4. Photosynthesis: antennas and reaction centers

1. 4. Photosynthesis: antennas and reaction centers

2. 6 CO2 + 6 H2O C6H12O6 + 6 O2 6 CO2 + 6 H2O + 48 hn C6H12O6 + 6 O2 Synthesizing carbohydrates from CO2 and water presents a formidable thermodynamic problem: DGo = 679 kcal/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 = Nhn = 41 kcal/einstein*. DGo = -1290 kcal/mol (Keq = 10942) ! *An einstein is a mol of photons. N = Avogadro’s number; h = Planck’s constant; n = frequency.

3. LIGHT When light raises a molecule to an excited electronic state, the molecule becomes a stronger reductant LUMO LUMO HOMO HOMO electrons A* B A B

4. O N Mg Mg N N N N N O Fe N N N N O N N The photochemically reactive pigments are chlorins or bacteriochlorins, which are structurally related to hemes Hemes symmetrical p systems; absorb blue light Chlorophylls asymmetrical p systems; absorb blue & red light Bacteriochlorophylls more asymmetrical p systems; absorb blue, orange & near-IR light

5. The photochemical reactions of photosynthesis take place in integral membrane proteins thylakoid lumen outer membrane inner membrane stroma thylakoid membrane chloroplast

6. 0.0004 O2/Chl 0.0002 0 0.004 0.008 0 Light absorbed (photons/Chl) 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. At high light intensity, the maximum O2 released per flash was about 1 O2 per 2400 Chls. R. Emerson & W. Arnold measured the amount of O2 formed when they excited algae with short flashes of light. At low light intensity, 1 O2 is formed for ~each 8 photons absorbed (yellow dashed line).

7. The antenna system of purple photosynthetic bacteria has circular pigment-protein complexes The LH1 complex, with 31 transmembrane a-helical peptides (yellow) and 30 BChls (CPK colors), surrounds the reaction center (red & orange). view normal to the membrane view in the plane of the membrane A. W. Roszak et al. Science 302: 1969 (2003); 1pyh.pdb

8. 0.1 - 0.2 ps 1.2 ps When the antenna is excited with light, excitations are transferred to the reaction center within ~40 ps The smaller LH2 complexes transfer energy rapidly to LH1 LH2 LH2 LH2 35 ps View normal to the membrane 1 ps = 10-12 s RC LH1

9. The LHC-II (light-harvesting complex II) antenna protein of plants has 12 chlorophylls and 2 carotenoids per protein subunit Chla Chlb

10. Two subunits are integral membrane proteins with homologous structures The reaction center of purple photosynthetic bacteria has 3 to 4 subunits, depending on the species A relatively small number of bacteriochlorophylls and other electron carriers are bound to the proteins membrane phospholipid bilayer view parallel to the membrane

11. BChl dimer BChl BChl BPh BPh Q Q Fe 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 (BChl with 2 H in place of Mg) Q = ubiquinone Axis of approximate rotational symmetry

12. l1 l2 Absorbance change Delay between excitation and probe pulses The sequence and kinetics of the initial electron-transfer reactions can be studied by exciting RCs with short pulses of light beam splitter train of short pulses Laser change the optical path to vary the delay between the excitation & probe pulses excitation pulses sample probe pulses (wavelength l1orl2) photodiode measures the probe pulses The curves labeled l1andl2 are measurements at two wavelengths

13. When the complex is excited with light, an electron moves from the BChl dimer (P870) to a BPh and then to a quinone LIGHT BChl dimer 3 ps BChl 1 ps BPh 200 ps Q Fe

14. The photosynthetic electron-transfer system in purple bacteria is cyclic P870* BChl BPh QA 1. Reduced ubiquinone (QBH2) dissociates from the RC and diffuses to the cytochrome bc1 complex, where it is reoxidized. QB Light 1. cytochrome bc1 cytochrome c 2. Reduced cytochrome c diffuses back to the RC to complete the cycle. 2. P870

15. The cytochrome bc1 complex of photosynthetic bacteria is homologous to the mitochondrial bc1 complex and the cytochrome b6f complex of chloroplasts and cyanobacteria cytochrome c1 or f Fe-S center Chl & carotenoid (b6f only) membrane cytochrome b with two hemes quinone (UQ or PQ) This is cytochrome b6f. It’s a homodimer with multiple subunits H. Zhang et al. Proc. Natl. Acad. Sci. 100: 5160 (2003); 1vf5.pdb

16. P700* Plants have two photosystems that work in series to move electrons from water to NADP Chl QK1 P680* Fe-S centers Phe Ferredoxin Light NADPH NADP PQA, PQB Light Cyt b6f complex Photosystem I O2 plastocyanin H2O P700 Photosystem II Mn center PQA, PQB = plastoquinone QK1 = phyloquinone plastocyanin is a Cu protein P680

17. The reaction center of Photosystem II has 19 subunits It binds 32 molecules of Chl-a (green). Most of these are part of the antenna. view parallel to the membrane K. N. Ferreira et al. Science 303: 1831 (2004). pdb file 1s5l.pdb

18. The core of the Photosystem II reaction center is very similar to that of purple bacteria The polypeptide backbones of the two main subunits are shown in red and orange, chlorophylls in green, pheophytins in blue and quinones in yellow.

19. Mn4O4Ca complex HCO3- P680 Tyr Chl a Chl a Pheo Pheo PQ Fe PQ HCO3- Electron carriers in the Photosystem II reaction center Mn4O4Ca complex and the surrounding amino acid side chains The cluster of 4 Mn atoms is the O2-evolution site. Note the tyrosine residue located between the Mn cluster and P680. K. N. Ferreira et al. Science 303: 1831 (2004); 1s5l.pdb

20. The Photosystem 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 Chl electron carriers (blue) again is very similar to that in bacterial RCs. view normal to the membrane

21. Two of the Fe-S centers are in subunit C on the stromal side of the membrane view parallel to the membrane surface