Lecture 6 - Photosynthesis: The Light Reactions

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

5. 1 Mitochondria • Mitochondria are bound by a double membrane

6. 2. Electron Transfer • Light absorption by chlorophyll induces electron transfer

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

12. 2.2 The Bacterial Photosynthetic Reaction Center

13. 2.3 Electron Transfer • The rate of electron transfer is dependent up two factors: Distance Driving Force

14. 3.3 Q-Cytochrome c Oxidoreductase • The Q cycle:

15. 3. Two Photosystems of Plants • In green plants there are two photosynthetic reaction centers.

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

17. 3.1 Photosystem II

18. 3.1 Photosystem II • Four photons are required to generate one oxygen molecule

19. 3.1 Photosystem II • The 4 Manganese center is where the electrons are extracted from the water.

20. 3.1 Photosystem II • An equivalent of 4 H+ are moved across the thylakoid membrane by PSII

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

22. 3.2 Cytochrome bf

23. 3.3 Q-Cytochrome c Oxidoreductase • The Q cycle:

24. 3.3 Photosystem I • Though larger, the core of PSI is also similar to the bacterial system.

25. 3.3 Photosystem I • The receptor of the the electrons from PSI is the small one-electron redox protein, Ferredoxin.

26. 3.3 Photosystem I

27. 3.3 Photosystem I • The Z-scheme of photosynthesis

28. 3.4 Ferredoxin-NADP+ Reductase • Ferredoxin-NADP+ reductase

29. 3.4 Ferredoxin-NADP+ Reductase • Ferredoxin-NADP+ reductase contains the coenzyme FAD.

30. 3.4 Ferredoxin-NADP+ Reductase • Ferredoxin-NADP+ reductase contains the coenzyme FAD.

31. 4. The Proton Gradient • PSII, Cytochrome bf and ferredoxin-NADP+ reductase each contribute to the proton gradient.

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:

37. 4.3 Stoichiometry of Photosynthesis • 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