5.Photophosphorylation - 5. 5. Photosynthesis: electron...

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Unformatted text preview: 5. 5. Photosynthesis: electron transfer (continued) and photophosphorylation 1 Plants Plants have two photosystems that work in series to move electrons from water to NADP P700* Chl QK1 P680* Fe-S centers Phe Light Light NADP PQA, PQB Cyt b6f complex H2O O2 Ferredoxin NADPH Photosystem I plastocyanin P700 Mn center Photosystem II P680 PQA, PQB: plastoquinone QK1: phylloquinone plastocyanin: a Cu protein ferredoxin: a soluble Fe-S protein 2 Experimental evidence for the Z scheme (1) The efficiency of O2 evolution drops at (1) wavelengths above 680 nm, but can be increased by green background light with background light O2 released per photon absorbed without background light 660 680 700 720 Wavelength (nm) R. Emerson et al. Proc. Natl. Acad. Sci. USA 43: 133 (1957) The green background light has this effect even if it is turned off just before the red light is turned on. 3 The Z-scheme can explain these results if only one of the two photosystems absorbs far red light P700* P680* NADP Photosystem I Photosystem II P700 H2O P680 Measurements of partial reactions (e.g., P700 photooxidation) showed that both photosystems absorb below 680 nm, but only PS I absorbs at longer wavelengths. 3 (2) Green light can (2) either oxidize or reduce components between PS II & PS I, but far red light only oxidizes them P700* P680* NADP Cyt f H2O PS II PS I P700 L. Duysens & J. Amesz Biochim. Biophys. Acta 64: 243 (1962) P680 cyt f oxidation state time 4 (3) (3) Far-red background light decreases fluorescence from PS II P700* Chl* P680* Phe energy transfer NADP PS I P700 LIGHT PS II Chl P680 PS II antenna 7 PS II complexes are located mainly in stacked regions of the thylakoid membrane; PS I complexes are mainly in unstacked lamellae stacked membranes (granal membranes) Cyt b 6f ATP synthase unstacked membranes (stromal lamellae) PS I PS II LHC-II The LHC-II antenna complex can move between these two regions. 7 Phosphorylation causes the LHC-II antenna complex to move to unstacked regions & associate with PS I This increases the fraction of the absorbed light energy that goes to PS I PS I ATP kinase O O-P=O O ADP OH Pi 8 phosphatase The kinase that phosphorylates LHC-II is activated when the ratio [PQH2]/[PQ] is high, reflecting overexcitation of photosystem II P700* P680* Light PQH2 NADP PQ Light Photosystem I P700 H2O Photosystem II P680 LHC II then moves to PS I, balancing the rates of excitation of the two systems. 9 O2 evolution requires accumulation of four oxidizing equivalents S4 O2 evolved per flash hν S3 O2 hν S2 hν S1 hν 0 4 8 12 16 flash number 20 24 S0 Each step removes one electron from the Mn4O4Ca complex. 10 S1 S0 e- Ca S2 e- Mn(III) Mn(IV ) Likely Likely mechanism of O2 evolution e- oxyl radical P. Siegbahn Acc. Chem. Res. 42: 1871 I(2009)expect you to don’t remember these structures eS4 S3 11 The chloroplast cytochrome b6f complex is homologous to the bc1 complexes of mitochondria and photosynthetic bacteria cytochrome cytochrome f “Rieske” Fe-S center thylakoid membrane Cytochrome b6f is a homodimer with multiple subunits cytochrome b6 with two hemes (red) Chl (green) & carotenoid (orange) found only in b6f quinone (plastoquinone in b6f, ubiquinone in mitochondria) H. Zhang et al. Proc. Natl. Acad. Sci. USA 100: 5160 (2003); 1vf5.pdb 12 Oxidation of PQH2 by the cytochrome b6f complex pumps protons across the thylakoid membrane plastocyanin thylakoid lumen 2 H+ cyt f thylakoid membrane Fe-S bL bH ee- ePQH2 PQ 1/2 PQ 1/2 PQH2 H+ stromal space Proton pumping generates an electrochemical potential gradient that is used to make ATP. 13 The “Q cycle” enables the cytb6f complex to pump 4 protons for each QH2 that is oxidized plastocyanin 4 H+ cyt f thylakoid membrane thylakoid lumen Fe-S bL 2 PQH2 2 PQ PQH2 PQ bH PQ PQH2 2 H+ stromal space The mitochondrial & bacterial cyt-bc1 complexes also carry out a Q cycle. 14 Proton and electron circuits in chloroplasts The electron-transfer reactions driven by light acidify the thylkoid lumen relative to the stroma NADP+ + H+ light light 4 H+ PQ PS II cyt b6 f PQH2 Mn Fd PS I PC 4 H+ H2 O NADPH 1/2 O2 2 H+ thylakoid lumen thylakoid membrane stroma 2 ADP + 2 Pi + H+ 6 H+ ATP 2 ATP synthase 15 The ATP synthases of chloroplasts and photosynthetic bacteria are very similar to those of mitochondria and nonphotosynthetic bacteria inner membrane matrix Mitochondrion Chloroplast ATP synthase inter-membrane space E. coli lumen stroma thylakoid membrane ATP synthase 16 The orientation of the ATP synthase relative to the electrochemical potential gradient generated by electron transfer is the same in all cases ADP + Pi ADP + Pi ATP H+ ATP H+ matrix E.T. E.T. H+ inner membrane inter-membrane space Mitochondrion stroma H+ thylakoid membrane thylakoid lumen Chloroplast 17 ...
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