Photosystems I and II in 3-D

Data derived from the Science Watch/Hot Papers database and the Web of Science (ISI, Philadelphia) show that Hot Papers are cited 50 to 100 times more often than the average paper of the same type and age. P. Jordan et al., "Three-dimensional structure of cyanobacterial photosystem I at 2.5 angstrom resolution," Nature, 411:909-17, 2001. (cited in 176 papers) A. Zouni et al., "Crystal structure of photosystem II from Synechococcus elongatus at 3.8 angstrom resolution," Nature, 409:739-43, 2001. (cited in 276 papers) At the heart of photosynthesis, photosystems I and II hold a special place, not just in plant biology, but also as the source of oxygen for all aerobic life. Therefore, researchers were not surprised when two papers, describing the detailed, three-dimensional crystal structure of the photoreaction centers of the thermophilic cyanobacterium, Synechococcus elongatus, received high acclaim.1,2 A single Berlin group published both structures within five months. "These structures represent a major leap forward in photosynthesis research, and for one team to produce two such structures in quick succession caused a huge stir in the field," says Mike Jones, Bristol University, UK, who specializes in bacterial reaction centers. The team from Freie Unversitat and Technische Unversitat exploited the same X-ray crystallography advances to produce sufficiently stable crystals of the two photocenters, particularly photosystem I (PSI), for high-resolution diffraction. While the photosystem II (PSII) resolution is slightly lower, the molecule's significance as the source of atmospheric oxygen drew much attention, as its structure will help researchers elucidate the elegant machinery through which molecules harness the sun's power without being destroyed. Courtesy of Paul Fyfe and Mike Jones, University of Bristol PHOTOSYSTEMS REVEALED: X-ray crystal structures of Photosystem I and Photosystem II (left to right) from Synechococcus elongatus, as determined by the Hot Papers' authors. The reduced level of detail on the Photosystem II reflects the more modest resolution of the structure. The horizontal bar shows the membrane's approximate position. THE LOW-RES SOLUTION X-ray crystallography has become the gold standard for deriving protein structure. About 82% of 3-D data in the Protein Data Bank was obtained from X-ray crystallography, compared with 16% from nuclear magnetic resonance imaging, and 2% from theoretical modeling. Crystals are exposed to X-rays, which are diffracted by the electron clouds of the constituent atoms, yielding a pattern comprising thousands of spots. These are the raw data from which the detailed 3D structure is inferred. X-rays are well suited to the task, because their wavelength matches the atomic diameters in the order of 1 to 10 angstrom units (i.e., 1 to 10 x 10-10 meters). Yet technical difficulties, notably the need to obtain sufficiently stable and high-quality single crystals, make the work challenging. On average, there is an uncertainty in the position of an atom equal to 10% to 20% of the resolution. Therefore, 5 Å resolution fails to resolve accurately the position of constituent atoms, especially smaller inorganics such as manganese atoms, in the case of PSII. At 2.5 Å resolution, more atomic positions can be resolved with adequate precision, while 1.2 Å resolution is high for a protein, enabling even hydrogen atoms to be placed reasonably accurately. The major breakthroughs that enabled the studies were in crystallization techniques. PSI had been studied longer and thus proved easier to crystallize; the group obtained 2.5 Å resolution for PSI, compared with 3.8 Å resolution for PSII. Consequently, the PSI paper has had the more immediate and profound influence on other scientists' work, according to biochemistry professor Peter Heathcote, Queen Mary College, University of London. "PSII needs a higher resolution structure in order that we can actually model its amino acid side chains and see ligands of oxygen-evolving complex of manganese [and calcium]." Petra Fromme, a coauthor on both papers who is now at Arizona State University, concedes the need for a higher resolution 3-D model of PSII: "PSII has an extra-complicating factor in that it is close to the thermodynamic limit, so that it is extremely oxidizing and has a high chance of destroying itself. This makes it extremely hard to crystallize, but we are now working on a closer structure," says Fromme, who expects her group to publish the high-resolution structure of PSII before year's end. That, according to Jones, will have just as great an impact as the two 2001 papers. Yet, the PSII's essentially half-done structure has been more highly cited. "I was a little bit surprised about the impact of the PSII paper, which has dominated the citations," says another coauthor, Norbert Krauss. The reason for the discrepancy, perhaps, is that it provides a first hazy glimpse of the core oxygen-evolving structure that enabled aerobic life on earth. Fromme notes that it is the only enzyme capable of extracting electrons from water to yield protons (H+ ions) and oxygen. Although one needs higher resolution to determine the exact structure of the oxygen-producing manganese cluster, "It is very exciting, especially to see where these manganese clusters are located," says Fromme. PSII uses elaborate systems to protect itself from oxidative damage and subsequently to repair the crucial D1 protein at its center. "When a plant is in bright sunlight, PSII has a half-life of just half an hour," says Fromme. "Plants have a very complex system for recognizing damage and repairing the protein." Cofactors and noncovalently bound molecular subunits within the PS complex also perform various functions, such as protecting the complex and assisting with light energy conversion. These cofactors, which include antioxidants such as carotenoids, react with the dangerous singlet oxygen produced in bright light, and safely dissipate excess energy as heat. "We have now found there are 22 carotenoids distributed all over the channel system," Fromme adds. A Japanese team also investigated the vulnerability of PSII to excess light and its preservation mechanism; the team produced a complete PSII structure of similar resolution this year.3 ROBUST REVELATIONS In the case of PSI's structure,2 the greatest revelation, according to Krauss, was the system's sheer sophistication, which he says astonished the entire team. "It's so complex that just getting the structure won't answer many of the questions," says Krauss. But, with the structure resolved, groups now can use site-directed mutagenesis of individual residues to probe functional-structural relationships. The team had been expecting a simpler PSI configuration comprising cofactors located around a ring structure. "Instead, there are all sorts of orientations, and pigments which catch the light [and] connect with several other pigments," says Fromme, who suspects that the complexity, based more on a network structure, confers greater robustness than a ring. "With a network, when you remove one cofactor, it doesn't matter. Energy can reach the center another way," Fromme adds. The PSI paper also resolved a major debate over whether one or both of two chains of chlorophyll cofactors, which connect the outside with the center of the complex, was engaged in electron transfer. It turns out they both are. The question to settle now is why both are involved and how they are coordinated. In the longer term, both papers herald important developments in several key fields, notably genetic modification of plants to improve yields when sunlight is in short supply. Other directions include harnessing artificial reaction centers to power novel tiny devices. The knowledge gained from the structural resolution of PSI and PSII also will be applied to the wider study of membrane proteins, whose malfunction is responsible for many diseases. Philip Hunter (phunter@philiphunter.com) is a freelance writer in London. References 1. P. Jordan et al., "Three-dimensional structure of cyanobacterial photosystem I at 2.5 angstrom resolution," Nature, 411:909-17, 2001. (cited in 176 papers) 2. A. Zouni et al., "Crystal structure of photosystem II from Synechococcus elongatus at 3.8 angstrom resolution," Nature, 409:739-43, 2001. (cited in 276 papers) 3. N. Kamiya et al., "Crystal structure of oxygen-evolving photosystem II from Thermosynechococcus vulcanus at 3.7 Å resolution," Proc Natl Acad Sci, 100:98-103, 2003. THE CENTER OF IT ALL Some organisms, including a variety of bacteria, derive their energy from anoxygenic photosynthesis involving just a single reaction center that does not produce oxygen. The evolution of oxygenic photosynthesis--the source of energy in all plants, green algae, and cyanobacteria--resulted in atmospheric oxygen as a byproduct of photosynthesis via two reaction centers (photosystems). PSI and PSII, operating in tandem and both located in the thylakoid photosynthetic membranes, exploit the energy from sunlight (or in principle any photon source) to create an electrical potential difference across the charge-impermeable membrane. This potential powers a variety of metabolic reactions. PSII's evolution provided the cue for aerobic life. By exploiting light energy to transfer electrons from water to soluble carriers such as plastoquinone, PSII yields oxygen as a by-product. A cluster of four manganese atoms catalyzes this process. PSI, the largest and most complex membrane protein, comprises 100,000 atoms with a molecular weight of about 1 million (PSII is less than half that size). PSI has the key role of transferring light energy with extraordinary efficiency (>99.99%) from a sophisticated system of antennae pigments, such as chlorophyll, to the reaction core. There, it drives the electron transfer from the soluble electron carriers generated by PSI to ferredoxin or flavodoxin on the cytoplasmic side of the thylakoid membrane. The charge differential drives ATP synthesis and ultimately the reduction of carbon dioxide to carbohydrates, the source of energy for most organisms. function sendData() { document.frm.pathName.value = location.pathname; result = false if (document.frm.score[0].checked) result = true; if (document.frm.score[1].checked) result = true; if (document.frm.score[2].checked) result = true; if (document.frm.score[3].checked) result = true; if (document.frm.score[4].checked) result = true; if (!result) alert("Please select") return result; } .myradio{background-color : #94bee5} Please indicate on a 1 - 5 scale how strongly you would recommend this article to your colleagues? Not recommended 1 2 3 4 5 Highly recommended Please register your vote

Photosystems I and II in 3-D

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