2pp_4.Photosynthesis - 1/8/2010 4. Photosynthesis: antennas...

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Unformatted text preview: 1/8/2010 4. Photosynthesis: antennas and reaction centers Synthesizing carbohydrates from (:02 and water presents a formidable thermodynamic problem: 6 co2 + 6 H20 —.- cfiHmo6 + s 02 AG° = +679 kcallmol (+2480 lemol}. Ken = 113‘496 Photosynthetic organisms use the energy of light to drive carbohydrate synthesis against this enormous gradient. The energy of red light (700 nm} is E = th = 41 kcalleinstein* {172 leeinstein} 6 co2 + 6 H20 +43 m —. CBH1ZOE+ a 02 AG° = 4290 kcallmol (—5398 kJimol}. Kecl = 1'09“2 1 'An einstein is a mol of photons. N = Avogadro’s number (6x103): h = Planck’s constant {6.63x10'3‘ Jls}; v = frequency {5"}. 1/8/2010 When light raises a molecule to an excited electronic state, the molecule becomes a stronger reductant f LUMO Ea“ LUMO S '-'-' HOMO El .5 m g Homo LIGHT —I .E.’ electrons A* B The photochemically reactive pigments are chlorins or bacteriochlorins, which are structurally related to hemes Hemes Chlorophylls Bacteriochlorophylls symmetrical asymmetrical more asymmetrical :r: a: systems; a: systems; systems; absorb blue absorb blue absorb blue, orange light & red light & near-IR light The photochemical reactions of photosynthesis take place in integral membrane proteins thylakoid outer membrane lumen inner membrane stroma thylakoid " membrane Most of the pigments in photosynthetic cells do not participate in the electron-transfer reactions of photosynthesis. Instead, they serve as an antenna that increases the absorption of light. R. Emerson 8. W. Arnold measured the amount of 02 formed when they excited algae with short flashes of light. At high light intensity, the maximum (1|2 released per flash was about 1 02 per 2400 Chls. mooo4 oycm 0.0002 At low light intensity, 1 O2 is formed for ~each 8 photons fl absorbed (yellow dashed line}. a 0304 0mm Light absorbed (photonlehU 1/8/2010 The antenna system of purple photosynthetic bacteria has circular pigment-protein complexes The LH1 complex, with 31 transmembrane u-helical peptides {yellow} and 30 BChls (CPK colors), surrounds the reaction center (red, orange & brown}, where the electron-transfer reactions occur. view normal to the membrane View in the plane of the membrane A. W. Roszak et al. Science 302: 1969 (2003]; 1pyh.pdb When the antenna is excited with light, excitations are transferred to the reaction center within ~40 ps Smaller “LH2" antenna 4;. v 3. complexes transfer - - ' energy rapidly to LH1 View normal to 1\_, 2 s antenna BChls are the membrane ' p green and blue in 1 pa = 10-12 5 this figure. 1/8/2010 1/8/2010 The LHC-ll {light-harvesting complex ll) antenna protein of plants is a trimer with 14 chlorophylls and 3 carotenoids per protein subunit Z. Liu et al. Nature 428: 287 (2004}; 1erpr The reaction center of purple photosynthetic bacteria has 3 to 4 subunits, depending on the species The two central subunits are integral membrane proteins with homologous structures A small number of bacteriochlorophylls and other electron carriers are bound to the proteins membrane phospholipid bflayer View parallel to the membrane 10 The electron carriers in the bacterial reaction center are arranged around an axis of approximate rotational symmetry Two of the four BChls form a dimer {P870} that acts as the initial electron donor BChl = bacteriochlorophyll BPh = bacteriopheophytin Q J (BChl with 2 H in place of Mg} . - Q = ubiquinone : Fe Axis of approximate E The sidechains of BChI. rotational symmetry BPh and Q are truncated for clarity in this figure. 11 The sequence and kinetics of the initial electron-transfer reactions can be studied by exciting RCs with short pulses of light A beam train of short pulses Splitter Laser varying the path length changes the delay between the excitation and probe pulses excitation pulses 1K1 and 12 : different probe wavelengths are used to detect different electron carriers. Absorbance change measure the intensity of the transmitted Delay between excitation ‘ probe beam averaged and probe pulses over many pulses 1/8/2010 1/8/2010 When the complex is excited with light, an electron moves from the BChl dimer (P870) to a BPh and then to a quinone 13 The photosynthetic electron-transfer system in purple bacteria is cyclic P870; BChl BPh GA _ \0 1. Reduced ubiquinone (QaHzl nght B 1 dissociates from the RC and l ' diffuses to the cytochrome tic1 cytochrome bc1 complex, where it is reoxidized. cytochrome c 2. Reduced cytochrome c A/Z. diffuses back to the RC to pan complete the cycle. Electron flow pumps protons across the membrane 14 Plants have two photosystems P700“ that work in series to move \c electrons from water to NADP I: “t: P53? Fe-S centers ‘~ \ Phe Li ht ferredoxin \ 9 \ . PGA.PQB NADP NADPH Ll ht 9 \ U Cyt bar complex Photosystem I 0; plastocyanin WV “‘1 P700 Mn center Photosystem ll P0» P03: P'asmqumona \ 0K1: phyloquinone plastocyanin: a Cu protein P680 ferredoxin: an Fe-S protein 15 The reaction center of Photosystem II has 20 subunits It binds 32 molecules of Chi-a (green). Most of these are part of the antenna. View parallel to the \ membrane K.N. Ferreira etall Science 303: 1831 (2004); 155Lpdb. B. Loll etall Nature 438: 1040 (2005}; Zaxtpdb. 16 1/8/2010 The core of the Photosystem ll reaction center is very similar to that of purple bacteria 1551.pdb, Zaxi. pdb. The polypeptide backbones of the two main subunits are shown in red and orange, chlorophylls in green, pheophytins in blue and quinones in yellow. 1? Electron carriers In the Photosystem II reaction center A cluster ofd Mn atoms and a Ca forms the Oz-evolution site and is unique to P3 |l. Note the tyrosine residue between the Mn cluster and P630. Pushkar et al. Proc. Natl. Acad. Sci. USA 105: 182199008). 18 1/8/2010 1/8/2010 The Photosyslem I reaction center has 12 subunits It binds about 100 molecules of Chl. Those shown in yellow are part of the antenna. The arrangement of the Chi electron carriers (blue) again is very similar to that in bacterial RCs. View normal to the membrane 19 Two of the Fe-S centers in Photosystem l are in subunit 0, on the stromal side of the membrane View parallel to the membrane surface 20 10 ...
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This note was uploaded on 02/08/2010 for the course LING 100 taught by Professor Kova during the Spring '10 term at University of Warsaw.

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