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Photo-Phosphorylation

Lehninger 5th ed. Chapter 19

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Photosynthesis

•  The source of food, and therefore life on earth.

•  It uses water to produce O2. •  However E´0 of water is 0.816V

(NADH’s is -0.32V). •  Thus input of energy (light) is needed.

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Energy of a photon

The energy of a photon at 700 nm

The energy of an einstein of photons at 700 nm

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Life’s food chain

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“Dark reactions”

Photosynthesis reactions

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Chloroplasts

Note 3 separate regions inside!

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Hill reaction

•  When leaf extracts containing chloroplasts are illuminated, they: –  evolve O2. –  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+.

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The players: 1

Chlorophylls a and b and bacteriochlorophyll are the primary gatherers of light energy.

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The players: 2

Phycoerythrobilin and phycocyanobilin (phycobilins) are the antenna pigments in cyanobacteria and red algae.

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The players: 3

(c) β-Carotene (a carotenoid) and (d) lutein (a xanthophyll) are accessory pigments in plants.

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The players: 4

(c) β-Carotene (a carotenoid) and (d) lutein (a xanthophyll) are accessory pigments in plants.

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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.

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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.

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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)

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Photosynthesis action spectrum

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3 photosynthetic schemes

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Purple bacteria (type II RC)

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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, QBH2, which is free to diffuse in the membrane bilayer, away from the reaction center.

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Type II RC cont.

•  The QBH2 then enters the Cyt bc1, which is analogous to complex III (Q cycle).

•  Here the soluble e- acceptor is Cyt c1. •  The ultimate acceptor is the e- depleted

P870 •(Chl)2+.

•  This all takes place in the solid state.

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Fe-S type RC •  Similar to type II, but some electrons do not reach Cyt

bc1, 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 H2S to elemental S, then to SO4

2-. •  Therefore the name “green sulfur bacteria”. •  This oxidation of H2S by bacteria is chemically

analogous to the oxidation of H2O by oxygenic plants.

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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.

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Photosynthetic efficiency

Remember slide # 4

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Plants, algae & cyanobacteria

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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 O2.

– PSI is similar to green sulfur bacteria. •  Contributes to the PMF. •  Produces reducing power (NADPH) for future

sugar making.

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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 H2O.

•  Oxygenic photosynthesis.

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Bacteria PSTII plants

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Bacteria PSTI plants

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PS2

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PSI scheme

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PSI structure

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Cyt b6f

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Cyt b6f

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Q cycle in Cyt b6f

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Cyclic vs. noncyclic PSI

•  If e- move from Fd to Cyt b6f then: –  ATP is made –  NADPH isn’t made. –  O2 isn’t evolved.

•  Thus, plants can regulate ATP production versus C assimilation.

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42 •  The separate locations of PSI and PSII ensures that both get excited.

•  Otherwise P680 would excite P700.

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43 •  LHCII can associate with PSI or PSII according to light intensity and wavelength.

•  This leads to state transitions

• Under intense blue light, that favors PSII, more PQH2 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

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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:

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Water splitting (O2 evolving)

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ATP synthesis

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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 O2 evolved.

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Ox-Phos vs. Photo-Phos

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Evolution of oxygenic photosynthesis

•  2.5 billion years ago oxygenic photosynthesis changed the biosphere!

•  Chloroplasts evolved from ancient photosynthetic bacteria.

•  D is for donor

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Dual role of complex III in cyanobacteria

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One protein can do it all!

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