BBA - Bioenergetics (v.1507, #1-3)

Type I photosynthetic reaction centres by Peter Heathcote (1-2).

Jan Amesz by A.J Hoff; T.J Aartsma (3-4).

Structure of photosystem I by Petra Fromme; Patrick Jordan; Norbert Krauß (5-31).
In plants and cyanobacteria, the primary step in oxygenic photosynthesis, the light induced charge separation, is driven by two large membrane intrinsic protein complexes, the photosystems I and II. Photosystem I catalyses the light driven electron transfer from plastocyanin/cytochrome c 6 on the lumenal side of the membrane to ferredoxin/flavodoxin at the stromal side by a chain of electron carriers. Photosystem I of Synechococcus elongatus consists of 12 protein subunits, 96 chlorophyll a molecules, 22 carotenoids, three [4Fe4S] clusters and two phylloquinones. Furthermore, it has been discovered that four lipids are intrinsic components of photosystem I. Photosystem I exists as a trimer in the native membrane with a molecular mass of 1068 kDa for the whole complex. The X-ray structure of photosystem I at a resolution of 2.5 Å shows the location of the individual subunits and cofactors and provides new information on the protein–cofactor interactions. [P. Jordan, P. Fromme, H.T. Witt, O. Klukas, W. Saenger, N. Krauß, Nature 411 (2001) 909-917]. In this review, biochemical data and results of biophysical investigations are discussed with respect to the X-ray crystallographic structure in order to give an overview of the structure and function of this large membrane protein.
Keywords: X-ray crystallography; Photosystem I; Structure; Electron transfer chain; Core antenna; Membrane protein;

Proteins of the cyanobacterial photosystem I by Wu Xu; Huadong Tang; Yingchun Wang; Parag R Chitnis (32-40).
Cyanobacterial photosystem (PS) I is remarkably similar to its counterpart in the chloroplast of plants and algae. Therefore, it has served as a prototype for the type I reaction centers of photosynthesis. Cyanobacterial PS I contains 11–12 proteins. Some of the cyanobacterial proteins are modified post-translationally. Reverse genetics has been used to generate subunit-deficient cyanobacterial mutants, phenotypes of which have revealed the functions of the missing proteins. The cyanobacterial PS I proteins bind cofactors, provide docking sites for electron transfer proteins, participate in tertiary and quaternary organization of the complex and protect the electron transfer centers. Many of these mutants are now being used in sophisticated structure–function analyses. Yet, the roles of some proteins of the cyanobacterial PS I are unknown. It is necessary to examine functions of these proteins on a global scale of cell physiology, biogenesis and evolution.
Keywords: Cyanobacteria; Mutant; Posttranslational modification; Structure–function study;

Role of subunits in eukaryotic Photosystem I by Henrik Vibe Scheller; Poul Erik Jensen; Anna Haldrup; Christina Lunde; Juergen Knoetzel (41-60).
Photosystem I (PSI) of eukaryotes has a number of features that distinguishes it from PSI of cyanobacteria. In plants, the PSI core has three subunits that are not found in cyanobacterial PSI. The remaining 11 subunits of the core are conserved but several of the subunits have a different role in eukaryotic PSI. A distinguishing feature of eukaryotic PSI is the membrane-imbedded peripheral antenna. Light-harvesting complex I is composed of four different subunits and is specific for PSI. Light-harvesting complex II can be associated with both PSI and PSII. Several of the core subunits interact with the peripheral antenna proteins and are important for proper function of the peripheral antenna. The review describes the role of the different subunits in eukaryotic PSI. The emphasis is on features that are different from cyanobacterial PSI.
Keywords: Photosynthesis; Photosystem I; Light harvesting complex; Plant; Topology; Electron transport;

P700: the primary electron donor of photosystem I by Andrew N Webber; Wolfgang Lubitz (61-79).
The primary electron donor of photosystem I, P700, is a chlorophyll species that in its excited state has a potential of approximately −1.2 V. The precise chemical composition and electronic structure of P700 is still unknown. Recent evidence indicates that P700 is a dimer of one chlorophyll (Chl) a and one Chl a′. The Chl a′ and Chl a are axially coordinated by His residues provided by protein subunits PsaA and PsaB, respectively. The Chl a′, but not the Chl a, is also H-bonded to the protein. The H-bonding is likely responsible for selective insertion of Chl a′ into the reaction center. EPR studies of P700+⋅ in frozen solution and single crystals indicate a large asymmetry in the electron spin and charge distribution towards one Chl of the dimer. Molecular orbital calculations indicate that H-bonding will specifically stabilize the Chl a′-side of the dimer, suggesting that the unpaired electron would predominantly reside on the Chl a. This is supported by results of specific mutagenesis of the PsaA and PsaB axial His residues, which show that only mutations of the PsaB subunit significantly alter the hyperfine coupling constants associated with a single Chl molecule. The PsaB mutants also alter the microwave induced triplet-minus-singlet spectrum indicating that the triplet state is localized on the same Chl. Excitonic coupling between the two Chl a of P700 is weak due to the distance and overlap of the porphyrin planes. Evidence of excitonic coupling is found in PsaB mutants which show a new bleaching band at 665 nm that likely represents an increased intensity of the upper exciton band of P700. Additional properties of P700 that may give rise to its unusually low potential are discussed.
Keywords: Photosystem I; P700; Electron nuclear double resonance;

Energy transfer and trapping in photosystem I by Bas Gobets; Rienk van Grondelle (80-99).
Keywords: Photosystem I; Excitation energy transfer; Trapping; Time-resolved fluorescence; Target analysis;

Electron transfer in photosystem I by Klaus Brettel; Winfried Leibl (100-114).
This mini-review focuses on recent experimental results and questions, which came up since the last more comprehensive reviews on the subject. We include a brief discussion of the different techniques used for time-resolved studies of electron transfer in photosystem I (PS I) and relate the kinetic results to new structural data of the PS I reaction centre.
Keywords: Photosystem I; Electron transfer; Phylloquinone; Iron–sulfur cluster;

The photosystem (PS) I photosynthetic reaction center was modified thorough the selective extraction and exchange of chlorophylls and quinones. Extraction of lyophilized photosystem I complex with diethyl ether depleted more than 90% chlorophyll (Chl) molecules bound to the complex, preserving the photochemical electron transfer activity from the primary electron donor P700 to the acceptor chlorophyll A0. The treatment extracted all the carotenoids and the secondary acceptor phylloquinone (A1), and produced a PS I reaction center that contains nine molecules of Chls including P700 and A0, and three Fe-S clusters (FX, FA and FB). The ether-extracted PS I complex showed fast electron transfer from P700 to A0 as it is, and to FeS clusters if phylloquinone or an appropriate artificial quinone was reconstituted as A1. The ether-extracted PS I enabled accurate detection of the primary photoreactions with little disturbance from the absorbance changes of the bulk pigments. The quinone reconstitution created the new reactions between the artificial cofactors and the intrinsic components with altered energy gaps. We review the studies done in the ether-extracted PS I complex including chlorophyll forms of the core moiety of PS I, fluorescence of P700, reaction rate between A0 and reconstituted A1, and the fast electron transfer from P700 to A0. Natural exchange of chlorophyll a to 710–740 nm absorbing chlorophyll d in PS I of the newly found cyanobacteria-like organism Acaryochloris marina was also reviewed. Based on the results of exchange studies in different systems, designs of photosynthetic reaction centers are discussed.
Keywords: Electron transfer; Chlorophyll; Quinone; P700; Photosystem I; Reaction center;

Iron–sulfur clusters in type I reaction centers by Ilya R. Vassiliev; Mikhail L. Antonkine; John H. Golbeck (139-160).
Type I reaction centers (RCs) are multisubunit chlorophyll–protein complexes that function in photosynthetic organisms to convert photons to Gibbs free energy. The unique feature of Type I RCs is the presence of iron–sulfur clusters as electron transfer cofactors. Photosystem I (PS I) of oxygenic phototrophs is the best-studied Type I RC. It is comprised of an interpolypeptide [4Fe–4S] cluster, FX, that bridges the PsaA and PsaB subunits, and two terminal [4Fe–4S] clusters, FA and FB, that are bound to the PsaC subunit. In this review, we provide an update on the structure and function of the bound iron–sulfur clusters in Type I RCs. The first new development in this area is the identification of FA as the cluster proximal to FX and the resolution of the electron transfer sequence as FX→FA→FB→soluble ferredoxin. The second new development is the determination of the three-dimensional NMR solution structure of unbound PsaC and localization of the equal- and mixed-valence pairs in FA and FB . We provide a survey of the EPR properties and spectra of the iron–sulfur clusters in Type I RCs of cyanobacteria, green sulfur bacteria, and heliobacteria, and we summarize new information about the kinetics of back-reactions involving the iron–sulfur clusters.
Keywords: Photosystem I; Electron paramagnetic resonance (EPR); Nuclear magnetic resonance (NMR); Electron transfer; Charge recombination; Kinetics; Equilibrium; Iron–sulfur cluster; FA; FB; FX; PsaC;

Ferredoxin and flavodoxin are soluble proteins which are reduced by the terminal electron acceptors of photosystem I. The kinetics of ferredoxin (flavodoxin) photoreduction are discussed in detail, together with the last steps of intramolecular photosystem I electron transfer which precede ferredoxin (flavodoxin) reduction. The present knowledge concerning the photosystem I docking site for ferredoxin and flavodoxin is described in the second part of the review.
Keywords: Photosystem I; Ferredoxin; Flavodoxin; Electron transfer; Iron–sulfur cluster; Docking site;

The vibrational properties of the primary electron donors (P) of type I photosynthetic reaction centers, as investigated by Fourier transform infrared (FTIR) difference spectroscopy in the last 15 years, are briefly reviewed. The results obtained on the microenvironment of the chlorophyll molecules in P700 of photosystem I and of the bacteriochlorophyll molecules in P840 of the green bacteria (Chlorobium) and in P798 of heliobacteria are presented and discussed with special attention to the bonding interactions with the protein of the carbonyl groups and of the central Mg atom of the pigments. The observation of broad electronic transitions in the mid-IR for the cationic state of all the primary donors investigated provides evidence for charge repartition over two (B)Chl molecules. In the green sulfur bacteria and the heliobacteria, the assignments proposed for the carbonyl groups of P and P+ are still very tentative. In contrast, the axial ligands of P700 in photosystem I have been identified and the vibrational properties of the chlorophyll (Chl) molecules involved in P700, P700+, and 3P700 are well described in terms of two molecules, denoted P1 and P2, with very different hydrogen bonding patterns. While P1 has hydrogen bonds to both the 9-keto and the 10a-ester CO groups and bears all the triplet character in 3P700, the carbonyl groups of P2 are free from hydrogen bonding. The positive charge in P700+ is shared between the two Chl molecules with a ratio ranging from 1:1 to 2:1, in favor of P2, depending on the temperature and the species. The localization of the triplet in 3P700 and of the unpaired electron in P700+ deduced from FTIR spectroscopy is in sharp contrast with that resulting from the analysis of the magnetic resonance experiments. However, the FTIR results are in excellent agreement with the most recent structural model derived from X-ray crystallography of photosystem I at 2.5 Å resolution that reveals the hydrogen bonds to the carbonyl groups of the Chl in P700 as well as the histidine ligands of the central Mg atoms predicted from the FTIR data.
Keywords: Photosynthesis; Photosystem I; Type I reaction center; Primary electron donor; Fourier transform infrared spectroscopy;

The application of pulsed electron paramagnetic resonance spectroscopy on short-lived intermediates in Photosystem I is reviewed. The spin polarization in light-induced radical pairs gives rise to a phase shifted ‘out-of-phase’ electron spin echo signal. This echo signal shows a prominent modulation of its intensity as a function of the spacing between the two microwave pulses. Its modulation frequency is determined by the electron–electron spin couplings within the radical pair. Thereby, the measurement of the dipolar coupling gives direct information about the spin–spin distance and can therefore be used to determine cofactor distances with high precision. Application of this technique to the radical pair P⋅+ 700A⋅− 1 in Photosystem I is discussed. Moreover, if oriented samples (e.g. single crystals) are used, the angular dependence of the dipolar coupling can be used to derive the orientation of the axis connecting donor and acceptor with respect to an external (crystal) axes system. Using out-of-phase electron spin echo envelope modulation spectroscopy, the localization of the secondary acceptor quinone A1 has become possible.
Keywords: Pulsed EPR spectroscopy; Short-lived intermediates; Angular dependence; Radical pair P⋅+ 700A⋅− 1;

The use of light-induced spin polarization to study the structure and function of type I reaction centres is reviewed. The absorption of light by these systems generates a series of sequential radical pairs, which exhibit spin polarization as a result of the correlation of the unpaired electron spins. A description of how the polarization patterns can be used to deduce the relative orientation of the radicals is given and the most important structural results from such studies on photosystem I (PS I) are summarized. Quinone exchange experiments which demonstrate the influence of protein–cofactor interactions on the polarization patterns are discussed. The results show that there are significant differences between the binding sites of the primary quinone acceptors in PS I and purple bacterial reaction centres and suggest that π–π interactions probably play a more important role in PS I. Studies using spin-polarized EPR transients and spectra to investigate the electron transfer pathway and kinetics are also reviewed. The results from PS I, green-sulphur bacteria and Heliobacteria are compared and the controversy surrounding the role of a quinone in the electron transfer in the latter two systems is discussed.
Keywords: Electron spin resonance; Electron transfer; Quinone; Radical pair; Iron–sulphur center;

Electron spin echo envelope modulation spectroscopy in photosystem I by Yiannis Deligiannakis; A.W. Rutherford (226-246).
The applications of electron spin echo envelope modulation (ESEEM) spectroscopy to study paramagnetic centers in photosystem I (PSI) are reviewed with special attention to the novel spectroscopic techniques applied and the structural information obtained. We briefly summarize the physical principles and experimental techniques of ESEEM, the spectral shapes and the methods for their analysis. In PSI, ESEEM spectroscopy has been used to the study of the cation radical form of the primary electron donor chlorophyll species, P700 +, and the phyllosemiquinone anion radical, A1 , that acts as a low-potential electron carrier. For P700 +, ESEEM has contributed to a debate concerning whether the cation is localized on a one or two chlorophyll molecules. This debate is treated in detail and relevant data from other methods, particularly electron nuclear double resonance (ENDOR), are also discussed. It is concluded that the ESEEM and ENDOR data can be explained in terms of five distinct nitrogen couplings, four from the tetrapyrrole ring and a fifth from an axial ligand. Thus the ENDOR and ESEEM data can be fully accounted for based on the spin density being localized on a single chlorophyll molecule. This does not eliminate the possibility that some of the unpaired spin is shared with the other chlorophyll of P700 +; so far, however, no unambiguous evidence has been obtained from these electron paramagnetic resonance methods. The ESEEM of the phyllosemiquinone radical A1 provided the first evidence for a tryptophan molecule π-stacked over the semiquinone and for a weaker interaction from an additional nitrogen nucleus. Recent site-directed mutagenesis studies verified the presence of the tryptophan close to A1, while the recent crystal structure showed that the tryptophan was indeed π-stacked and that a weak potential H-bond from an amide backbone to one of the (semi)quinone carbonyls is probably the origin of the to the second nitrogen coupling seen in the ESEEM. ESEEM has already played an important role in the structural charaterization on PSI and since it specifically probes the radical forms of the chromophores and their protein environment, the information obtained is complimentary to the crystallography. ESEEM then will continue to provide structural information that is often unavailable using other methods.
Keywords: Electron spin echo envelope modulation; Hyperfine sublevel correlation spectroscopy; Photosystem I; 14N; 15N; A1; P700;

Keywords: Photosystem I; Green sulfur bacterium; Heliobacterium; A1; A0; P700; P840; P798;

The reaction center of green sulfur bacteria 1 Dedicated to the memory of Jan Amesz. 1 by G Hauska; T Schoedl; Hervé Remigy; G Tsiotis (260-277).
The composition of the P840-reaction center complex (RC), energy and electron transfer within the RC, as well as its topographical organization and interaction with other components in the membrane of green sulfur bacteria are presented, and compared to the FeS-type reaction centers of Photosystem I and of Heliobacteria. The core of the RC is homodimeric, since pscA is the only gene found in the genome of Chlorobium tepidum which resembles the genes psaA and -B for the heterodimeric core of Photosystem I. Functionally intact RC can be isolated from several species of green sulfur bacteria. It is generally composed of five subunits, PscA–D plus the BChl a-protein FMO. Functional cores, with PscA and PscB only, can be isolated from Prostecochloris aestuarii. The PscA-dimer binds P840, a special pair of BChl a-molecules, the primary electron acceptor A0, which is a Chl a-derivative and FeS-center FX. An equivalent to the electron acceptor A1 in Photosystem I, which is tightly bound phylloquinone acting between A0 and FX, is not required for forward electron transfer in the RC of green sulfur bacteria. This difference is reflected by different rates of electron transfer between A0 and FX in the two systems. The subunit PscB contains the two FeS-centers FA and FB. STEM particle analysis suggests that the core of the RC with PscA and PscB resembles the PsaAB/PsaC-core of the P700-reaction center in Photosystem I. PscB may form a protrusion into the cytoplasmic space where reduction of ferredoxin occurs, with FMO trimers bound on both sides of this protrusion. Thus the subunit composition of the RC in vivo should be 2(FMO)3(PscA)2PscB(PscC)2PscD. Only 16 BChl a-, four Chl a-molecules and two carotenoids are bound to the RC-core, which is substantially less than its counterpart of Photosystem I, with 85 Chl a-molecules and 22 carotenoids. A total of 58 BChl a/RC are present in the membranes of green sulfur bacteria outside the chlorosomes, corresponding to two trimers of FMO (42 Bchl a) per RC (16 BChl a). The question whether the homodimeric RC is totally symmetric is still open. Furthermore, it is still unclear which cytochrome c is the physiological electron donor to P840+. Also the way of NAD+-reduction is unknown, since a gene equivalent to ferredoxin-NADP+ reductase is not present in the genome.
Keywords: Green sulfur bacteria; Homodimeric P840 reaction center; FeS type reaction center; Photosynthetic electron transport; Energy transfer; Bacteriochlorophyll protein; Menaquinone; Cytochrome; Scanning transmission electron microscopy particle analysis;

A survey is given of various aspects of the photosynthetic processes in heliobacteria. The review mainly refers to results obtained since 1995, which had not been covered earlier. It first discusses the antenna organization and pigmentation. The pigments of heliobacteria include some unusual species: bacteriochlorophyll (BChl) g, the main pigment, 81 hydroxy chlorophyll a, which acts as primary electron acceptor, and 4,4′-diaponeurosporene, a carotenoid with 30 carbon atoms. Energy conversion within the antenna is very fast: at room temperature thermal equilibrium among the approx. 35 BChls g of the antenna is largely completed within a few ps. This is then followed by primary charge separation, involving a dimer of BChl g (P798) as donor, but recent evidence indicates that excitation of the acceptor pigment 81 hydroxy chlorophyll a gives rise to an alternative primary reaction not involving excited P798. The final section of the review concerns secondary electron transfer, an area that is relatively poorly known in heliobacteria.
Keywords: Heliobacterium; Electron transfer; Energy transfer; Bacteriochlorophyll g;

Daddy, where did (PS)I come from? by Frauke Baymann; Myriam Brugna; Ulrich Mühlenhoff; Wolfgang Nitschke (291-310).
The reacton centre I (RCI)-type photosystems from plants, cyano-, helio- and green sulphur bacteria are compared and the essential properties of an archetypal RCI are deduced. Species containing RCI-type photosystems most probably cluster together on a common branch of the phylogenetic tree. The predicted branching order is green sulphur, helio- and cyanobacteria. Striking similarities between RCI- and RCII-type photosystems recently became apparent in the three-dimensional structures of photosystem I (PSI), PSII and RCII. The phylogenetic relationship between all presently known photosystems is analysed suggesting (a) RCI as the ancestral photosystem and (b) the descendence of PSII from RCI via gene duplication and gene splitting. An evolutionary model trying to rationalise available data is presented.
Keywords: Photosystem I; RCI type reaction centre; Bioenergetics; Electron transport; Evolution;

Author Index (311).

Contents (312).