694:408/115:512 - Molecular Bi ology & Biochemistry Lecture#2 -- Light Reactions of Photosynthesis November 4, 2008 1.) Overall photosynthetic process -- light converted into chemical energy for biosphere, provides atmospheric O2, fixes atmospheric CO2 into CH2O (1011 tons worldwide/yr.). 2.) Photosynthesis consists of light reactions (drive O2 evolution, generate NADPH and ATP) and carbon assimilation reactions (NADPH and ATP drive CO2 fixation via ribulose-1,5-bis-P carbox-ylase & Calvin cycle to regenerate ribulose-1,5-bis-P). Improved understanding of light reactions is basis for harnessing solar energy, a sustainable and renewable source of clean energy 3.) Photosynthesis occurs in chloroplasts in higher phototrophs -- light reactions in thylakoid membrane (stacked grana and un-stacked stroma lamellae regions), thylakoid lumen (accumulates H+), CO2 fixation occurs in stroma. 4.) Distinguishing oxygenic photosynthesis (H2O as e- donor -- algae, plants) and anoxygenic (H2S, organic acids/alcohols as e- donors – green and purple bacteria). 5.) Photosynthetic pigments -- associated noncovalently in pigment-protein complexes: a.) Chlorophyll (Chl)(Mg tetrapyrrole) -- has light-harvesting role (funnels collected energy to reaction center) and photochemical reaction center role (catalyzes charge separation); b.) Carotenoids (β-carotene) -- accessory light-harvesting and photoprotective roles. 6.) Fates of photoexcitations -- resonance energy transfer (in light-harvesting proteins, e.g. light-harvesting complex II, LHC-II), electron transfer (initiated in reaction center), loss as fluoresc-ence (useful in energy transfer studies), and radiationless decay. 7.) Molecular architecture and role of LHC-II (major light-harvesting complex of higher plants), X-ray structure reveals how LHC-II Chls funnel excitations to PSII 8.) Oxygenic phototrophs use two photosystems -- PSI and PSII (multisubunit, integral membrane complexes with reaction center and electron transport components, surrounded by light-harvest-ing Chls): a.) PS I reaction center (P700, absorbs 700 nm photons) -- strong reductant generating NADPH, weak oxidant; b.) PS II reaction center (P680, absorbs 680 nm photons) -- strong oxidant photolyzing H2O, weak reductant; c.) PSII reductant and PSI oxidant connected by the cyto-chrome b6f complex in Z-scheme of non-cyclic electron flow from H2O to NADP 9.) Oxygen evolution by oxygen evolving complex (OEC): a.) Accumulation of 4 oxidizing equivalents by PSII; b.) Cycles through 5 Mn oxidation states. 10.) Photophosphorylation: a.) Driven by H+ gradient generated arising from light-driven e- transport reactions; b.) Catalyzed by CF1/CFo-ATPase. 11.) Structures of the thylakoid membrane complexes have been determined by X-ray crystallography a.) Cyanobacterial PSII structure revealed detailed organization of protein subunits and cofactors and OEC; separate core light-havesting proteins CP43 and CP47 localized next to reaction center D1 and D2 proteins, respectively, in arrangement similar to PsaA and PsaB of PSI; OEC assumes trigonal pyra-mid–like structure b.) PSI structure determination in cyanobacteria (trimeric) and pea (monomeric) reveals how extensive Chl arrays use solar energy to transport electrons; both complexes have12 core subunits containing core antenna and reaction center Chls; pea complex also contains 4 LHCI light-harvesting proteins on one side of core; all cofactors were localized in the struct-ures, permitting elucidation of pathways of excitation energy and electron transfer c.) Structure of thylakoid membrane complexes completed by determination of cytochrome b6f complex in a thermophilic cyanobacterium –- forms a dimer like bc1 complex, but has different domain arrangement outside core and additional prosthetic groups (Chl a , β-carotene of unknown function, and heme x, which may mediate cyclic electron flow from FNR). Reading: Lehninger, Nelson, Cox (2008) Principles of Biochemistry, Chapter 19, pp. 742-764 (4th Edition, Chapter 19, pp. 723-743).1The Light Reactions ofThe Light Reactions ofPhotosynthesisPhotosynthesis• Convert 1% of the energy of sunlight to chemical energy serving asmain energy input for the biosphere• Also the main source of O2 in atmosphere-- Needed for all aerobic metabolism by heterotrophic organisms-- O2 arising from cyanobacterial photosynthesis drove evolution of animal life ~2 billion years ago• Provide energy to fix atmospheric CO2 into CH2O at the bottom of thefood chain (1011 tons worldwide/yr)• Harnessing of solar energy is essential for meeting future needs for asustainable and renewable source of clean energy as the world runsout of fossil fuel-- Requires an improved understanding of the light reactions of photosynthesis-- Basis for construction of biosolar photoelectric cells for conversion of sunlight into electric current and molecular hydrogen as a fuel sourcePhotosynthetic and Heterotrophic Organisms Exist in aPhotosynthetic and Heterotrophic Organisms Exist in aBalanced Steady State in the BiosphereBalanced Steady State in the Biosphere• Photosynthetic organisms serve as the ultimatesource of all biological energy• They trap solar energy and form NADPH andATP as energy sources for making carbohydr-ates and other reduced organic compoundsfrom CO2 and H2O, releasing O2 into theatmosphere• Aerobic heterotrophs use the O2 as the terminalelectron acceptor in the catabolism of energyrich products of photosynthesis to CO2 andH2O, generating ATP• The CO2 is recycled back into atmosphere to befixed into new organic compounds by photosyn-thesis• The overall equation for photosynthesis in high-er phototrophs represents a redox reaction withH2O providing the reducing equivalents for CO2reduction: 6CO2 + 6H2O Light C6H12O6 + 6O2• The evolved O2 arises from photolysis of H2OPhotosynthesis can be Separated into Light-Dependent andPhotosynthesis can be Separated into Light-Dependent andCarbon-Assimilation ReactionsCarbon-Assimilation Reactions• The light reactions result in O2 evolutionand the generation of NADPH and ATP atthe expense of solar energy• The reducing power of NADPH and theenergy of ATP hydrolysis are used todrive the carbon assimilation reactions inwhich CO2 is fixed into triose-P to formstarch and sucrose (occur both in lightand in darkness)• Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalyzes the firstmajor step of carbon fixation in the