Cytochrome b6-f complex : The carburettor of exciton distribution in oxygenic photosynthesis

Efficient oxygenic photosynthesis not only requires synchronous turover and operation of photosystem I (PS I) and photosystem II (PS II) but also the preferential turnover of PS I for cyclic photophosphorylation to maintain required ATP and NADPH ratio during carbon dioxide reduction. Ohe initial higher rate of turnover of PS IIin viva is accounted by the fact that (i) PS I contains only about one-third of total chlorophylls, (ii) about 90% of light harvesting a/b protein (LAC) which accounts for about 50% of the total chlorophylls, remains associated with PS II as PS II-LHC II complexes (PS II_α and (iii) the ratio of PS II/PS I is always greater than unity, in the range of 1–2 : 1 under different environmental regimes. Ohe initial preferential feeding of PS II, due to its larger antenna, is bound to result in faster rate of turn over of PS II than PS I, leading to higher rate of reduction of an intersystem carrier than the rate of its oxidation by PS I. Ohe light dependent phosphorylation of a ‘mobile’ and small pool (−20%) of LHC II of PS II_α (possibly located at the edge of appressed regions of the membranes) increases the repulsive forces of LHC II resulting in its migration to non-appressed region associating itself with PS 1. Ohe phosphorylation itself is controlled by the redox state of an intermediate of electron transport. Several experimental approaches have provided evidence which suggest that (i) phosphorylation of LAC II involves interaction of cyt b5-f complex with LAC II kinase and the interaction of QA with cyt b₅-f complex and (ii) different kinases may be involved in phosphorylation of LHC IIversus PS II polypeptides. Ohe major purpose of light dependent LAC II phosphorylation and its consequent migration close to PS I appears to balance the rate of cyclicversus non-cyclic photophosphorylation. Ohe mechanism by which cyt b₅-f complex controls the activation of LAC II is not known. Ohe role of membrane bound ealmodulin, electron transfer through cyt b₆-f complex in activation of LAC II kinase should be explored.     

The enigmatic cytochrome b-559 of oxygenic photosynthesis

The ubiquitous and obligatory association of cytochrome b -559 with the photosystem II reaction center of oxygenic photosynthesis is a conundrum since it seems not to have a function in the primary electron transport pathway of oxygen evolution. A model for the cytochrome structure that satisfies the cis -positive rule for membrane protein assembly consists of two short, non-identical hydrophobic membrane-spanning polypeptides (α and β), each containing a single histidine residue, as ligands for the bridging heme prosthetic group that is on the side of the membrane opposite to the water splitting apparatus. The ability of the heterodimer, but not the single α-subunit, to satisfy the cis -positive rule implies that the cytochrome inserts into the membrane as a heterodimer, with some evidence implicating it as the first membrane inserted unit of the assembling reaction center. The very positive redox potential of the cytochrome can be explained by a position for the heme in a hydrophobic niche near the stromal aqueous interface where it is also influenced by the large positive dipole potential of the parallel α-helices of the cytochrome. The requirement for the cytochrome in oxygenic photosynthesis may be a consequence of the presence of the strongly oxidizing reaction center needed for H₂O-splitting. This may lead to the need, under conditions of stress or plastid development, for an alternate source of electrons when the H₂O-splitting system is not operative as a source of reductant for the reaction center.     

Carotenoids,versatile components of oxygenic photosynthesis

Carotenoids (CARs) are a group of pigments that perform several important physiological functions in all kingdoms of living organisms. CARs serve as protective agents, which are essential structural components of photosynthetic complexes and membranes, and they play an important role in the light harvesting mechanism of photosynthesizing plants and cyanobacteria. The protection against reactive oxygen species, realized by quenching of singlet oxygen and the excited states of photosensitizing molecules, as well as by the scavenging of free radicals, is one of the main biological functions of CARs. X-ray crystallographic localization of CARs revealed that they are present at functionally and structurally important sites of both the PSI and PSII reaction centers. Characterization of a CAR-less cyanobacterial mutant revealed that while the absence of CARs prevents the formation of PSII complexes, it does not abolish the assembly and function of PSI. CAR molecules assist in the formation of protein subunits of the photosynthetic complexes by gluing together their protein components. In addition to their aforementioned indispensable functions, CARs have a substantial role in the formation and maintenance of proper cellular architecture, and potentially also in the protection of the translational machinery under stress conditions.     

The age of Rubisco: the evolution of oxygenic photosynthesis

The evolutionary history of oxygenesis is controversial. Form I of ribulose 1,5‐bisphosphate carboxylase/oxygenase (Rubisco) in oxygen‐tolerant organisms both enables them to carry out oxygenic extraction of carbon from air and enables the competitive process of photorespiration. Carbon isotopic evidence is presented from ~2.9 Ga stromatolites from Steep Rock, Ontario, Canada, ~2.9 Ga stromatolites from Mushandike, Zimbabwe, and ~2.7 Ga stromatolites in the Belingwe belt, Zimbabwe. The data imply that in all three localities the reef‐building autotrophs included organisms using Form I Rubisco. This inference, though not conclusive, is supported by other geochemical evidence that these stromatolites formed in oxic conditions. Collectively, the implication is that oxygenic photosynthesizers first appeared ~2.9 Ga ago, and were abundant 2.7–2.65 Ga ago. Rubisco specificity (its preference for CO₂ over O₂) and compensation constraints (the limits on carbon fixation) may explain the paradox that despite the inferred evolution of oxygenesis 2.9 Ga ago, the Late Archaean air was anoxic. The atmospheric CO₂:O₂ ratio, and hence greenhouse warming, may reflect Form I Rubisco's specificity for CO₂ over O₂. The system may be bistable under the warming Sun, with liquid oceans occurring in either anoxic (H₂O with abundant CH₄ plus CO₂) or oxic (H₂O with more abundant CO₂, but little CH₄) greenhouse states. Transition between the two states would involve catastrophic remaking of the biosphere. Build‐up of a very high atmospheric inventory of CO₂ in the 2.3 Ga glaciation may have allowed the atmosphere to move up the CO₂ compensation line to reach stability in an oxygen‐rich system. Since then, Form I Rubisco specificity and consequent compensation limits may have maintained the long‐term atmospheric disproportion between O₂ and CO₂, which is now close to both CO₂ and O₂ compensation barriers.     

Recent advances on the structure and function of NDH-1: The complex I of oxygenic photosynthesis

Photosynthetic NADH dehydrogenase-like complex type-1 (a.k.a, NDH, NDH-1, or NDH-1L) is a multi-subunit, membrane-bound oxidoreductase related to the respiratory complex I. Although originally discovered 30 years ago, a number of recent advances have revealed significant insight into the structure, function, and physiology of NDH-1. Here, we highlight progress in understanding the function of NDH-1 in the photosynthetic light reactions of both cyanobacteria and chloroplasts from biochemical and structural perspectives. We further examine the cyanobacterial-specific forms of NDH-1 that possess vectorial carbonic anhydrase (vCA) activity and function in the CO₂-concentrating mechanism (CCM). We compare the proposed mechanism for the cyanobacterial NDH-1 vCA-activity to that of the DAB (DABs accumulates bicarbonate) complex, another putative vCA. Finally, we discuss both new and remaining questions pertaining to the mechanisms of NDH-1 complexes in light of these recent advances.     

Hydrogen peroxide and the evolution of oxygenic photosynthesis

The early atmosphere of the Earth is considered to have been reducing (H₂ rich) or neutral (CO₂-N₂). The present atmosphere by contrast is highly oxidizing (20% O₂). The source of this oxygen is generally agreed to have been oxygenic photosynthesis, whereby organisms use water as the electron donor in the production of organic matter, liberating oxygen into the atmosphere. A major question in the evolution of life is how oxygenic photosynthesis could have evolved under anoxic conditions — and also when this capability evolved. It seems unlikely that water would be employed as the electron donor in anoxic environments that were rich in reducing agents such as ferrous or sulfide ions which could play that role. The abiotic production of atmospheric oxidants could have provided a mechanism by which locally oxidizing conditions were sustained within spatially confined habitats thus removing the available reductants and forcing photosynthetic organisms to utilize water as the electron donor. We suggest that atmospheric H₂O₂ played the key role in inducing oxygenic photosynthesis because as peroxide increased in a local environment, organisms would not only be faced with a loss of reductant, but they would also be pressed to develop the biochemical apparatus (e.g., catalase) that would ultimately be needed to protect against the products of oxygenic photosynthesis. This scenario allows for the early evolution of oxygenic photosynthesis while global conditions were still anaerobic.     

The evolutionary consequences of oxygenic photosynthesis: a body size perspective

The high concentration of molecular oxygen in Earth??s atmosphere is arguably the most conspicuous and geologically important signature of life. Earth??s early atmosphere lacked oxygen; accumulation began after the evolution of oxygenic photosynthesis in cyanobacteria around 3.0?C2.5 billion years ago (Gya). Concentrations of oxygen have since varied, first reaching near-modern values ~600 million years ago (Mya). These fluctuations have been hypothesized to constrain many biological patterns, among them the evolution of body size. Here, we review the state of knowledge relating oxygen availability to body size. Laboratory studies increasingly illuminate the mechanisms by which organisms can adapt physiologically to the variation in oxygen availability, but the extent to which these findings can be extrapolated to evolutionary timescales remains poorly understood. Experiments confirm that animal size is limited by experimental hypoxia, but show that plant vegetative growth is enhanced due to reduced photorespiration at lower O₂:CO₂. Field studies of size distributions across extant higher taxa and individual species in the modern provide qualitative support for a correlation between animal and protist size and oxygen availability, but few allow prediction of maximum or mean size from oxygen concentrations in unstudied regions. There is qualitative support for a link between oxygen availability and body size from the fossil record of protists and animals, but there have been few quantitative analyses confirming or refuting this impression. As oxygen transport limits the thickness or volume-to-surface area ratio??rather than mass or volume??predictions of maximum possible size cannot be constructed simply from metabolic rate and oxygen availability. Thus, it remains difficult to confirm that the largest representatives of fossil or living taxa are limited by oxygen transport rather than other factors. Despite the challenges of integrating findings from experiments on model organisms, comparative observations across living species, and fossil specimens spanning millions to billions of years, numerous tractable avenues of research could greatly improve quantitative constraints on the role of oxygen in the macroevolutionary history of organismal size.     

Geological constraints on the origin of oxygenic photosynthesis

This article examines the geological evidence for the rise of atmospheric oxygen and the origin of oxygenic photosynthesis. The evidence for the rise of atmospheric oxygen places a minimum time constraint before which oxygenic photosynthesis must have developed, and was subsequently established as the primary control on the atmospheric oxygen level. The geological evidence places the global rise of atmospheric oxygen, termed the Great Oxidation Event (GOE), between ~2.45 and ~2.32 Ga, and it is captured within the Duitschland Formation, which shows a transition from mass-independent to mass-dependent sulfur isotope fractionation. The rise of atmospheric oxygen during this interval is closely associated with a number of environmental changes, such as glaciations and intense continental weathering, and led to dramatic changes in the oxidation state of the ocean and the seawater inventory of transition elements. There are other features of the geologic record predating the GOE by as much as 200–300 million years, perhaps extending as far back as the Mesoarchean–Neoarchean boundary at 2.8 Ga, that suggest the presence of low level, transient or local, oxygenation. If verified, these features would not only imply an earlier origin for oxygenic photosynthesis, but also require a mechanism to decouple oxygen production from oxidation of Earth’s surface environments. Most hypotheses for the GOE suggest that oxygen production by oxygenic photosynthesis is a precondition for the rise of oxygen, but that a synchronous change in atmospheric oxygen level is not required by the onset of this oxygen source. The potential lag-time in the response of Earth surface environments is related to the way that oxygen sinks, such as reduced Fe and sulfur compounds, respond to oxygen production. Changes in oxygen level imply an imbalance in the sources and sinks for oxygen. Changes in the cycling of oxygen have occurred at various times before and after the GOE, and do not appear to require corresponding changes in the intensity of oxygenic photosynthesis. The available geological constraints for these changes do not, however, disallow a direct role for this metabolism. The geological evidence for early oxygen and hypotheses for the controls on oxygen level are the basis for the interpretation of photosynthetic oxygen production as examined in this review.     

Photoevolution of hydrogen during oxygenic photosynthesis of cyanobacteriumNostoc muscorum

Summary Nitrogen fixing cultures of the cyanobacteriumNostocmuscorum lacked hydrogen evolution but cultures infected with cyanophage N-1 showed significant hydrogen evolution and inactive nitrogenase, suggesting that nitrogenase activity is not responsible for the observed oxygen-resistant photoproduction of hydrogen. Significant oxygen-resistant hydrogen production by nitrate or ammonium assimilating cultures deficient in both nitrogenase and uptake hydrogenase activity supports this conclusion. These findings suggest a role of uptake hydrogenase in blocking the production of hydrogen during aerobic photosynthetic conditions.     

Carbon dioxide assimilation in oxygenic and anoxygenic photosynthesis

This article represents a summary of our contemporary understanding of carbon dioxide assimilation in photosynthesis, including both the oxygen-evolving (oxygenic) type characteristic of cyanobacteria, algae and higher plants, and the non-oxygen-evolving (anoxygenic) type characteristic of other bacteria. Mechanisms functional in the regulation of the reductive pentose phosphate cycle of oxygenic photosynthesis are emphasized, as is the reductive carboxylic acid cycle-the photosynthetic carbon pathway functional in anoxygenic green sulfur bacteria. Thioredoxins, an ubiquitous group of low molecular weight proteins with catalytically active thiols, are also described in some detail, notably their role in regulating the reductive pentose phosphate cycle of oxygenic photosynthesis and their potential use as markers to trace the evolutionary development of photosynthesis.Abbreviations NADP-GAPDH-NADP glyceraldehyde 3-phosphate dehydrogenase - FBPase fructose 1,6-bisphosphatase - FTR ferredoxin-thioredoxin reductase - Rubisco ribulose 1,5-bisphosphate carboxylase/oxygenase - SBPase sedoheptulos 1,7-bisphosphatase - PRK phosphoribulokinase - NADP-MDH-NADP malate dehydrogenase - CF1-ATPase chloroplast coupling factor - G6PDH glucose 6-phosphate dehydrogenase Most of the references cited in this article are reviews. For references to specific material, readers should consult the appropriate review.     

Characterization of the high-spin heme x in the cytochrome b6f complex of oxygenic photosynthesis

X-ray structures at 3.0-3.1 A resolution of the cytochrome b(6) f complex from the cyanobacterium Mastigocladus laminosus [Kurisu, G., Zhang, H., Smith, J. L., and Cramer, W. A. (2003) Science 302, 1009-1014] and the green alga Chlamydomonas reinhardtii [Stroebel, D., Choquet, Y., Popot, J.-L., and Picot, D. (2003) Nature 426, 413-418] showed the presence of a unique heme, hemex, that is covalently linked by a single thioether bond to a Cys residue (Cys35) on the electrochemically negative (n) side of the cytochrome b(6) polypeptide. Heme x faces the intermonomer quinone exchange cavity. The only axial ligand associated with this heme is a H(2)O or OH(-) that is H-bonded to the propionate of the stromal side heme b(n), showing that it is pentacoordinate. The spectral properties of this heme were hardly defined at the time of the structure determination. The pyridine hemochromagen redox difference spectrum for heme x covalently bound to the cytochrome b polypeptide isolated from SDS-PAGE displays a low-amplitude broad spectrum with a peak at 553 nm, similar to that of other hemes with a single thioether linkage. The binding of CO and a hydrophobic cyanide analogue, butyl isocyanide, to dithionite-reduced b(6) f complex perturbs and significantly shifts the redox difference visible spectrum. Together with EPR spectra displaying g values of the oxidized complex of 6.7 and 7.4, heme x is defined as a ferric high-spin heme in a rhombic environment. In addition to a possible function in photosystem I-linked cyclic electron transport, the five-coordinate state implies that there is at least one more function of heme x that is related to axial binding of a physiological ligand.     

Evolution and unique bioenergetic mechanisms in oxygenic photosynthesis

Oxygenic photosynthesis is one example of the many bioenergetic pathways utilized by different organisms to harvest energy from the environment. These pathways revolve around a theme of coupling oxidation-reduction reactions to the formation of membrane potential and subsequent ATP synthesis. Although the basic principles underlying bioenergetics are universally conserved, the constituents of the bioenergetic pathways in different organisms have evolved unique aspects to fill an evolutionary niche. Three-dimensional structures of all of the membrane-spanning components of the electron-transfer chain of oxygenic photosynthesis have revealed those unique aspects of this fascinating process, including the unique metallocofactor for catalysis, the determinants of the uniquely high voltage cofactor, and the numerous photoprotective mechanisms that guard against radical damage.     

Tryptophan-heme pi-electrostatic interactions in cytochrome f of oxygenic photosynthesis

Cytochrome f of oxygenic photosynthesis has an unprecedented structure, including the N-terminus being a heme ligand. The adjacent N-terminal heme-shielding domain is enriched in aromatic amino acids. The atomic structures of the chloroplast and cyanobacterial cytochromes f were compared to explain spectral and redox differences between them. The conserved aromatic side chain in the N-terminal heme-shielding peptide at position 4, Phe and Tyr in plants and algae, respectively, and Trp in cyanobacteria, is in contact with the heme. Mutagenesis of cytochrome f from the eukaryotic green alga Chlamydomonas reinhardtii showed that a Phe4 --> Trp substitution in the N-terminal domain was unique in causing a red shift of 1 and 2 nm in the cytochrome Soret (gamma) and Q (alpha) visible absorption bands, respectively. The resulting alpha band peak at 556 nm is characteristic of the cyanobacterial cytochrome. Conversely, a Trp4 --> Phe mutation in the expressed cytochrome from the cyanobacterium Phormidium laminosum caused a blue shift to the 554 nm alpha band peak diagnostic of the chloroplast cytochrome. Residue 4 was found to be the sole determinant of this 60 cm(-)(1) spectral shift, and of approximately one-half of the 70 mV redox potential difference between cytochrome f of P. laminosum and C. reinhardtii (E(m7) = 297 and 370 mV, respectively). The proximity of Trp-4 to the heme implies that the spectral and redox potential shifts arise through differential interaction of its sigma- or pi-electrostatic potential with the heme ring and of the pi-potential with the heme Fe orbitals, respectively. The dependence of the visible spectrum and redox potential of cytochrome f on the identity of aromatic residue 4 provides an example of the use of the relatively sharp cytochrome spectrum as a "spectral fingerprint", and of the novel structural connection between the heme and a single nonliganding residue.     

Concerning the enigmatic cytochrome b-559 of oxygenic photosynthesis

Photosynthesis Research - Although there is an extensive literature on the properties and possible electron transfer pathways of cytochrome b-559, which is a prominent subunit of the multi-subunit...     

Stoichiometric determination of pheophytin in photosystem II of oxygenic photosynthesis

Pheophytin and chlorophyll extracted from oxygen-evolving photosystem II particles, chloroplast thylakoids and cyanobacterial cells were separated by column chromatography with DEAE-Toyopearl, and quantitatively determined by spectrophotometry. The molecular ratio of chlorophyll a+b to pheophytin a was about 100 in spinach photosystem II particles and about 140 in spinach thylakoids. Using flash spectrophotometry of P680 and measurement of flash-induced oxygen yield, the molecular ratio of the chlorophyll to the photochemical reaction center II was determined to be about 200 in the photosystem II particles. These findings suggest that the stoichiometry in photosystem II particles is one reaction center II and two pheophytin a molecules per about 200 chlorophyll molecules. The same stoichiometry for pheophytin to the reaction center II was obtained in the cyanobacteria, Anacystis nidulans and Synechocystis PCC 6714. A quantitative determination of pheophytin a and the electron donor P700 in stroma thylakoids from pokeweed suggests that photosystem I does not contain pheophytin.Dedicated to Prof. L.N.M. Duysens on the occasion of his retirement.