American Society for Photobiology

ASP Conference 2016: 21-26 May 2016
Tampa Marriott Waterside Hotel & Marina

Single Session

[Schedule Grid]

16 - Antenna Processes in Photosynthesis

Florida 1   08:00 - 11:30

Chair(s): Bob Blankenship
16-1   08:00  Molecular Mechanisms of Photosynthetic Antenna Regulation RE Blankenship*, Washington Univ.

Abstract: All photosynthetic organisms contain a light-harvesting antenna system. Photosynthetic antenna systems are extremely diverse in terms of structural organization and type of pigment utilized. In addition to the light absorption function of the antenna, it is essential for all photosynthetic organisms to have regulatory mechanisms that serve to protect them against excess light. Several of these regulatory mechanisms involve excited state quenching processes. These mechanisms are referred to as Non-Photochemical Quenching (NPQ) to distinguish them from the normal excited state quenching by photochemistry that leads to productive energy storage. Remarkably, there are several different types of NPQ that are distinct mechanistically and are almost certainly independent evolutionary inventions. This talk will center on two of the mechanisms of NPQ. These are the Orange Carotenoid Protein (OCP) that is found in photosynthetic cyanobacteria and the redox-induced quenching that has recently been discovered in the Fenna-Matthews-Olson (FMO) protein found in Green Sulfur Bacteria. Cyanobacterial OCP serves to regulate energy collection in the phycobilisome antenna complex, and contains a photoactivated 3'-hydroxyechinenone carotenoid molecule as the pigment, which is photoconverted from an orange form to a red form. While in the red form, the OCP binds to the phycobilisome and quenches excitations. The regulation in the FMO protein takes place via a pair of redox-active cysteine residues, which are near to two of the bacteriochlorophyll pigments. Under oxidizing conditions, these residues oxidize to form thiyl radicals, which directly quench the excited states of the nearby bacteriochlorophylls by electron transfer processes. These regulatory systems have been investigated using an interdisciplinary approach involving ultrafast spectroscopy, mass spectrometry, mutational analysis, molecular modeling, X-ray and neutron crystallography, EPR spectroscopy and electrochemistry.

16-2   08:30  Elucidation of the Photoprotective Mechanisms in Algal Light Harvesting G.S. Schlau-Cohen*, MIT

Abstract: In photosynthetic light harvesting, absorbed energy migrates through a protein network to reach a dedicated location for conversion to chemical energy. In green algae, this energy flow is efficient, directional, and regulated. The regulatory response involves complex and complicated multi-timescale processes that safely dissipate excess energy, thus protecting the system against deleterious photoproducts. We explore the mechanisms behind this photoprotective process in a light-harvesting complex implicated in dissipation, light-harvesting complex stress response (LHCSR). By characterizing the conformational states and dynamics of individual proteins, we identify the extent of energy dissipation in single LHCSR proteins and how the extent of dissipation changes in response to pH and carotenoid composition, two components known to play a role in photoprotection. From this information, we explore how individual complexes contribute to the balance between efficiency and adaptability in photosynthetic light harvesting.

16-3   09:00  Quantitative Imaging of Photosynthetic Pigments and Proteins in Live Cells JA Timlin*, Sandia National Laboratories

Abstract: In order to facilitate efficient energy harvesting and transfer, the endogenous pigments in photosynthetic organisms such as chlorophylls and carotenoids are collocated in pigment-protein complexes and have an inherently high degree of spectral overlap. Photosynthetic pigments and proteins vary within and between organisms and are dynamically regulated in response to changing environmental conditions. The identity, abundance, and localization of photosynthetic pigments are critical to understanding light harvesting and the reactions that produce energy within the cell. Spectral imaging methods coupled with multivariate analysis are uniquely suited to untangle the highly overlapped spectral signatures from photosynthetic pigments and reveal global pigment localization and dynamics in intact, living cells. In this talk, I will introduce the state-of-the-art in spectral imaging and multivariate analysis methods with an emphasis on preprocessing techniques that we have developed for robust analysis. Examples will highlight applications of hyperspectral confocal fluorescence microscopy and hyperspectral confocal Raman microscopy to identify and map multiple photosynthetic pigments in cyanobacteria, green algae, and land plants in response to changing conditions (environmental, genetic differences, etc.). These methods provide increased fundamental understanding of global pigment dynamics and function within and across photosynthetic organisms. The results have important implications for synthetic biology, genetic engineering, and development of biohybrid or bio-inspired devices.

16-4   09:30  Artificial photosynthesis with a single redox centre based on the engineered flavoprotein LOV Nozomi Ueda*, Nagoya Institute of Technology ; Yukiko Ono, Nagoya Institute of Technology; Tatsuya Iwata, Nagoya Institute of Technology; Masayo Iwaki, Nagoya Institute of Technology; Hideki Kandori, Nagoya Institute of Technology

Abstract: One of the key concepts of artificial photosynthesis is a light-induced charge separation, of which the lifetime is long enough for the redox center to react with external chemical species or solid electrodes. To achieve this, a molecular unit that is made of a photosensitizable electron donor-spacer-acceptor (D-S-A) triad has been regarded as a principle molecular design after inspired by the molecular structure of natural photosynthetic reaction centres. In this presentation, we propose a different type of molecular design of artificial photosynthesis, in which a single photosensitizable redox center can function as both electron donor and acceptor. A plant photosensor, LOV (light-oxygen-voltage) protein, which contains a neutral oxidized form of FMN (flavin mononucleotide) molecule embedded in a 10-kDa polypeptide, was used as a template model compound to demonstrate the possibility of artificial photosynthesis with a single redox center. In the wild type LOV, light irradiation generates the excited state of FMN, followed by the adduct formation between FMN and the thiol group of the nearby Cys residue. When the Cys was mutated to the Ala (denoted as LOV C/A), the excited state of FMN oxidized the external electron donor ferrocyanide to ferricyanide and the semiquinone form of FMN (FMNH) was accumulated in the anaerobic condition. In the presence of methylene blue (MB) as an electron acceptor in addition to ferrocyanide, the reduction of MB was observed, which was concomitant with the formation of ferricyanide and less accumulation of FMNH. In the aerobic condition or in the absence of LOV protein, the redox reduction of MB was abolished. The results suggests that the single photosensitizer FMN molecule in LOV C/A functions as the electron acceptor first, and then the FMNH as the electron donor. Compared to the D-S-A triad, the single redox catalyst may have the advantage of easy manufacturing and broad application.

16-5   10:15  Thylakoid Membrane Dynamics in Cyanobacteria VS Urban*, Oak Ridge National Laboratory ; LR Stingaciu, Julich Centre for Neutron Science; H O'Neill, Oak Ridge National Laboratory; M Liberton, Washington University, St. Louis; HB Pakrasi, Washington University, St. Louis; M Ohl, Julich Centre for Neutron Science

Abstract: Cyanobacteria are photosynthetic prokaryotes that make major contributions to the production of the oxygen in the Earth atmosphere. The photosynthetic machinery in cyanobacterial cells is housed in flattened membrane structures called thylakoids. The structural organization of cyanobacterial cells and the arrangement of the thylakoid membranes in response to environmental conditions have been widely investigated. However, there is limited knowledge about the internal dynamics of these membranes in terms of their flexibility and motion during the photosynthetic process. We present a direct observation of thylakoid membrane undulatory motion in vivoand show a connection between membrane mobility and photosynthetic activity. High-resolution inelastic neutron scattering experiments on the cyanobacterium Synechocystis sp. PCC 6803 assessed the flexibility of cyanobacterial thylakoid membrane sheets and the dependence of the membranes on illumination conditions. We observed softer thylakoid membranes in the dark that have three to four fold excess mobility compared to membranes under high light conditions. Our analysis indicates that electron transfer between photosynthetic reaction centers and the associated electrochemical proton gradient across the thylakoid membrane result in a significant driving force for excess membrane dynamics. These observations contribute to a deeper understanding of the relationship between photosynthesis and cellular architecture.

16-6   10:45  Towarsd a full atomic resolution model of a photosynthetic antennae complex Myles DA, Oak Ridge National Laboratory ; Cuneo MJ*, Oak Ridge National Laboratory

Abstract: An understanding of the molecular details that underlie solar energy capture in photosynthetic organisms will lead to a better understanding of energy transfer in natural photosynthetic systems, and, in addition, will aid in the development of biohybrid photosynthetic systems. Current x-ray crystal structures of photosynthetic machinery reveal details of the pigment"protein architecture and interactions that regulate and control energy transfer but do not resolve hydrogen atoms. This is significant because, in many cases, hydrogen bonding interactions are likely to be important in the stabilization of antenna pigments and in fine-tuning and control of their site-energies. Important questions remain on how the individual site energies of protein bound chlorophyll molecules are modulated and tuned by local hydrogen bonding and electrostatic interactions with the protein scaffold. The extra level of detail provided by neutron diffraction experiments can contribute to better understanding of the spatio-energetic landscape and exquisitely tuned properties of the photosynthetic apparatus.

16-7   11:15  Incorporation of Algal Carotenoids into the Light-Harvesting System from a Purple Photosynthetic Bacterium H Hashimoto*, Kwasnei Gakuin University ; H Yukihira, Kwasnei Gakuin University; Y Sugai, Kwasnei Gakuin University; M Fujiwara, Kwasnei Gakuin University; M Iha, South Product; K Sakaguchi, Osaka City University; S Katsumura, Osaka City University; AT Gardiner, University of Glasgow; RJ Cogdell, University of Glasgow

Abstract: Fucoxanthin (Fx) is a carotenoid that is mainly bound to the light-harvesting complexes (LHCs) from algae. It produces intra-molecular charge transfer (ICT) excited-state under polar environment following photoexcitation up to the excited state. This ICT state plays a key role for the highly efficient energy-transfer from Fx to chlorophylls in the LHC from algae. In this study we were successful, for the first time, to incorporate Fx into the LHCs from a purple photosynthetic bacterium, Rhodospirillum rubrum G9+ (a carotenoidless strain). Femtosecond pump-probe spectroscopy was applied thus fabricated reconstituted LH1 complex in order to discuss the Fx to bacteriochlorophyll energy-transfer.

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