Alexander N. Melkozernov

Structure and function of photosystem I: interaction with its soluble electron carriers and external antenna systems

Photosystem I (PS I) is a large membrane protein complex that catalyzes the first step of solar conversion, the light‐induced transmembrane electron transfer, and generates reductants for CO2 assimilation. It consists of 12 different proteins and 127 cofactors that perform light capturing and electron transfer. The function of PS I includes inter‐protein electron transfer between PS I and smaller soluble electron transfer proteins. The structure of PS I is discussed with respect to the potential docking sites for the soluble electron acceptors, ferredoxin/flavodoxin, at the stromal side and the soluble electron donors, cytochrome c 6/plastocyanin, at the luminal side of the PS I complex. Furthermore, the potential interaction sites with the peripheral antenna proteins are discussed.

Excitation energy transfer in Photosystem I from oxygenic organisms.

This Review discusses energy transfer pathways in Photosystem I (PS I) from oxygenic organisms. In the trimeric PS I core from cyanobacteria, the efficiency of solar energy conversion is largely determined by ultrafast excitation transfer processes in the core chlorophyll a (Chl a) antenna network and efficient photochemical trapping in the reaction center (RC). The role of clusters of Chl a in energy equilibration and photochemical trapping in the PS I core is discussed. Dimers of the longest-wavelength absorbing (red) pigments with strongest excitonic interactions localize the excitation in the PS I core antenna. Those dimers that are located closer to the RC participate in a fast energy equilibration with coupled pigments of the RC. This suggests that the function of the red pigments is to concentrate the excitation near the RC. In the PS I holocomplex from algae and higher plants, in addition to the red pigments of the core antenna, spectrally distinct red pigments are bound to the peripheral Chl a/b-binding light-harvesting antenna (LHC I), specifically to the Lhca4 subunit of the LHC I-730 complex. Intramonomeric energy equilibration between pools of Chl b and Chl a in Lhca1 and Lhca4 monomers of the LHC I-730 heterodimer are as fast as the energy equilibration processes within the PS I core. In contrast to the structural stability of the PS I core, the flexible subunit structure of the LHC I would probably determine the observed slow excitation energy equilibration processes in the range of tens of picoseconds. The red pigments in the LHC I are suggested to function largely as photoprotective excitation sinks in the peripheral antenna of PS I.

Ultrafast excitation dynamics of low energy pigments in reconstituted peripheral light-harvesting complexes of photosystem I

Ultrafast dynamics of a reconstituted Lhca4 subunit from the peripheral LHCI‐730 antenna of photosystem I of higher plants were probed by femtosecond absorption spectroscopy at 77 K. Intramonomeric energy transfer from chlorophyll (Chl) b to Chl a and energy equilibration between Chl a molecules observed on the subpicosecond time scale are largely similar to subpicosecond energy equilibration processes within LHCII monomers. However, a 5 ps equilibration process in Lhca4 involves unique low energy Chls in LHCI absorbing at 705 nm. These pigments localize the excitation both in the Lhca4 subunit and in LHCI‐730 heterodimers. An additional 30–50 ps equilibration process involving red pigments of Lhca4 in the heterodimer, observed by transient absorption and picosecond fluorescence spectroscopy, was ascribed to intersubunit energy transfer.

Structural and functional organization of the peripheral light-harvesting system in photosystem I.

This review centers on the structural and functional organization of the light-harvesting system in the peripheral antenna of Photosystem I (LHC I) and its energy coupling to the Photosystem I (PS I) core antenna network in view of recently available structural models of the eukaryotic Photosystem I–LHC I complex, eukaryotic LHC II complexes and the cyanobacterial Photosystem I core. A structural model based on the 3D homology of Lhca4 with LHC II is used for analysis of the principles of pigment arrangement in the LHC I peripheral antenna, for prediction of the protein ligands for the pigments that are unique for LHC I and for estimates of the excitonic coupling in strongly interacting pigment dimers. The presence of chlorophyll clusters with strong pigment–pigment interactions is a structural feature of PS I, resulting in the characteristic red-shifted fluorescence. Analysis of the interactions between the PS I core antenna and the peripheral antenna leads to the suggestion that the specific function of the red pigments is likely to be determined by their localization with respect to the reaction center. In the PS I core antenna, the Chl clusters with a different magnitude of low energy shift contribute to better spectral overlap of Chls in the reaction center and the Chls of the antenna network, concentrate the excitation around the reaction center and participate in downhill enhancement of energy transfer from LHC II to the PS I core. Chlorophyll clusters forming terminal emitters in LHC I are likely to be involved in photoprotection against excess energy.

Spectral inhomogeneity of photosystem I and its influence on excitation equilibration and trapping in the cyanobacterium Synechocystis sp. PCC6803 at 77 K.

Ultrafast transient absorption spectroscopy was used to probe excitation energy transfer and trapping at 77 K in the photosystem I (PSI) core antenna from the cyanobacterium Synechocystis sp. PCC 6803. Excitation of the bulk antenna at 670 and 680 nm induces a subpicosecond energy transfer process that populates the Chl a spectral form at 685--687 nm within few transfer steps (300--400 fs). On a picosecond time scale equilibration with the longest-wavelength absorbing pigments occurs within 4-6 ps, slightly slower than at room temperature. At low temperatures in the absence of uphill energy transfer the energy equilibration processes involve low-energy shifted chlorophyll spectral forms of the bulk antenna participating in a 30--50-ps process of photochemical trapping of the excitation by P(700). These spectral forms might originate from clustered pigments in the core antenna and coupled chlorophylls of the reaction center. Part of the excitation is trapped on a pool of the longest-wavelength absorbing pigments serving as deep traps at 77 K. Transient hole burning of the ground-state absorption of the PSI with excitation at 710 and 720 nm indicates heterogeneity of the red pigment absorption band with two broad homogeneous transitions at 708 nm and 714 nm (full-width at half-maximum (fwhm) approximately 200--300 cm(-1)). The origin of these two bands is attributed to the presence of two chlorophyll dimers, while the appearance of the early time bleaching bands at 683 nm and 678 nm under excitation into the red side of the absorption spectrum (>690 nm) can be explained by borrowing of the dipole strength by the ground-state absorption of the chlorophyll a monomers from the excited-state absorption of the dimeric red pigments.

Orientation and excitonic interactions of the Fenna-Matthews-Olson bacteriochlorophyll a protein in membranes of the green sulfur bacterium Chlorobium tepidum

Linear and circular dichroism spectra of isolated bacteriochlorophyll a proteins (FMO proteins) and membrane vesicles containing FMO protein from the green sulfur bacterium Chlorobium tepidum were measured at room temperature and 77 K. The orientation of membranes and isolated FMO protein was obtained by gel squeezing. Linear dichroism (LD) data indicate that isolated FMO protein and membrane vesicles associated with the FMO protein are oriented in a similar way in a squeezed polyacrylamide gel. Both samples show a characteristic negative LD band around 814 nm with flanking positive bands at 802 and 824 nm ascribed to the Qy excitonic transitions of BChl a of the FMO protein. This confirms that the C3 symmetry axis of the trimer is perpendicular to the membrane plane, which is supported by the model of the disc-like structure of FMO protein trimers of Cb. tepidum [Li Yi-Fen, Zhou W, Blankenship RE, and Allen JP (1997) J Mol Biol 272: 456–471]. The LD data are consistent with either BChl 3 or 6, but not 7 as the principal contributor to the low temperature band at 825 nm. The low temperature linear and circular dichroism spectra of FMO protein trimers from Chlorobium tepidum show significant differences from the low temperature LD and CD spectra of FMO protein trimers from Prosthecochloris aestuarii. The data are interpreted in terms of somewhat different pigment-protein and pigment-pigment interactions in the two complexes.

Structural modeling of the Lhca4 subunit of LHCI-730 peripheral antenna in photosystem I based on similarity with LHCII

Peripheral chlorophyll a/b binding antenna of photosystem I (LHCI) from green algae and higher plants binds specific low energy absorbing chlorophylls (red pigments) that give rise to a unique red-shifted emission. A three-dimensional structural model of the Lhca4 polypeptide from the LHCI from higher plants was constructed on the basis of comparative sequence analysis, secondary structure prediction, and homology modeling using LHCII as a template. The obtained model of Lhca4 helps to visualize protein ligands to nine chlorophylls (Chls) and three potential His residues to extra Chls. Central domain of the Lhca4 comprising the first (A) and the third (C) transmembrane (TM) helices that binds 6 Chl molecules and two carotenoids is conserved structurally, whereas the interface between the first and the second TM helices and the outer surface of the second TM helix differ significantly among the LHCI and LHCII polypeptides. The model of Lhca4 predicts a histidine residue in the second TM helix, a potential binding site for extra Chl in close proximity to Chls a5 and b5 (labeling by Kühlbrandt). The interpigment interactions in the formed pigment cluster are suggested to cause a red spectral shift in absorption and emission. Modeling of the LHCI-730 heterodimer based on the model structures of Lhca1 and Lhca4 allowed us to suggest potential sites of pigment-pigment interactions that might be formed upon heterodimerization or docking of the LHCI dimers to the surface of PSI.

Excitation energy transfer in thylakoid membranes from Chlamydomonas reinhardtii lacking chlorophyll b and with mutant Photosystem I

Energy trapping in Photosystem I (PS I) was studied by time-resolved fluorescence spectroscopy of PS II-deleted Chl b-minus thylakoid membranes isolated from site-directed mutants of Chlamydomonas reinhardtii with specific amino acid substitutions of a histidine ligand to P700. In vivo the fluorescence of the PS I core antenna in mutant thylakoids with His-656 of PsaB replaced by asparagine, serine or phenylalanine is characterized by an increase in the lifetime of the fast decay component ascribed to the energy trapping in PS I (25 ps in wild type PS I with intact histidine-656, 50 ps in the mutant PS I with asparagine-656 and 70 ps in the mutant PS I with phenylalanine-656). Assuming that the excitation dynamics in the PS I antenna are trap-limited, the increase in the trapping time suggests a decrease in the primary charge separation rate. Western blot analysis showed that the mutants accumulate significantly less PS I than wild type. Spectroscopically, the mutations lead to a decrease in relative quantum yield of the trapping in the PS I core and increase in relative quantum yield of the fluorescence decay phase ascribed to uncoupled chlorophyll–protein complexes which suggests that improper assembly of PS I and LHC in the mutant thylakoids may result in energy uncoupling in PS I.

Energy Equilibration in the Antenna of Photosystem I from Cyanobacterium Synechocystis SP . PCC 6803

The Photosystem I core complex of cyanobacteria, green algae and higher plants is a unique membrane Chl a-binding protein that combines features of both an effective solar energy antenna and a converter of Chl excitation into the chemical energy of transmembrane charge separation via electron transfer reactions of the RC. The recent 4 A crystal structure of cyanobacterial PS I (1) revealed a network of about 80 Chl a molecules that surround the electron transfer cofactors of the RC. The distances between Chl a molecules in the core antenna lie in the range of 10–20 A, which enable fast energy transfer reactions. The excitation of a Chl a molecule results in a fast 0.2 Ps excitation hopping between neighboring Chls (2) followed by the energy equilibration over the antenna that for cyanobacteria is reported to be in the range of 2–4 ps (3). Finally, the excitation of the thermally equilibrated antenna is trapped by P700 within 20–30 ps in Synechocystis sp. PS I(3, 4) or 35 Ps in Synechococcus PS I(5).

From Ionizing Radiation to Photosynthesis

This chapter focuses on the driving forces of the origin and the evolution of the early anoxygenic photosynthesis before the onset of the oxygenic cyanobacterial photosynthesis 2.8–2.4 billion years ago. The early Earth conditions were sustained by the global cycling of carbon dioxide among the atmosphere, primordial crust, and the ocean. High-energy ultraviolet radiation played a decisive role in this interaction, maintaining the temperature of the ocean and promoting mildly oxidizing conditions in the atmosphere and the upper layers of the ocean. Constrained by the sterilizing effect of the UV radiation and the evaporating consequences of the Late Heavy Bombardment, chemolithotrophic life was likely to emerge at the seafloor under excess of carbon dioxide and a constant flux of reducing hydrothermal fluids. Trophic diversification had launched the expansion of the chemolithotrophic lineages to the photic zone. Alternative hypotheses of the terrestrial origin of life with implications for the origin of photosynthesis are discussed. The review reconstructs the development of the ancient photosynthetic habitats under the strong selective pressure of the UV radiation and discusses available hypotheses of the origin of photosynthesis with regard to emergence of workable photosynthetic mechanism. The evolution of the homodimeric proto-photosystem and its divergence to the homodimeric iron–sulfur-type and the homodimeric quinone-type reaction centers were probably driven by the availability of the exogenous electron donors in a series of ecological successions from the oceanic photic zone to the shallow water habitats and microbial mats. This process had eventually resulted in the advent of independence of the photosynthetic organisms from the geochemical conditions in cyanobacterial lineages that were able to oxidize water.