1. Lecture 6 - Photosynthesis: The Light Reactions
Chem 454: Regulatory Mechanisms in Biochemistry
University of Wisconsin-Eau Claire

2. Introduction
Photosynthesis represents for the biosphere the major source of
Free energy (1017 kcal/year stored)
Carbon (1010 tons/year assimilated)
Oxygen

3. Overview
Analogous to the combination of oxidative phosphorylation and the citric acid cycle.

4. 1. Chloroplasts
Like oxidative phosphorylation, photosynthesis is compartmentalized
- Three membranes: outer, inner and thylakoid.
- The inner membrane surrounds the stroma.
- Thylakoid is analogous to the cristae in mitochondria
• Probably derived from the inner membrane
- The stacks of thylakoid membrane sacs are called grana.
- The grana are connected by stroma lamellae.
- The thylakoid membraned partitions the stroma space from the thylakoid space.
• ATP and NADPH produced are used to convert CO2 to sugar in the stroma space.
• The light harvesting, electron transport, and ATP synthesis occur in the thylakoid membrane.
- Like mitochondria, chloroplasts were once thought to be free living organisms, probably cyanobacteria, that took up an endsymbiotic relationship within a eukaryotic cell.

5. 1 Mitochondria Mitochondria are bound by a double membrane
- There are two compartments
• The Intermembrane space
• The matrix
- Oxidative phosphorylation takes place in the inner membrane
- The citric acid cycle takes place in the matrix.
- Outer membrane is quite permeable because it contains porin (VDAC - voltage-dependent anion channel) - regulates flux of anionic species including phosphate, chloride, organic ions, nucleotides
- Inner membrane is impermeable to most substances. Transporters are required to move materials in and out of the matrix.
- N (negative, matrix) and P (positive, cytosolic) sides of membrane
- There is evidence that mitochondria derived from prokaryotes and became part of eukaryotic cells as a result of an endosymbiotic event.

6. 2. Electron Transfer
Light absorption by chlorophyll induces electron transfer
- Like heme it is a tetrapyrrole, but one of its rings is reduced.
- Contains Mg instead of Fe
- Is a polyene, absorbs in visible region of light spectrum
- Has one of the highest extinction coefficients known (105 M-1 cm-1)

7. 2. Electron Transfer
Absorption of light leads to photoinduced charge separation

8. 2. Electron Transfer
Absorption of light leads to photoinduced charge separation

9. 2.1 Bacteria vs. Green Plants
Plants have two photosystems
Photosystem I
13 polypeptides chains
60 Chlorophylls
3 4Fe-4S centers
1 Quinone
Photosystem II
10 polypeptide chains
30 Chlorophylls
1 Non-heme Fe
4 Mn+2 (Mn+2, Mn+3, Mn+4 , Mn+5)

10. 2.1 Bacteria vs. Green Plants
Bacteria have a single system
4 Bacteriochlorophylls b
2 Bacteriopheophytin b
2 Quinones
1 Fe2+

11. 2.1 Bacteria vs. Green Plants
Bacteria have a single system
- The structure of bacterial photoreaction center was determined in the late 1980's, J. Deisenhofer and R. Huber ("
- The L and M subunits each contain 5 transmembrane helices
- The H contains one transmembrane helix

12. 2.2 The Bacterial Photosynthetic Reaction Center
- The light energy causes an electron to flow from the special pair to the bacteropheophytin.
- The charge separation is stabilized by three factors
• The Q(A) is close to the BPh- (10Å)
• The charge on the P960(+) can e readily neutralized by an electron from the cyt c that is only 10Å away.
• The recombination with the P960(+) is so thermodynamically favorable that it is in the inverted region (See Fig18.8 on next slide)
- The electon then moves on the more loosely bound Q(B) quinone, picking up a proton from the cytoplasmic side of the membrane. (The caption to Fig says the periplasmic side?)
- The QH(2) is reoxidized by Compled III of the electron transport chain
- The cyt c(b) that is reduced by Complex III transfers the electrons back to the cyt c of the BPC on the periplasmic side of the membrane.
- The proton gradient that results is used to drive the synthesis of ATP

13. 2.3 Electron Transfer
The rate of electron transfer is dependent up two factors:
Distance
Driving Force
- The area to the right of the maximum in the driving force is called the inverted region

14. 3.3 Q-Cytochrome c Oxidoreductase
The Q cycle:
- Complex III is involved in recycling the electrons back to the cyt c, by way of a mobile cyt c(b)
-The pattern of electron flow is reminicient to what we saw with the Q-Cytochrome c oxidoreductase

15. 3. Two Photosystems of Plants
In green plants there are two photosynthetic reaction centers.
- Instead of cycling electrons, as is done in the bacterial system, they are removed from water and used to make a high-energy NADPH
- This photo synthesis is oxygenic
- The two systems respond to light of shorter wavelength than the bacterial system: 680 for PSII and 700 for PSI
- The cytochrome bf complex that connects the two systems is homologous to the Q-cyt c oxidoreductase (Complex III) of the electron transport chain, with plastocyanin being the acceptor of the electrons instead of cyt c.

16. 3.1 Photosystem II
The core of PSII is similar to the bacterial system.

17. 3.1 Photosystem II
- The source of the electrons is H2O instead of cyt c.
- The 4 Manganese center is where the electrons are removed from the water.

18. 3.1 Photosystem II
Four photons are required to generate one oxygen molecule
- The 4 Manganese center is where the electrons are removed from the water.

19. 3.1 Photosystem II
The 4 Manganese center is where the electrons are extracted from the water.
- The 4 Manganese center is where the electrons are removed from the water.
- The site of water oxidation is on the thylakoid side of the membrane
- The reduced plastoquinone is produced on the stroma side of the membrane.

20. 3.1 Photosystem II
An equivalent of 4 H+ are moved across the thylakoid membrane by PSII
- The 4 Manganese center is where the electrons are removed from the water.
- The site of water oxidation is on the thylakoid side of the membrane
- The reduced plastoquinone is produced on the stroma side of the membrane.

21. 3.2 Cytochrome bf
The cytochrome bf complex transfers the electrons from plastoquinone to platocyanin:

22. 3.2 Cytochrome bf
- This complex works very much like the Complex III of the electron transport chain.
• Uses a Q-cycle to transfer electrons from the two-electron QH(2) to the one-electron carrier, platocyanin.
• 2 protons are transferred across the thylakoid membrane, and along with the two protons released by QH(2), contribute to the proton gradient across the thylakoid membrane.

23. 3.3 Q-Cytochrome c Oxidoreductase
The Q cycle:
- Complex III is involved in recycling the electrons back to the cyt c, by way of a mobile cyt c(b)
-The pattern of electron flow is reminicient to what we saw with the Q-Cytochrome c oxidoreductase

24. 3.3 Photosystem I
Though larger, the core of PSI is also similar to the bacterial system.
- The psaA (83kd) and psaB(82kd) are larger than the D1 and D2 subunits of PSI (32kd) and the L(31kd) and M(32kd) subunits of the bacterial photoreaction center.
• Terminal 40% is similar to the D1 and D2 subunits in the PSII system.
- The special pair of chlorophyl molecules absorb at 700nm

25. 3.3 Photosystem I
The receptor of the the electrons from PSI is the small one-electron redox protein, Ferredoxin.
- Ferredoxin contains a 2Fe-2S center that is ligated by 4 cysteines.

26. 3.3 Photosystem I
- From the reduction potentials for plastocyanin (+0.37V) and for ferredoxin (-0.45V), the standard free energy for the the reduction of ferredoxin by plastocyanin is 18.9 kcal/mol.
• The energy for a mole of photons with a wavelength of 700nm is 40.9 kcal/mol.
- A single electron flows from the special pair to ferredoxin, through a chlorophyll, a quinone and 3 4Fe-4S centers.

27. 3.3 Photosystem I The Z-scheme of photosynthesis
-The reduced ferredoxin is used to reduce NADP(+)
• As a one-electron carrier, ferredoxin is not a good reducing agent for most biosynthetic reactions.
- The reduction of NADP(+) by ferredoxin is carried out by the enzyme ferredoxin-NADP(+) reductase.

28. 3.4 Ferredoxin-NADP+ Reductase
- The reduction of NADP(+) by ferredoxin is carried out by the enzyme ferredoxin-NADP(+) reductase.
• The reductase contains an FAD, which can accept one electron at a time.

29. 3.4 Ferredoxin-NADP+ Reductase
Ferredoxin-NADP+ reductase contains the coenzyme FAD.
- The reduction of NADP(+) by ferredoxin is carried out by the enzyme ferredoxin-NADP(+) reductase.
• The reductase contains an FAD, which can accept one electron at a time.

30. 3.4 Ferredoxin-NADP+ Reductase
Ferredoxin-NADP+ reductase contains the coenzyme FAD.
- The H(+) ions are picked up from the stroma side of the thylakoid membrane and so contribute to the proton gradient.

31. 4. The Proton Gradient
PSII, Cytochrome bf and ferredoxin-NADP+ reductase each contribute to the proton gradient.
- The is no concomitant membrane potential as with mitochondria because the thylakoid membrane is permeable to Cl(-) and Mg(2+).

32. 4. The Proton Gradient André Jagendorf's demostration
1966, was one of the earliest pieces of evidence to support Peter Mitchell's chemiosmotic hypothesis.

33. 4.1 ATP Synthase
The ATP synthase closely resembles the ATP synthase of mitochondria

34. 4.1 ATP Synthase
Comparing photosynthesis to oxidative phosphorylation:

35. 4.2 Flow of Electrons Through Photosystem I
Normally photosynthesis produces both ATP and NADPH.
When there is not NADP+ available, cyclic flow of electrons through PSI can be used to produce ATP without reducing NADP+

36. 4.2 Flow of Electrons Through Photosystem I
Cyclic flow of electrons through PSI:
- Only PS1 is involved, no O2 is evolved.

37. 4.3 Stoichiometry of Photosynthesis
-Protons
• 4 protons released to thylakoid lumen per O2 produced
• 2 QH(2) in Q-cycle of cyt bf compled release 8 protons
• Pc to NADPH absorb 4 more from stroma side.
12 protons are expected to be used for one turn of the ATP synthase, therefore, producing 3 AtP's from 3ADP's and 3Pi's