understanding photosynthesis the most important process on the planet

1. understanding photosynthesis the most important process on the planet John Gray Department of Plant Sciences University of Cambridge

2. Life on earth depends on plants for photosynthetic CO2 fixation and O2 evolution

3. Photosynthesis • a highly efficient energy transduction process • conversion of light energy into chemical energy light 6CO2 + 6H2O  C6H12O6 + 6O2 6CO2 + 6H2O C6H12O6 + 6O2 energy respiration

4. Cross-section of a leaf 100 mm Mesophyll cells

5. Thylakoid membrane • chlorophyll • light-harvesting • electron transfer • O2 evolution • energy production • Stroma • Rubisco • CO2 fixation • sugar and starch • synthesis Pea chloroplast 1 mm

6. Schematic chloroplast membrane-enclosed stroma sealed thylakoid membrane

7. Photosynthetic processes in the thylakoid membrane The Light Reactions

8. Structures of thylakoid membrane complexes

9. Light absorption bychlorophylls • All chlorophyll is associated • with proteins to form • light-harvesting complexes • in the thylakoid membrane • There is no free chlorophyll

10. Structure of LHCII trimer Kühlbrandt et al. (1994) Liu et al. (2004)

11. LHCII trimers in grana stack

12. Energy transfer in light-harvesting complexes Light is absorbed by individual chlorophylls in the light-harvesting complexes Energy is transferred from one pigment to another via Resonance Energy Transfer This transfer funnels the energy to a reaction centre where electron transfer starts

13. arrangement in thylakoid membrane • Low resolution structures of photosystem II • electron microscopy • membrane preparations • single particles - negative stain

14. Photosystem II - at 3.5Å resolution

15. D1 and D2 polypeptides - the core of PSII • 5 transmembrane spans • similar to purple bacterial reaction centre D1 is the product of the chloroplast psbA gene

16. Prosthetic groups of PSII core

17. OXYGEN EVOLUTION 2H2O  O2 + 4H+ + 4e by analogy to sulphur bacteria (van Niel 1930) H2S  S + 2H+ + 2e 1970 Joliot and Kok - measured O2 yield from saturating light flashes O2 evolution every 4th flash - system for accumulating 4 positive charges

18. Structure of the manganese cluster 'Dangler' model cubane Mn3CaO4 cluster + fourth Mn linked via O

19. Photosynthetic electron transfer

20. ATP synthesis coupled to electron transfer

21. Structure of ATP synthase ,  and  subunits cross section side view

22. Mechanism of ATP synthesis • NOBEL PRIZE 1997: • Paul Boyer (UCLA) • Rotational catalysis • John Walker (Cambridge) • X-ray structure showing • 3 different conformations • for 3  subunit dimers

23. Rotary catalysis by ATP synthase

24. Models of H+ translocation proton translocation through a subunit drives rotation of c subunit ring and g subunit b subunits (b and b' in CFo) act as stator to prevent rotation of ab subunits

25. Light reactions of photosynthesis • Light absorption by chlorophylls in light-harvesting • complexes • Electron transfer initiated at reaction centres in • photosystem II and photosystem I • Electron transfer from H2O to NADP+ • generating O2 and reducing power • Coupled H+ liberation in thylakoid lumen provides • driving force for ATP synthesis

26. 10 8 6 1000 4 Rubisco appears 2 800 CO2 fixation had a massive impact on global climate 600 CO2 Atmospheric partial pressure 400 O2 200 0 4 3 2 1 0 0.6 0.4 0.2 0 Time before present (billion years) The dark reactions: capturing CO2 • Light reactions generate ATP and NADPH • Provide energy for fixing CO2

27. The dark reactions: capturing CO2 The numbers are HUGE • Atmospheric CO2 is 0.035% (and rising!) • Total CO2 in atmosphere 700 x 109 tonnes • Photosynthesis fixes ~100 x 109 tonnes per year • ~15% of total atmospheric CO2moves into • photosynthetic organisms each year!

28. Active site • Rubisco • Ribulose 1,5-bisphosphate (RuBP) carboxylase-oxygenase • catalyses CO2 fixation into C3 compounds • is the most abundant protein on the planet Rubisco is made from 8 small and 8 large subunits

29. CH2OP C=O H-C-OH H-C-OH CH2OP CH2OP H-C-OH COOH COOH H-C-OH CH2OP + Rubisco reaction H2O CH2OP H-C-OH CHO CO2 ATP NADPH CH2OP C=O CH2OH RuBP C5 sugar 3-PGA 2 x C3 acid 2 x C3 sugars

30. 12C3 6C5 6C5 12C3 C6 C6 Regeneration via C4 C5 C6 & C7 sugar phosphates 10C3 6CO2 6 ATP sucrose 6 ATP 6 NADPH 6 cycles export from chloroplast 2C3 C6 starch

31. Photosynthesis • Light-driven electron transfer from H2O to NADP+ • generating O2 and reducing power • Coupled H+ translocation into thylakoid lumen used • to generate ATP • CO2 fixation into sugars using energy from ATP • and NADPH • Requires chloroplasts with intact thylakoid membranes

32. Plant cell stained with DAPI (a DNA fluorochrome)

33. 1 mm • Chloroplast DNA • Each chloroplast contains up to 100 copies of chloroplast DNA • Leaf mesophyll cells contains ~100 chloroplasts • Leaf mesophyll cells contains ~10000 copies of chloroplast DNA

34. Genes in land plant chloroplast DNA Rubisco LSrbcL 1 Photosystem IIpsb 13 Cytochrome bfpet 5 Photosystem Ipsa 6 ATP synthaseatp 6 NADH dehydrogenasendh 13 Ribosomal RNA rrn 4 (x 2) Transfer RNA trn ~32 Ribosomal proteins rpl or rps 19 RNA polymerase rpo 4 Translation initiation factor infA 1 Acetyl CoA carboxylase accD 1 ATP-dependent protease clpP 1 Unknown ycf 3 44 64

35. Assembly of photosynthesis complexes chloroplast gene product nuclear gene product • All complexes contain at least one nuclear-encoded subunit • Requires coordination of plastid and nuclear gene expression

36. Nuclear gene products structural & regulatory proteins Plastid signals Coordination of nuclear and chloroplast gene expression nucleus Expression of nuclear genes for chloroplast proteins is regulated by plastid signals reporting the functional state of the chloroplasts chloroplast

37. STROMULES(stroma-filled tubules)

38. STROMULES stroma-filled tubules interconnecting plastids