4.Photosynthesis - 4 4 Photosynthesis antennas and reaction...

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Unformatted text preview: 4. 4. Photosynthesis: antennas and reaction centers 1 Synthesizing carbohydrates from CO2 and water presents a formidable thermodynamic problem: 6 CO2 + 6 H2O C6H12O6 + 6 O2 ∆ Go = +679 kcal/mol (+2480 kJ/mol). Keq = 10-496 Photosynthetic organisms use the energy of light to drive carbohydrate synthesis against this enormous gradient. The energy of red light (700 nm) is E = Nhν = 41 kcal/einstein* (172 kJ/einstein) 6 CO2 + 6 H2O + 48 hν C6H12O6 + 6 O2 ∆ Go = -1290 kcal/mol (-5398 kJ/mol). Keq = 10942 ! *An einstein is a mol of photons. N = Avogadro’s number (6x1023); h = Planck’s constant (6.63x10-34 J/s); ν = frequency (s-1). 2 When light raises a molecule to an excited electronic state, the molecule becomes a stronger reductant Increasing Energy LUMO LUMO HOMO LIGHT HOMO electrons A B A* B 3 The photochemically reactive pigments in plants and photosynthetic bacteria are chlorins or bacteriochlorins, which are structurally related to hemes O N N Fe N N N N Hemes N Mg N N N O Mg N N Chlorophylls Bacteriochlorophylls symmetrical π systems; asymmetrical π systems; more asymmetrical π systems; absorb blue light absorb blue & red light O absorb blue, orange & near-IR light 4 The photochemical reactions of photosynthesis take place in integral membrane proteins outer membrane thylakoid lumen inner membrane stroma chloroplast thylakoid membrane 5 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 & W. Arnold R. measured the amount of O2 formed when they excited algae with short flashes of light. At high light intensity, the maximum O2 released per flash was about 1 O2 per 2400 Chls. 0.0004 O2/Chl 0.0002 At At low light intensity, 1 O2 is formed for ~each 8 photons absorbed (yellow dashed line). 0 0.004 0.008 0 Light absorbed (photons/Chl) 6 The antenna system of purple photosynthetic bacteria has circular pigment-protein complexes The LH1 complex, with 31 transmembrane α -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 7 When the antenna is excited with light, excitations are transferred to the reaction center within ~40 ps 0.1 - 0.2 ps Smaller “LH2” antenna complexes transfer energy rapidly to LH1 LH2 LH2 LH2 35 ps view normal to the membrane 1 ps = 10-12 s RC 1.2 ps LH1 antenna BChls are green and blue in this figure. 8 The LHC-II (light-harvesting complex II) antenna protein of plants is a trimer with 14 chlorophylls and 3 carotenoids per protein subunit Chl-b Chl-a carotenoid Z. Liu et al. Nature 428: 287 (2004); 1rwt.pdb 9 The reaction center of purple photosynthetic bacteria has 3 to 4 subunits, depending on the species A small number of bacteriochlorophylls and other electron carriers are bound to the proteins The The two central subunits are integral membrane proteins with homologous structures membrane phospholipid bilayer view parallel to the membrane 10 The electron carriers in the bacterial reaction center are arranged around an axis of approximate rotational symmetry BChl dimer BChl BChl BPh Two of the four BChls form a dimer (P870) that acts as the initial electron donor BPh Q Axis of approximate rotational symmetry QQ Fe BChl = bacteriochlorophyll BChl BPh = bacteriopheophytin (BChl with 2 H in place of Mg) Q = ubiquinone The sidechains of BChl, 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 train of short pulses Laser Absorbance change varying varying the path length changes the delay between the excitation and probe pulses λ1 beam splitter probe pulses λ2 λ 1 and λ 2 : different probe wavelengths are used to detect different electron carriers. Delay between excitation and probe pulses excitation pulses sample measure the intensity of the transmitted probe beam averaged over many pulses 12 When the complex is excited with light, an electron moves from the BChl dimer (P870) to a BPh and then to a quinone BChl dimer LIGHT 3 ps BChl 1 ps BPh 200 ps Fe Q 10 μs 13 The photosynthetic electron-transfer system in purple bacteria is cyclic P870* BChl BPh QA 1. Reduced ubiquinone (QBH2) QB Light 1. cytochrome bc1 dissociates from the RC and diffuses to the cytochrome bc1 complex, where it is reoxidized. 2. Reduced cytochrome cytochrome c 2. P870 c diffuses back to the RC to complete the cycle. Electron flow pumps protons across the membrane 14 Plants have two photosystems that work in series to move electrons from water to NADP P700* Chl QK1 P680* Fe-S centers Phe Light ferredoxin Light NADP PQA, PQB Cyt b6f complex H2O O2 NADPH Photosystem I plastocyanin P700 Mn center Photosystem II P680 PQ PQA, PQB: plastoquinone QK1: phylloquinone plastocyanin: a Cu protein ferredoxin: an Fe-S protein 15 The reaction center of Photosystem II has 20 subunits PS II binds 32 molecules of Chl-a (green). Most of these are part of the antenna. view parallel to the membrane B. Loll et al. Nature 438: 1040 (2005); 2axt.pdb. K. Kawakami et al. Proc. Natl. Acad. Sci. USA 106: 8567 (2009); 3aob.pdb. A. Guskov et al. Nature Struct. Mol. Biol. 16: 334 (2009); 3bz1.pdb. 16 The core of the Photosystem II reaction center is very similar to that of purple bacteria 1s5l.pdb, 2axt.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. 17 Electron carriers in the Photosystem II reaction center D E Mn4Ca complex P680 E C-term E D H H cluster A cluster of 4 Mn atoms and a Ca forms the O2-evolution site and is unique to PS II. Note the tyrosine residue between the Mn cluster and P680. Tyr Chl a Chl a Pheo Pheo PQ PQ Fe HCO3- Pushkar et al. Proc. Natl. Acad. Sci. USA 105: 1879 (2008). 18 The Photosystem I reaction center has 12 subunits PS I binds about 100 PS molecules of Chl. Those shown in yellow are part of the antenna. The arrangement of the Chl 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 I are in subunit C, on the stromal side of the membrane view parallel to the membrane surface P. Jordan et al. Nature 411: 909 (2001); 1jb0.pdb A. Amunts et al. J. Biol. Chem. (2010); 2wsc.pdb 20 ...
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