Photosynthetic reaction centers Part I: Hsiu-An Chu ( 朱修安 )

Transcription

1 Photosynthetic reaction centers Part I: 10/12/2006 Hsiu-An Chu ( 朱修安 ) Assistant Research Fellow Institute of Plant and Microbial Biology, Academia Sinica

2 Topics Oct 12 Photosynthetic Reaction centers Oct 17 Structural and mechanism of photosynthetic water oxidation Oct 19 Spectroscopy techniques Oct 24 Protein chemistry, chemical and posttranslational modification Oct 26 Proteomics References: 1. Biochemistry, Voet et al. Chapter 19 and Protein structure and Function, Petsko and Ringe et al. Chapter 2, 3 and 4.

3 Oxygenic Photosynthesis Light reaction 2H 2 O + light O 2 + 4H + + 4e - The reaction is not only the main energy source but also the oxygen source for sustaining almost all the life on the earth. The underlying principle of the natural system might inspire the construction of biomimetic energyconversion devices with the better efficiency---- Artificial photosynthesis.

4 Photosynthesis in higher plants : take place in the chloroplast

5 Photosynthetic electron transport chain Nature 2005, 3.0Å resolution Science 2004, 3.5Å PCCP 2004, 3.2Å PNAS 2003, 3.7Å Nature 2001, 3.8Å 3Å resolution Nature 2003 Science Å resolution (cyanobacteria) Nature Å (plant) Nature 2003

6 Z scheme hν hν

7 Z scheme Monitored by EPR and Laser spectroscopy techniques

8 3D structure of Reaction center from the purple photosynthetic bacterium Rhodopseudomonas viridis 4 subunits Michel, Deisenhofer, Huber 1984 solved structure, 1988 Nobel prize

9 Arrangement of electron transfer cofactors in the bacterial RC Monitored by EPR and Laser spectroscopy techniques

10 structure and spectra of chlorophylls and bacteriochlorophylls BRCs PSII and PSI

11 Chemical structures of pheophytin a and bacteriopheophytin a

12 Structures of quinones found in various reaction centers PSII PSI

13 Photosynthetic electron-transported system of photosynthetic bacteria

14 Z scheme hν hν

15 3D structure of Reaction center from the purple photosynthetic bacterium Rhodopseudomonas viridis 4 subunits Michel, Deisenhofer, Huber 1984 solved structure, 1988 Nobel prize

16 Crystal structures of photosystem II from Thermosynechococcus elongatus Ferreira et al. 3.5Å (2004) Science 303, subunits were identified

17 Comparison of electron-transfer cofactors between PSII and Bacterial RC PSII Nature 2005, 3.0Å resolution Science 2004, 3.5Å PNAS 2003, 3.7Å Nature 2001, 3.8Å Purple Bacterial RC Michel, Deisenhofer, Huber 1988 Nobel prize

18 Exciton coupling When pigments are physically very close, e.g. chlorophylls is less than 10 Å. The absorption spectra of pigments are split. The magnitude of the splitting is dependent on the distance of the pigments and the relative orientation of the pigments. Energy level diagram of a monomer and an exciton-split dimer.

19 Scheme for primary charge separation in PSII PSII RC Weak exciton coupling Shallow trap Protective role? BRC Strong exciton coupling Deep trap Barber (2002) Bioelectrochemistry 55, 135 Near 100% efficiency

20 PSII 8.2 Differences in Mg-Mg distance and the headgroup orientation. Electrostatic and hydrophobic interaction from protein environment. BRC 7.6

21 Crystal structures of photosystem II from Thermosynechococcus elongatus Ferreira et al. 3.5Å (2004) Science 303, subunits were identified

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23 Arrangements of cofactors and pigments in PSII energy transfer efficiency depends on their relative orientation and distance

24 Energy transfer Energy transfer is intrinsically a longer-range process than is electron transfer. The structures of antenna complexes have been fined-tuned to minimize excited state electron transfer processes. This is accomplished by separating the pigment to a distance that is too large to permit rapid electron transfer, while at the same time keeping them close enough to efficiently transfer energy and ultimately deliver it to the reaction center.

25 Arrangements of cofactors and pigments in PSII

26 Comparison of PSII core structures between higher plants and cyanobacteria Spinach Electron microscope on 2 D crystal at 8Å resolution Cyanobacteria S. elongatus X-ray diffraction on 3 D crystal At 3.8 Å resolution PSII RC Structures are highly conserved through 1 billions year s revolution

27 The process of 2D crystal formation Electron cryo-crystallography: Bacteriorohodoposin 3.0Å, LHCII 3.4Å, aquaporin 3.8Å.

28 Cyanobacteria S. Elongatus X-ray diffraction on 3 D crystal at 3.5 Å resolution Spinach Electron microscope on 2 D crystal Barber BBA 2006

29 Extrinsic polypeptides of PSII/OEC

30 Comparison of PSII antenna between high plants and cyanobacteria

31 3D structure of spinach LHCII-PSII supercomplex Barber BBA 2006 Overlay of x-ray structure of cyanobacterial core and the spinach LHCII on to lumen top view of structure of spinach derived from cryo-em and single particle analysis.

32 The structural model of the OEC from 3.5Å crystal structure Mn 3 CaO 4 cubic structure Ferreira et al Science 303, 1831

33 Structural and functional studies of cytochrome b559 in photosystem II Ferreira et al Science Possible function might be involved in protecting PSII from the photoinhibition.

34 Photoinhibition and repairing process of photosystem II reaction centers

35 Photosynthetic electron transport chain Science 2004, 3.5Å resolution PCCP 2004, 3.2Å PNAS 2003, 3.7Å Nature 2001, 3.8Å 3Å reslution Nature 2003 Science Å resolution (cyanobacteria) Nature Å (plant) Nature 2003

36 Z scheme hν hν

37 Structure of cyanobacterial Photosystem I at 2.5Å resolution Jorden et al. (2001) Nature, 411, protein subunits and 127 cofactors

38 cyanobacteria Photosystem I is a trimer structure Jorden et al. (2001) Nature, 411, 909

39 Structural homology between PSI and PSII Barber Natutal Structural Biology 2001

40 Two classes of reaction centers?

41 Photosystem I: both sides active? PsaA PsaB ET in PSI involved both branches with different rate constants

42 Local environment of P700 ~80% spin density of P700 + is localized on non-hb chl a (ec-b1)

43 Photosystem I: both sides active? A 1 A 0 PsaA PsaB ET in PSI involved both branches with different rate constants

44 Ferredoxin-NADP + reductase FAD NADP+ Ferredoxin (Fd) Contains a [2Fe-2S] cluster

45 Structure of flavin adenine dinucleotide (FAD)

46 Structure of plant photosystem I at 4.4Å resolution Plant PS I is monomeric Almost total conservation of the protein and cofactor structure of the reaction cores except subunit H and G are only present in plant. 4 different LHC I proteins assembled in a half-moon shape on one side of the core. Ben-Sham et al. Nature (2003) 426, 630.

47 Structure of plant photosystem I at 4.4Å resolution Longer N-terminal domain of PsaF enable the more efficient plastocyanin binding and two orders of magnitude faster electron transfer from this copper protein to P700.

48 The major respiratory and photosynthetic e- transport components of cyanobacteria The molecular biology of cyanobacteria by DA Bryant

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50 Light-Collecting Pigments Chlorophyll a Phycobilin proteins

51 Absorption spectra of light-collecting pigments

52 Nonphotochemical quenching (NPQ) Very shallow trap This process decrease the energy transfer to PSII and reduced the formation of triplet chlorophyll in LHCII, diminishing the production of the reactive oxygen species.

53 How to measure NPQ

54 NPQ = (Fm 0 -Fm )/Fm NPQ is linearly related to heat dissipation. NPQ = q E (energy dependent) + q S (state transition) +q I (photoinhibition).

55 NPQ Mutants Genetic approach using a digital video imaging system led to the isolation and characterization of several NPQ mutants of Chlamydomonas and Arabidopsis. Niyogi et al, Plant cell 1997

56 The Xanthophyll cycle Zeaxanthin, energy sink NPQ STATE Limited light High light Violaxanthin Zeaxanthin molecules can act as direct quenchers of excess excitation by accepting singlet energy transfer from chlorophyll

57 Chlamydomonas NPQ mutants deficient in the xanthophyll cycle Xanthophyll cycle do not account for all ΔpH dependent NPQ Niyogi et al, Plant cell 1997

58 Photoprotective roles of carotenoid molecules

59 PsbS NPQ Mutants One arabidopsis mutant, npq4, encode a protein, PsbS, associated with PSII. Belongs to LHC superfamily; contain 4 tansmembrane helix. Li et al. (2000) Nature 403,

60 PSII and LHC in the thylakoid membrane

61 Image from electron microscope and image processing of purified PSII particles from Arabidopsis thaliana Yakushevska et al. 2003

62 PsbS in Non- photochemical quenching Mutagenesis of two lumen-exposed glutamate residues of PsbS abolished the rapidly reducible NPQ. PsbS senses the ph change in the lumen, undergo conformational change, and induce NPQ

63 Non-photochemical quenching The xanthophyll cycle PsbS and ph dependent conformational changes Aggregation of LHCII induced fluorescence quenching

64 Crystal structure of spinach major light harvesting complex at 2.72 Å resolution Liu et al 2004 Nature, 428, chl a, 6 chl b, 3-4 cartenoid, 1 phospolipid

65 Stroma Lumen The inner ring has an important role in intermonomeric energy transfer. The outer ring favors the efficient absorption of incident light energy from all the direction. The light energy is quickly focus on Chla 612/Chla 611 and further transfer to the near exit.

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67 Spectral evolution in LHCII at 77K by fs laser spectroscopy

68 This crystal structure represents a dissipative state of LHCII Two interacting trimers from the icosahedral vesicle in the crystal The enlargement shows the two pairs of their their peripheril pigments, chla614 and chlb 605. Pascal et al (2005) Nature This structure provide a molecular basis of photoprotection and contol of photosynthetic light-havesting.

69 Structure-based NPQ model in LHC-II Liu et al 2004 Nature, 428, Zeaxanthin molecules act as direct quenchers of excess excitation by accepting singlet energy transfer from chlorophyll. Protonation of lumen-exposed acidic residues on PsbS and/or LHCII trigger the conformational changes and aggregation of LHCII. Efficient NPQ are established upon aggregation of LHC-II trimer.

70 State transition Plants can balance the distribution of absorbed light energy between the two photosystems. When the PSII is favored, a mobile pool of LHCII moves from PSII to PSI. This short-term and reversible redistribution is known as a state transition.

71 State transition In land plants, 15-20% of LHC is mobile during state transition. Transition from state I to state II is accompanied by a large fluorescence decrease.

72 State transition mutants Nature (2005) 433, p Phosphorylation of LHCII is diminished in STN7 mutant under state 2 conditions.

73 STN7 mutants impaired in state transition Wild-type stn7

74 Growth of the STN7 mutant is impaired under changing light conditions 8h/16h light/dark, alternatively illuminated for 1h at 50 and 1 h at 240 μe light intensity.

75 Antenna complex of plant PS I Subunit H is suggested to be the docking site for LHC II during state transition. Ben-Sham et al. Nature, 2003

76 State transition When subunit H is absent, LHCII is phosphorylated but it stays connected to PSII. Phosphorylation of LHCII induced a structural change that change the specificity of LHCII for each of two photosystems. PsaH is part of a high affinity binding site for phosphorylated LHCII.

77 Electron microscope image of Structure of PSI-LHCI supercomplex from the green alga C. reinhardtii Kargul et al (2005) FEBS Let 272,

78 Structural models of PSI-LHCI supercomplexes in state 2 Green algae C. reinhardtii, Phospho-CP29 Higer plants, Ar. Thaliana, LHCII trimer Species-dependent functional differences in state 2? Melkozernov et al (2006) Biochemistry

79 Species-dependent functional differences in state 2? In high plant, only 10-15% of LHCII is transfer from PSII to PSI during a state 1 to state 2 transition. In green alga C. reinhardtii, 80% of the LHCII displaced to PSI in state 2. Involvement of state transitions in the switch from linear to cyclic electron flow.

80 State transition Cyclic electron transfer

81 Cyclic electron transfer from PSI Fd is proposed to reduces a ferredoxin-plastoquinone reductase (FQR).

82 Cytochrome bf complex Dr. Popot (France) and Dr. Cramer (USA) research teams present crystal structure of cytochrome bf complex from Chamydomonas and cyanobacteria, respectively, to about 3Å resolution. Stroebel, Choquet, Popot and Picot, Nature (2003) 426, Kurisu, Zhang, Smith and Cramer, Science (2003) 302,

83 Q cycle Buchanan

84 Cytochrome bf complex Kurisu et al. (2003) science

85 Comparsion of cyt bf and cyt bc complex

86 The missing link in cyclic electron transfer Heme X in cyt bf complex is very likely to be the missing link, the Fdx-PQ oxidoreductase. The function of chlorophyll and carotenoid molecules in cyt bf complex are still not known.