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.     

Evolutionary relationships between "Q-type" photosynthetic reaction centres: hypothesis-testing using parsimony

Hypotheses concerning the evolutionary relationships between "Q-type" photosynthetic reaction centres are tested using amino acid parsimony analysis of subunit sequences and an alignment based on dot matrix comparisons. Strong evidence is found for independent gene duplications having produced the L and M subunits of the photosynthetic purple bacterial reaction centre and D1 and D2 of Photosystem-II. Much support is also found for the L and M subunits of the green filamentous bacterium Chloroflexus aurantiacus arising from the same gene duplication as the purple bacterial subunits, suggesting there was an ancestral bacterial heterodimeric reaction centre. These conclusions caution against over-extrapolation from the purple bacterial reaction centre to Photosystem-II, and suggest that the latter is more ancient than previously supposed.     

The design and synthesis of artificial photosynthetic antennas,reaction centres and membranes

Artificial antenna systems and reaction centres synthesized in our laboratory are used to illustrate that structural and thermodynamic factors controlling energy and electron transfer in these constructs can be modified to optimize performance. Artificial reaction centres have been incorporated into liposomal membranes where they convert light energy to vectorial redox potential. This redox potential drives a Mitchellian, quinone-based, proton-transporting redox loop that generates a Deltamu H(+) of ca. 4.4 kcal mol(-1) comprising DeltapH ca. 2.1 and Deltapsi ca. 70 mV. In liposomes containing CF(0)F(1)-ATP synthase, this system drives ATP synthesis against an ATP chemical potential similar to that observed in natural systems.     

Correlating ultrafast function with structure in single crystals of the photosynthetic reaction center

Femtosecond transient absorbance spectroscopy was applied to the study of primary electron transfer in single reaction center crystals from Rhodobacter sphaeroides. Polarized transient absorption spectra of individual crystals are shown to correlate with polarized ground-state absorption spectra and to track cofactor transition moment directions calculated from the crystallographic structure. Electron transfer from the bacteriochlorophyll dimer to the bacteriopheophytin acceptor was found to be multiphasic in crystals and approximately 2-fold slower than in solution. This work demonstrates the ability to resolve ultrafast photosynthetic function in single crystals and allows ultrafast function to be directly correlated with structure.     

The spectroscopy of the low-lying bands in the special-pair radical-cations of photosynthetic reaction centres

The spectra of the special-pair cation radicals P⁺ produced after photoexcitation of photosynthetic reaction centres and initial electron transfer can, in principle, provide important information concerning the function of the reaction centres. Extraction of this information requires detailed knowledge of the spectroscopy of the cations, however. We review our contributions to this field concerning the bands observed at near 2500 cm^–1 and 8000 cm^–1, and review results obtained from the study of porphyrin reaction-centre model complexes. We also consider the impact of recent experimental developments in these fields. However, our primary focus is to raise the possibility that the observed band at 2500 cm^–1 is either a composite of two independent electronic transitions or has both an allowed component and a forbidden component arising from vibronic coupling to intense high-energy transitions. The resolution of this dichotomy will have profound consequences for interpretation of the function of photosynthetic reaction centres.     

Structure and function of the photosynthetic reaction center fromRhodobacter sphaeroides

The three-dimensional structure of the photosynthetic reaction center fromRhodobacter sphaeroides is described. The reaction center is a transmembrane protein that converts light into chemical energy. The protein has three subunits: L, M, and H. The mostly helical L and M subunits provide the scaffolding and the finely tuned environment in which the chromophores carry out electron transfer. The details of the protein-chromophore interactions are from studies of a trigonal crystal form that diffracted to 2.65-Å resolution. Functional studies of the multi-subunit complex by site-specific replacement of key amino acid residues are summarized in the context of the molecular structure.This work was supported in part by the U.S. Department of Energy, Office of Health and Environmental Research, under Contract No. W-31-109-ENG-38 and by Public Health Service Grant GM36598.     

Evolution of photosynthetic reaction centers: insights from the structure of the heliobacterial reaction center

The proliferation of phototrophy within early-branching prokaryotes represented a significant step forward in metabolic evolution. All available evidence supports the hypothesis that the photosynthetic reaction center (RC)—the pigment-protein complex in which electromagnetic energy (i.e., photons of visible or near-infrared light) is converted to chemical energy usable by an organism—arose once in Earth’s history. This event took place over 3 billion years ago and the basic architecture of the RC has diversified into the distinct versions that now exist. Using our recent 2.2-Å X-ray crystal structure of the homodimeric photosynthetic RC from heliobacteria, we have performed a robust comparison of all known RC types with available structural data. These comparisons have allowed us to generate hypotheses about structural and functional aspects of the common ancestors of extant RCs and to expand upon existing evolutionary schemes. Since the heliobacterial RC is homodimeric and loosely binds (and reduces) quinones, we support the view that it retains more ancestral features than its homologs from other groups. In the evolutionary scenario we propose, the ancestral RC predating the division between Type I and Type II RCs was homodimeric, loosely bound two mobile quinones, and performed an inefficient disproportionation reaction to reduce quinone to quinol. The changes leading to the diversification into Type I and Type II RCs were separate responses to the need to optimize this reaction: the Type I lineage added a [4Fe–4S] cluster to facilitate double reduction of a quinone, while the Type II lineage heterodimerized and specialized the two cofactor branches, fixing the quinone in the Q_A site. After the Type I/II split, an ancestor to photosystem I fixed its quinone sites and then heterodimerized to bind PsaC as a new subunit, as responses to rising O₂ after the appearance of the oxygen-evolving complex in an ancestor of photosystem II. These pivotal events thus gave rise to the diversity that we observe today.     

The evolution of photosystem I in light of phage-encoded reaction centres

Recent structural determinations and metagenomic studies shed light on the evolution of photosystem I (PSI) from the homodimeric reaction centre of primitive bacteria to plant PSI at the top of the evolutionary development. The evolutionary scenario of over 3.5 billion years reveals an increase in the complexity of PSI. This phenomenon of ever-increasing complexity is common to all evolutionary processes that in their advanced stages are highly dependent on fine-tuning of regulatory processes. On the other hand, the recently discovered virus-encoded PSI complexes contain a minimal number of subunits. This may reflect the unique selection scenarios associated with viral replication. It may be beneficial for future engineering of productive processes to utilize ‘primitive’ complexes that disregard the cellular regulatory processes and to avoid those regulatory constraints when our goal is to divert the process from its original route. In this article, we discuss the evolutionary forces that act on viral reaction centres and the role of the virus-carried photosynthetic genes in the evolution of photosynthesis.     

A set-up to study photochemically induced dynamic nuclear polarization in photosynthetic reaction centres by solid-state NMR

Recently, solid-state NMR spectroscopy became a viable method to investigate photosynthetic reaction centres (RCs) on the atomic level. To study the electronic structure of the radical cation state of the RC, occurring after the electron emission, solid-state NMR using an illumination set-up can be exploited. This paper describes the illumination set-up we designed for a standard Bruker wide-bore MAS NMR probe. In addition we demonstrate its application to get information from the active site in photosynthetic reaction centres of Rhodobacter sphaeroides R-26 by photochemically induced dynamic nuclear polarization (photo-CIDNP). Solid-state NMR spectra of natural abundance 13C in detergent solubilized quinone depleted photosynthetic reaction centres under continuous illumination showed exceptionally strong nuclear spin polarization in NMR lines. Both enhanced-absorptive and emissive polarization were seen in the carbon spectrum which could be assigned to a bacteriochlorophyll a (BChl a) cofactor, presumably the special pair BChl a. The sign and intensities of the 13C NMR signals provide information about the electron spin density distribution of the transiently formed radical P.+ on the atomic level.     

Insights into the evolution of the antenna domains of Type-I and Type-II photosynthetic reaction centres through homology modelling

The (bacterio)chlorophylls of photosynthetic antenna and reaction centre complexes are bound to the protein via a fifth, axial ligand to the central magnesium atom. A number of the amino acids identified as providing such ligands are conserved between the large antenna of the cyanobacterial Type-I reaction centre and smaller antennas of the Type-I reaction centres of green sulphur bacteria and heliobacteria, and these numbers match closely the estimated number of antenna bacteriochlorophylls in the latter. The possible organisation of the antenna in the latter reaction centres is discussed, as is the mechanism by which the more pigment-rich antenna of the cyanobacterial reaction centre evolved. The homology modelling approach is also extended to the six-helix antenna proteins CP47 and CP43 associated with the Photosystem II reaction centre.     

Protein structure,electron transfer and evolution of prokaryotic photosynthetic reaction centers

Photosynthetic reaction centers from a variety of organisms have been isolated and characterized. The groups of prokaryotic photosynthetic organisms include the purple bacteria, the filamentous green bacteria, the green sulfur bacteria and the heliobacteria as anoxygenic representatives as well as the cyanobacteria and prochlorophytes as oxygenic representatives. This review focuses on structural and functional comparisons of the various groups of photosynthetic reaction centers and considers possible evolutionary scenarios to explain the diversity of existing photosynthetic organisms.Abbreviations BChl bacteriochlorophyll - Chl chlorophyll - Rb Rhodobacter - Rp Rhodopseudomonas