, which is followed by slow reorganization of the protein structure. Measuring the electric conductivity in a direct contact mode or in electrochemical cell indicates that there is an electronic interaction between the protein and the inorganic carrier matrices. This can be a basis of sensing element of bio-hybrid device for biosensor and/or optoelectronic applications.

Assembly of a photosynthetic reaction center with ABA tri-block polymersomes: highlights on protein localization

The micelle-to-vesicle transition technique was used to reconstitute the integral membrane protein photosynthetic reaction center (RC) and the position of the RC in the polymersome vesicle was investigated.

The binding of quinone to the photosynthetic reaction centers: kinetics and thermodynamics of reactions occurring at the QB-site in zwitterionic and anionic liposomes

Liposomes represent a versatile biomimetic environment for studying the interaction between integral membrane proteins and hydrophobic ligands. In this paper, the quinone binding to the QB-site of the photosynthetic reaction centers (RC) from Rhodobacter sphaeroides has been investigated in liposomes prepared with either the zwitterionic phosphatidylcholine (PC) or the negatively charged phosphatidylglycerol (PG) to highlight the role of the different phospholipid polar heads. Quinone binding (K Q) and interquinone electron transfer (L AB) equilibrium constants in the two type of liposomes were obtained by charge recombination reaction of QB-depleted RC in the presence of increasing amounts of ubiquinone-10 over the temperature interval 6-35 °C. The kinetic of the charge recombination reactions has been fitted by numerically solving the ordinary differential equations set associated with a detailed kinetic scheme involving electron transfer reactions coupled with quinone release and uptake. The entire set of traces at each temperature was accurately fitted using the sole quinone release constants (both in a neutral and a charge separated state) as adjustable parameters. The temperature dependence of the quinone exchange rate at the QB-site was, hence, obtained. It was found that the quinone exchange regime was always fast for PC while it switched from slow to fast in PG as the temperature rose above 20 °C. A new method was introduced in this paper for the evaluation of constant K Q using the area underneath the charge recombination traces as the indicator of the amount of quinone bound to the QB-site.

Hybrid assemblies of fluorescent nanocrystals and membrane proteins in liposomes

Because of the growing potential of nanoparticles in biological and medical applications, tuning and directing their properties toward a high compatibility with the aqueous biological milieu is of remarkable relevance. Moreover, the capability to combine nanocrystals (NCs) with biomolecules, such as proteins, offers great opportunities to design hybrid systems for both nanobiotechnology and biomedical technology. Here we report on the application of the micelle-to-vesicle transition (MVT) method for incorporation of hydrophobic, red-emitting CdSe@ZnS NCs into the bilayer of liposomes. This method enabled the construction of a novel hybrid proteo-NC-liposome containing, as model membrane protein, the photosynthetic reaction center (RC) of Rhodobacter sphaeroides. Electron microscopy confirmed the insertion of NCs within the lipid bilayer without significantly altering the structure of the unilamellar vesicles. The resulting aqueous NC-liposome suspensions showed low turbidity and kept unaltered the wavelengths of absorbance and emission peaks of the native NCs. A relative NC fluorescence quantum yield up to 8% was preserved after their incorporation in liposomes. Interestingly, in proteo-NC-liposomes, RC is not denatured by Cd-based NCs, retaining its structural and functional integrity as shown by absorption spectra and flash-induced charge recombination kinetics. The outlined strategy can be extended in principle to any suitably sized hydrophobic NC with similar surface chemistry and to any integral protein complex. Furthermore, the proposed approach could be used in nanomedicine for the realization of theranostic systems and provides new, interesting perspectives for understanding the interactions between integral membrane proteins and nanoparticles, i.e., in nanotoxicology studies.

Photosynthetic reaction centers/ITO hybrid nanostructure

Photosynthetic reaction center proteins purified from Rhodobacter sphaeroides purple bacterium were deposited on the surface of indium tin oxide (ITO), a transparent conductive oxide, and the photochemical/-physical properties of the composite were investigated. The kinetics of the light induced absorption change indicated that the RC was active in the composite and there was an interaction between the protein cofactors and the ITO. The electrochromic response of the bacteriopheophytine absorption at 771 nm showed an increased electric field perturbation around this chromophore on the surface of ITO compared to the one measured in solution. This absorption change is associated with the charge-compensating relaxation events inside the protein. Similar life time, but smaller magnitude of this absorption change was measured on the surface of borosilicate glass. The light induced change in the conductivity of the composite as a function of the concentration showed the typical sigmoid saturation characteristics unlike if the photochemically inactive chlorophyll was layered on the ITO. In this later case the light induced change in the conductivity was oppositely proportional to the chlorophyll concentration due to the thermal dissipation of the excitation energy. The sensitivity of the measurement is very high; few picomole RC can change the light induced resistance of the composite.

Cardiolipin binding in bacterial respiratory complexes: structural and functional implications

The structural and functional integrity of biological membranes is vital to life. The interplay of lipids and membrane proteins is crucial for numerous fundamental processes ranging from respiration, photosynthesis, signal transduction, solute transport to motility. Evidence is accumulating that specific lipids play important roles in membrane proteins, but how specific lipids interact with and enable membrane proteins to achieve their full functionality remains unclear. X-ray structures of membrane proteins have revealed tight and specific binding of lipids. For instance, cardiolipin, an anionic phospholipid, has been found to be associated to a number of eukaryotic and prokaryotic respiratory complexes. Moreover, polar and septal accumulation of cardiolipin in a number of prokaryotes may ensure proper spatial segregation and/or activity of proteins. In this review, we describe current knowledge of the functions associated with cardiolipin binding to respiratory complexes in prokaryotes as a frame to discuss how specific lipid binding may tune their reactivity towards quinone and participate to supercomplex formation of both aerobic and anaerobic respiratory chains. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

Prokaryotic diacylglycerol kinase and undecaprenol kinase

Prokaryotic diacylglycerol kinase (DAGK) and undecaprenol kinase (UDPK) are the lone members of a family of multispan membrane enzymes that are very small, lack relationships to any other family of proteins-including water soluble kinases-and exhibit an unusual structure and active site architecture. Escherichia coli DAGK plays an important role in recycling diacylglycerol produced as a by-product of biosynthesis of molecules located in the periplasmic space. UDPK seems to play an analogous role in gram-positive bacteria, where its importance is evident because UDPK is essential for biofilm formation by the oral pathogen Streptococcus mutans. DAGK has also long served as a model system for studies of membrane protein biocatalysis, folding, stability, and structure. This review explores our current understanding of the microbial physiology, enzymology, structural biology, and folding of the prokaryotic DAGK family, which is based on over 40 years of studies.

Lipid binding to the carotenoid binding site in photosynthetic reaction centers

Lipid binding to the carotenoid binding site near the inactive bacteriochlorophyll monomer was probed in the reaction centers of carotenoid-less mutant, R-26 from Rhodobacter sphaeroides. Recently, a marked light-induced change of the local dielectric constant in the vicinity of the inactive bacteriochlorophyll monomer was reported in wild type that was attributed to structural changes that ultimately lengthened the lifetime of the charge-separated state by 3 orders of magnitude (Deshmukh, S. S.; Williams, J. C.; Allen, J. P.; Kalman, L. Biochemistry 2011, 50, 340). Here in the R-26 reaction centers, the combination of light-induced structural changes and lipid binding resulted in a 5 orders of magnitude increase in the lifetime of the charge-separated state involving the oxidized dimer and the reduced primary quinone in proteoliposomes. Only saturated phospholipids with fatty acid chains of 12 and 14 carbon atoms long were bound successfully at 8 °C by cooling the reaction center protein slowly from room temperature. In addition to reporting a dramatic increase of the lifetime of the charge-separated state at physiologically relevant temperatures, this study reveals a novel lipid binding site in photosynthetic reaction center. These results shed light on a new potential application of the reaction center in energy storage as a light-driven biocapacitor since the charges separated by ∼30 Å in a low-dielectric medium can be prevented from recombination for hours.

Quantitative study of PNSB energy metabolism in degrading pollutants under weak light-micro oxygen condition

Contribution and relationship between oxidative phosphorylation and photophosphorylation pathways in purple non-sulfur bacteria (PNSB) wastewater treatment under weak light-micro oxygen condition were studied quantitatively. Results showed that under weak light-anaerobic condition, PNSB followed photophosphorylation with the first-order degradation kinetic constant k(3) of 0.0585. Under dark-micro aerobic condition, it followed oxidative phosphorylation with k(2) of 0.0896. Under weak light-micro oxygen condition, both pathways existed with k(1) of 0.108. When light and oxygen both existed, oxidative phosphorylation had a strong competitiveness, it played a dominative role and counted for 92.7% in pollutants degradation, and meanwhile photophosphorylation was restrained by 81.6%. Theoretical analysis showed the common part from coenzyme Q (CoQ) to cytochrome c2 (Cyt c2) in both respiration and photosynthetic chains might cause the competition. When oxygen existed, respiration electron transport would be enhanced. Other potential explanations included that oxygen might damage the pigment and membrane system vital to photophosphorylation.

Lipidic sponge phase crystal structure of a photosynthetic reaction center reveals lipids on the protein surface

Membrane proteins are embedded in a lipid bilayer and maintain strong interactions with lipid molecules. Tightly bound lipids are responsible for vertical positioning and integration of proteins in the membrane and for assembly of multisubunit complexes and occasionally act as substrates. In this work we present the lipidic sponge phase crystal structure of the reaction center from Blastochloris viridis to 1.86 A, which reveals lipid molecules interacting with the protein surface. A diacylglycerol molecule is bound, through a thioether bond, to the N-terminus of the tetraheme cytochrome c subunit. From the electron density recovered at the Q(B) site and the observed change in recombination kinetics in lipidic sponge phase-grown crystals, the mobile ubiquinone appears to be displaced by a monoolein molecule. A 36 A long electron density feature is observed at the interface of transmembrane helices belonging to the H- and M-subunits, probably arising from an unidentified lipid.

Characterisation of RC-proteoliposomes at different RC/lipid ratios

Reconstitution of membrane proteins in phospholipid vesicles allows the investigation of such macromolecules in a biomimetic simplified environment. The often employed micelle-to-vesicle-transition method for proteoliposome preparation is a fast and reproducible technique. In this, communication is shown that the lipid/protein ratio influences the size of the proteoliposomes and the actual protein reconstitution. The results indicate that for photosynthetic reaction centres, the best conditions for ligand-interaction experiments are achieved with a lipid/protein value of 1000:1, while for complete protein incorporation, the 2000:1 ratio should be chosen.

Spectrally silent light induced conformation change in photosynthetic reaction centers

Spectrally silent conformation change after photoexcitation of photosynthetic reaction centers isolated from Rhodobacter sphaeroides R-26 was observed by the optical heterodyne transient grating technique. The signal showed spectrally silent structural change in photosynthetic reaction centers followed by the primary P+BPh- charge separation and this change remains even after the charge recombination. Without bound quinone to the RC, the conformation change relaxes with about 28micros lifetime. The presence of quinone at the primary quinone (QA) site may suppress this conformation change. However, a weak relaxation with 30-40micros lifetime is still observed under the presence of QA, which increases up to 40micros as a function of the occupancy of the secondary quinone (QB) site.

Dynamics of electron transfer from high-potential cytochrome c to bacteriochlorophyll dimer in photosynthetic reaction centers as probed using laser-induced temperature jump

Laser-induced temperature jump experiments were used for testing the rates of thermoinduced conformational transitions of reaction center (RC) complexes in chromatophores of Chromatium minutissimum. The thermoinduced transition of the macromolecular RC complex to a state providing effective electron transport from the multiheme cytochrome c to the photoactive bacteriochlorophyll dimer within the temperature range 220-280 K accounts for tens of seconds with activation energy 0.166 eV/molecule. The rate of the thermoinduced transition in the cytochrome-RC complex was found to be three orders of magnitude slower than the rate of similar thermoinduced transition of the electron transfer reaction from the primary to secondary quinone acceptors studied in the preceding work (Chamorovsky et al. in Eur Biophys J 32:537-543, 2003). Parameters of thermoinduced activation of the electron transfer from the multiheme cytochrome c to the photoactive bacteriochlorophyll dimer are discussed in terms of cytochrome c docking onto the RC.

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.

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.

Lipids in photosystem II: interactions with protein and cofactors

Photosystem II (PSII) is a homodimeric protein-cofactor complex embedded in the thylakoid membrane that catalyses light-driven charge separation accompanied by the oxidation of water during oxygenic photosynthesis. Biochemical analysis of the lipid content of PSII indicates a number of integral lipids, their composition being similar to the average lipid composition of the thylakoid membrane. The crystal structure of PSII at 3.0 A resolution allowed for the first time the assignment of 14 integral lipids within the protein scaffold, all of them being located at the interface of different protein subunits. The reaction centre subunits D1 and D2 are encircled by a belt of 11 lipids providing a flexible environment for the exchange of D1. Three lipids are located in the dimerization interface and mediate interactions between the PSII monomers. Several lipids are located close to the binding pocket of the mobile plastoquinone Q(B), forming part of a postulated diffusion pathway for plastoquinone. Furthermore two lipids were found, each ligating one antenna chlorophyll a. A detailed analysis of lipid-protein and lipid-cofactor interactions allows to derive some general principles of lipid binding pockets in PSII and to suggest possible functional properties of the various identified lipid molecules.

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.

Photosynthesis research in Italy: a review

This historical review was compiled and edited by Giorgio Forti, whereas the other authors of the different sections are listed alphabetically after his name, below the title of the paper; they are also listed in the individual sections. This review deals with the research on photosynthesis performed in several Italian laboratories during the last 50 years; it includes research done, in collaboration, at several international laboratories, particularly USA, UK, Switzerland, Hungary, Germany, France, Finland, Denmark, and Austria. Wherever pertinent, references are provided, especially to other historical papers in Govindjee et al. [Govindjee, Beatty JT, Gest H, Allen JF (eds) (2005) Discoveries in Photosynthesis. Springer, Dordrecht]. This paper covers the physical and chemical events starting with the absorption of a quantum of light by a pigment molecule to the conversion of the radiation energy into the stable chemical forms of the reducing power and of ATP. It describes the work done on the structure, function and regulation of the photosynthetic apparatus in higher plants, unicellular algae and in photosynthetic bacteria. Phenomena such as photoinhibition and the protection from it are also included. Research in biophysics of photosynthesis in Padova (Italy) is discussed by G.M. Giacometti and G. Giacometti (2006).

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.

Proline mutations induce negative-dosage effects on uptake velocity of the dopamine transporter

Ala and Gly substitutions for Pro 101 (P101) located in transmembrane domain 2 of the dopamine transporter (DAT) abolished transport activity but did not disrupt plasma membrane expression. Due to the high conservation of P101 in all neurotransmitter transporters and the capability of Pro to add flexibility to helices, we hypothesized that P101 contributes to the dynamic feature of substrate translocation. To test this hypothesis, here we analysed transport activity for DAT mutants where this Pro was mutated into different amino acids, including Ser, Val, Leu and Phe. The transmembrane domain 2 helix of P101F, unlike the other mutants, was computationally predicted to have a Van der Waals energy threefold higher than the wild-type helix. P101F mutant expression was consistently disrupted in COS cells. Among all the other mutants that express normally, P101V, with a side-chain size close to that of Pro, restores the transport activity of P101A by sevenfold. Most importantly, P101V, P101L and P101S display negative-dosage effects on dopamine (DA) transport, i.e. the velocity-concentration curve for DA uptake does not show a plateau with increasing [DA] but rather peaks and then goes down. These data support the view that P101 of DAT plays an essential role in DA translocation.

Proteins, chlorophylls and lipids: X-ray analysis of a three-way relationship

Photosynthetic reaction centres and light harvesting complexes have been at the forefront of crystallographic studies of integral membrane proteins. In recent years, there have been spectacular advances in our understanding of the structure of (bacterio)chlorophyll-containing membrane proteins from oxygenic and anoxygenic phototrophs. In these complex structures, the protein scaffold encases different combinations of cofactors and interacts with several tightly bound lipid species that play a variety of hitherto unrecognized structural roles. Some of these lipids have relevance to the physiological function of the protein, whereas others are important for the formation of highly ordered crystals. The first site-directed mutagenesis studies of individual lipid binding sites have now underlined the importance of the lipid component for the structural stability of protein-cofactor-lipid complexes.