Photo-Phosphorylation. Photosynthesis 11/29/10. Lehninger 5 th ed. Chapter 19

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1 1 Photo-Phosphorylation Lehninger 5 th ed. Chapter 19 2 Photosynthesis The source of food, and therefore life on earth. It uses water to produce O 2. However E 0 of water is 0.816V (NADH s is -0.32V). Thus input of energy (light) is needed. 3 1

2 4 Energy of a photon The energy of a photon at 700 nm The energy of an einstein of photons at 700 nm Life s food chain 5 Photosynthesis reactions 6 Dark reactions 2

3 Chloroplasts 7 Note 3 separate regions inside! 8 9 Hill reaction When leaf extracts containing chloroplasts are illuminated, they: evolve O 2. Reduce a non-biological electron acceptor added to the medium (A below). None of the above happens in the dark! The biological acceptor is NADP +. 3

4 10 The players: 1 11 Chlorophylls a and b and bacteriochlorophyll are the primary gatherers of light energy. 12 The players: 2 Phycoerythrobilin and phycocyanobilin (phycobilins) are the antenna pigments in cyanobacteria and red algae. 4

5 13 The players: 3 (c) β-carotene (a carotenoid) and (d) lutein (a xanthophyll) are accessory pigments in plants. 14 The players: 4 (c) β-carotene (a carotenoid) and (d) lutein (a xanthophyll) are accessory pigments in plants. 15 Plants are green because their pigments absorb light from the red and blue regions of the spectrum, leaving primarily green light to be reflected or transmitted. 5

6 16 Antenna There are numerous pigments that capture the light. They in turn transfer the light to the reaction center. This exciton transfer is 95% efficient. They are called light harvesting complexes. 17 Phycobilisome Extension of the action spectrum by accessory pigments. Biological niches can thus be utilized. Found in cyanobacteria and red algae. phycoerythrin (PE), phycocyanin (PC), and allophycocyanin (AP) Photosynthesis action spectrum 18 6

7 19 3 photosynthetic schemes 20 Purple bacteria (type II RC) 21 7

8 22 23 Purple bacteria (type II RC) The pheophytin radical now passes its e - to a tightly bound QA. The semiquinone radical immediately donates its extra e - to a second, loosely bound QB. Two such electron transfers convert QB to its fully reduced form, QBH 2, which is free to diffuse in the membrane bilayer, away from the reaction center. 24 Type II RC cont. The QBH 2 then enters the Cyt bc 1, which is analogous to complex III (Q cycle). Here the soluble e - acceptor is Cyt c 1. The ultimate acceptor is the e - depleted P870 (Chl) 2+. This all takes place in the solid state. 8

9 25 Fe-S type RC Similar to type II, but some electrons do not reach Cyt bc 1, rather ferredoxin. Ferredoxin, an Fe-S protein passes the electrons to ferredoxin:nad reductase producing NADH. The electrons taken from the reaction center to reduce NAD + are replaced by the oxidation of H 2 S to elemental S, then to SO Therefore the name green sulfur bacteria. This oxidation of H 2 S by bacteria is chemically analogous to the oxidation of H 2 O by oxygenic plants. 26 Dissipation: the enemy Why doesn t the excited state decay to the ground state by internal conversion? The proteins hold the chromophors in a precise orientation to ensure maximal charge separation. Charge separation takes place < 100ps with >90% efficiency. The combination of fast kinetics and favorable thermodynamics makes the process virtually irreversible and highly efficient. 27 Photosynthetic efficiency Remember slide # 4 9

10 Plants, algae & cyanobacteria Two RCs in tandem In plants there are 2 photosystems that resemble a combination of the 2 different bacterial systems: PSII is similar to that of purple bacteria. Contributes to the PMF. Produces O 2. PSI is similar to green sulfur bacteria. Contributes to the PMF. Produces reducing power (NADPH) for future sugar making. 30 Connection between PSI & PSII Plastocyanin, a one-e - carrier functionally similar to cytochrome c of mitochondria. The Z-scheme. To replace the electrons that move from PSII to PSI to NADP +, cyanobacteria and plants oxidize H 2 O. Oxygenic photosynthesis. 10

11 Bacteria PSTII plants 31 Bacteria PSTI plants PS2 11

12 11/29/ PSI scheme 36 PSI structure 12

13 Cyt b 6 f 37 Cyt b 6 f 38 Q cycle in Cyt b 6 f 39 13

14 40 Cyclic vs. noncyclic PSI If e - move from Fd to Cyt b 6 f then: ATP is made NADPH isn t made. O 2 isn t evolved. Thus, plants can regulate ATP production versus C assimilation. 41 The separate locations of PSI and PSII ensures that both get excited. Otherwise P680 would excite P

15 LHCII can associate with PSI or PSII according to light intensity and wavelength. This leads to state transitions 43 Under intense blue light, that favors PSII, more PQH 2 is made than can be utilized by PSI. PQH2 activates a protein kinase that phosphorylates LHCII leading to its association with PSI. Under low light increase in PQ trigerss dephorsphorylation and association of LHCII with PSII. LHCII with PSII LHCII with PSI 44 Water splitting 1 2 Billion years ago bacteria came up with an e - donor that is always available: water. However the energy of a single photon does not have enough energy to break the bonds in water: Water splitting (O 2 evolving) 45 15

16 ATP synthesis Stoichiometry About 12 H + s are pumped from the stroma to the thylakoid lumen. This results in a ΔpH of 3 and a ΔG of 200 kj/ mol. Thus we get a ratio of 3ATPs per O 2 evolved. 16

17 Ox-Phos vs. Photo-Phos Evolution of oxygenic photosynthesis 2.5 billion years ago oxygenic photosynthesis changed the biosphere! Chloroplasts evolved from ancient photosynthetic bacteria. D is for donor Dual role of complex III in cyanobacteria 51 17

18 One protein can do it all!