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ScienceWeek
PHYSICAL CHEMISTRY: QUANTUM MECHANICS AND PHOTOSYNTHESIS
The following points are made by G.R. Fleming and G.D. Scholes (Nature 2004 431:256):
1) The first law of photosynthetic economics is: "A photon saved is a photon earned." Research into the factors behind this principle has been burgeoning, and has recently culminated in a report by Jang et al(1) in which the authors look at photosynthetic energy transfer at the quantum level.
2) Plants use solar antennae to capture incident photons and transmit the excitation energy to reaction centers, where it is used to initiate the primary electron transfer reactions of photosynthesis. These antennae are one of nature's supreme examples of nanoscale engineering, and are constructed from specialized light-harvesting complexes formed of proteins that bind chlorophylls and carotenoids. Photon collection involves up to several hundred light-absorbing molecules, or chromophores. Hundreds of energy-transfer steps over a hierarchy of time scales and distances, which often occur with near-perfect efficiency(2), are therefore required to collect and trap solar energy.
3) More than 50 years ago, Theodore Foerster described a method for calculating the rate of energy transfer between molecules from the overlap of the donor molecule's fluorescence spectrum and the acceptor molecule's absorption spectrum(3,4). The theory has had an enormous impact on biology, chemistry and physics. Collectively, high-resolution structural models, ultrafast spectroscopy, and quantum chemical calculations have helped to expose the complex and, in some cases, subtle relationships between structure and light-harvesting in photosynthetic systems. Indeed, it has turned out that there are only a few cases in which the energy transfer within photosynthetic light-harvesting complexes can be correctly characterized by conventional Foerster theory. Moreover, the realization that the concepts elucidated during the study of light-harvesting proteins are general principles that operate in molecular aggregates has inspired a change in thinking about energy transfer in confined geometries(5).
4) Various studies(2) have revealed that the nanoscale dimensions of the photosynthetic complexes are critical for light harvesting. Unlike most model systems, where the chromophores are spaced at distances that are large with respect to their size, chromophores in light-harvesting systems are densely packed. This means that the electronic interactions between the light-absorbing components are both qualitatively and quantitatively different from most other model systems. This realization has spurred the development of new theoretical methods, including the work of Jang et al(1), which provides a detailed prescription for calculating energy transfer in multichromophoric assemblies.
5) To understand the dynamics of light-harvesting and light-trapping in photosynthesis, certain design features must be taken into account. For example, the distances between the molecules are often smaller than the overall size of each molecule. In this confined geometry, energy transfer is governed by how the donor "sees" the acceptor on the submolecular scale at which the fine differences in the shape of the wavefunctions between the ground and excited states at the donor-acceptor junction become significant. At this level of spatial confinement, transitions and energy levels that would be ineffective, or even inoperative, in systems with widely spaced chromophores may be crucial in increasing energy-transfer efficiency.
References (abridged):
1. Jang, S., Newton, M. D. & Silbey, R. J. Phys. Rev. Lett. 92, 218301 (2004)
2. Sundstrom, V., Pullerits, T. & van Grondelle, R. J. Phys. Chem. B 103, 2327-2346 (1999)
3. Foerster, Th. Ann. Phys. 2, 55-75 (1948)
4. Van Der Meer, B. W., Coker, G. & Chen, S.-Y. S. Resonance Energy Transfer: Theory and Data (VCH, New York, 1994)
5. Scholes, G. D., Jordanides, X. J. & Fleming, G. R. J. Phys. Chem. B 105, 1640-1651 (2001)
Nature http://www.nature.com/nature
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BIOCHEMISTRY: ON PHOTOSYNTHESIS
The following points are made by M.R. Jones and P.K. Fyfe (Current Biology 2004 14:R320):
1) The thylakoid membranes of plant chloroplasts use sunlight to trigger a series of electron transfer reactions coupled to proton translocation. The resulting proton gradient is used to generate ATP, and under some circumstances electron flow produces NADPH. These molecules are then used for the fixation of carbon and the production of biomass. Three membrane-embedded protein complexes act in series to catalyze this process: Photosystem-I and Photosystem-II provide the sites at which light energy is used to drive electron flow, and these are connected by the cytochrome bf complex.
2) The reaction catalyzed by Photosystem-II is of particular interest and importance, as this complex uses water as its source of electrons and produces molecular oxygen as a waste product [1]. The evolution of this reaction in cyanobacteria is thought to have triggered large increases in the level of atmospheric oxygen, starting around 2.5 billion years ago [2]. The mechanism of light-driven water oxidation has been the subject of intense interest, and a recent report [3] on the X-ray crystal structure of Photosystem-II has brought new information on the structure of the protein-cofactor system responsible.
3) The photosystems of oxygenic photosynthetic organisms have been subject to an intensive spectroscopic examination, and the last three years have seen dramatic advances in our understanding of their structures. In 2001, a high-resolution X-ray crystal structure was reported for the Photosystem-I complex from the thermophilic cyanobacterium Thermosynechococcus elongatus[4], and lower resolution structures were reported for the Photosystem-II complex from T. elongatus[5] and from T. vulcanus. Late 2003 saw two new structures for the cytochrome b6f complex from the thermophilic cyanobacterium Mastigocladus laminosus and the alga Chlamydomonas reinhardtii, and a low resolution structure for the plant Photosystem-I complex and associated light-harvesting proteins which brings new insights into the longer-range organization of the photosynthetic membrane.
4) In the latest work, Ferriera et al.[3] describe the structure of Photosystem-II from T. elongatus, at a resolution of 3.5 angstroms. Although this is only a modest improvement on the resolution of previous structures -- 3.8 angstroms [5] and 3.7 angstroms -- the derived structural model differs markedly in the level of detail included. The earlier structures traced the backbone chains of the Photosystem-II polypeptides and provided locations for many of the cofactors of the electron transfer chain and light-harvesting "antenna". The new work includes, for the first time, structural models for the side-chains of 90% of the amino acids, including those that form the binding sites of the chlorophylls, pheophytins, and quinones of the electron transfer chain.
References (abridged):
1. Goussias, C., Boussac, A., and Rutherford, A.W. (2002). Photosystem II and photosynthetic oxidation of water: an overview. Phil. Trans. Roy. Soc. B 357, 1369-1381
2. Blankenship, R.E. (2002). Molecular Mechanisms of Photosynthesis. (Oxford, UK: Blackwell Science Ltd)
3. Ferreira, K.N., Iverson, T.M., Maghlaoui, K., Barber, J., and Iwata, S. (2004). Architecture of the photosynthetic oxygen-evolving centre. Science 303, 1831-1838
4. Jordan, P., Fromme, P., Witt, H.T., Klukas, O., Saenger, W., and Krauss, N. (2001). Three-dimensional structure of cyanobacterial photosystem I at 2.5 angstroms resolution. Nature 411, 909-917
5. Zouni, A., Witt, H.T., Kern, J., Fromme, P., Krauss, N., Saenger, W., and Orth, P. (2001). Crystal structure of Photosystem-II from Synechococcus elongatus at 3.8 angstroms resolution. Nature 409, 739-743
Current Biology http://www.current-biology.com
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PLANT BIOLOGY: ON PHOTOSYNTHESIS
Notes by ScienceWeek:
Photosynthesis is one of the more important biological processes on Earth, providing nearly all the oxygen we breathe, providing (either directly or indirectly) all the food we eat, and providing the most important net input of energy into the biosphere.
From the standpoint of chemistry, photosynthesis can be defined as the reductive carboxylation of organic substrates carried on by chlorophyll-containing biological cells capable of using light as their energy source. Fully oxidized carbon atoms in the form of carbon dioxide are covalently linked ("fixed") to organic acceptor molecules and are subsequently reduced and rearranged into sugars and other organic molecules, with light energy used to drive the fixation and provide the reducing power.
Many differences exist among photosynthetic organisms with respect to the components and organization of the photosynthetic apparatus and the end products of photosynthetic energy transduction. For example, the photosynthetic apparatus of oxygen-evolving organisms (*eukaryotic plants and *cyanobacteria), contains two different photosynthetic systems, whereas nonoxygenic photosynthesis in the *purple photosynthetic bacteria involves only one system. Nevertheless, there are a number of features and principles associated with photosynthetic energy transduction that are common to all organisms:
a) The process is always associated with membranes (*thylakoids in oxygen-evolving organisms; *chromatophore membranes in photosynthetic bacteria).
b) The process always involves multi-unit protein complexes. Initially, light energy must be captured and transferred to a photochemical reaction center, where it can be used to drive a reduction-oxidation (redox) reaction. This occurs in complexes, each complex containing a light-harvesting cluster of chlorophyll molecules ("antenna complex"; "antenna system") and a photochemical reaction center. The chemical redox energy resulting from the photochemical reaction at the reaction center can then drive a series of redox reactions, commonly termed "electron transport" reactions, which result in spatial separation of oxidized and reduced chemical species, and which provide a source of chemical energy in the form of reducing equivalents.
Coupled with electron transport is the pumping of protons across the membrane, and the free energy stored in the resulting electrochemical potential difference of protons across the membrane is used by an enzyme (ATP synthetase) to phosphorylate adenosine diphosphate (ADP) and provide *adenosine triphosphate (ATP) for the cell.
The following points are made by R.J. Cogdell et al (The Biochemist 2000 June):
1) The past few years have seen remarkable progress in our understanding of the very early light reactions in photosynthesis, and a large part of this research involves the study of the photosynthetic purple bacteria. These anaerobic prokaryotes have proved to be excellent model organisms in which to investigate the basic mechanisms of the primary light reactions of photosynthesis.
2) The light-absorbing pigments in purple-bacteria photosynthesis, mainly bacteriochlorophyll-a and carotenoids, are contained within two types of integral membrane pigment-protein complexes: light-harvesting complexes and reaction centers. Solar energy is absorbed by the light-harvesting components before being rapidly and efficiently transferred to the reaction centers. These reaction centers "trap" this light energy and convert it into chemical energy via a series of transmembrane redox reactions which initiate photosynthetic electron transport and lead to proton pumping and ATP synthesis.
3) Photosynthesis can occur in the absence of a light-harvesting system, but only in very bright sunlight. The antenna system associated with a reaction center increases the effective cross-sectional area available for photon capture, and this allows the reaction centers to be supplied with sufficient solar energy to drive photosynthesis even on dull days. The combination of light-harvesting complexes with a reaction center is the "photosynthetic unit". In purple bacteria, the size of this unit varies with the light intensity at which the bacteria are grown. In very bright light, the purple-bacteria photosynthetic unit can be as small as 30 bacteriochlorophyll-a molecules per reaction center, while at low light intensity the unit can be as large as several hundred such molecules per reaction center.
The Biochemist http://www.portlandpress.com/biochemist/biochemist.htm
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Notes by ScienceWeek:
eukaryotic plants: In general, plants whose cells contain nuclei and other intracellular membrane-bound organelles. (Cells without nuclei are "prokaryote" cells.)
thylakoids: A sac-like vesicle containing the photosynthetic pigments in photosynthetic organisms. In prokaryotes, the thylakoids are of various shapes and are attached to the plasma membrane; in eukaryotes, the thylakoids are flattened and located in chloroplasts; in the chloroplasts of higher plants, the thylakoids form dense stacks called "grana". Isolated thylakoids preparations can carry out photosynthetic electron transport and associated phosphorylation.
chromatophore membranes: In this context, the term "chromatophore" refers, in general, to any of the particles, isolated from photosynthetic organisms, that contain photosynthetic pigments.
adenosine triphosphate (ATP): ATP is the most important chemical energy source in all living cells, intimately involved in various cell functions and cell metabolism, and an entity in numerous cyclic chemical pathways involved in the synthesis of various cell components.
ScienceWeek http://scienceweek.com
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