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Topics: Regulation of the Calvin Cycle Photorespiration CO2 concentrating mechanisms Sucrose and starch synthesis

Topics: Regulation of the Calvin Cycle Photorespiration CO2 concentrating mechanisms Sucrose and starch synthesis. Regulation of the Calvin cycle. RuBP “activase” Light induction of Calvin cycle gene expression Enzyme activites regulated by redox state of the chloroplast.

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Topics: Regulation of the Calvin Cycle Photorespiration CO2 concentrating mechanisms Sucrose and starch synthesis

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  1. Topics: • Regulation of the Calvin Cycle • Photorespiration • CO2 concentrating mechanisms • Sucrose and starch synthesis

  2. Regulation of the Calvin cycle • RuBP “activase” • Light induction of Calvin cycle gene expression • Enzyme activites regulated by redox state of the chloroplast

  3. c: Redox state of stroma: The Ferredoxin-Thioredoxin System NADPH

  4. Topics: • Regulation of the Calvin Cycle • Photorespiration • CO2 concentrating mechanisms • Sucrose and starch synthesis

  5. “The Problem with Oxygen”: RUBISCO reacts with oxygen as well as CO2 (oxygenase/carboxylase) RUBISCO has a higher affinity for CO2 compared to O2 (lower Km) Rubisco : Km (CO2)= 15 μM Km (O2)= 550 μM But concentration of O2 is much higher: Atmophere: 20% O2 and only 0.03% CO2 In solution: CO2 = 12 μM, O2= 265 μM NET RESULT: a lot of O2 gets “fixed” instead of CO2 This process is called photorespiration.

  6. RuBis Carboxylase/Oxygenase

  7. Calvin Cycle 3xATP Rib15bisP (C5) + O2 2x NADPH 3-P-Glycerate (C3) P-Glycolate (C2) Chl. Fd ATP Glycerate (C3) Glycolate (C2) Glycolate (C2) Glycerate (C3) O2 ATP NH4 Hydroxypyruvate (C3) H2O2 Glycolate (C2) Per. Serine (C3-N) Glycine (C2-N) Glycine (C2-N) Serine (C3-N) CO2 NH4 Mit

  8. The cost of photorespiration 3x O2 needs 2x ATP and 2x Ferredoxin AND high temperature increases photorespiration: *Modifies Rubisco’s kinetics: oxygenation more favorable *Decreases the CO2/O2 ratio in solution

  9. What do plants do?

  10. CO2 Concentrating Mechanisms • CO2 and HCO3- Pumps: aquatic organisms • CO2 concentrating mechanisms: higher plants

  11. CO2 Concentrating Mechanisms • Clicker question: Is there only one type of CO2 concentrating mechanisms in higher plants? • A. Yes • B. No, there are two. • C. N, there are many

  12. CO2 Concentrating Mechanisms • Clicker question: Did these different mechanisms evolved from one common ancestor? • A. Yes, modifications occurred later. • B. No, there are two independent origins. • C. No , there were many independent origins. CO2 concentrating mechanisms evolved many times independently: Convergent evolution

  13. CO2 Concentrating Mechanisms • CO2 and HCO3- Pumps: aquatic organisms • CO2 concentrating mechanisms: higher plants PEP-Carboxylase CH2 II C-OPO32- + HCO3- I COO- COO- I CH2 + HPO42- I C=O I COO- Phosphoenolpyruvate Oxaloacetate

  14. Could plants just use PEP-carboxylase instead of Rubisco?

  15. Principles of CO2 concentration mechanisms HCO3- CO2 Fixation/carboxylation C3 + HCO3- C3-”recycling” C3 C4 C4 transport C3 + CO2 Decarboxylation RUBISCO

  16. HCO3- CO2 Fixation/carboxylation C3 + HCO3- C3-”recycling” C3 C4 C4 transport C3 + CO2 Decarboxylation RUBISCO

  17. CO2 Concentrating Mechanisms • C4 Photosynthesis: spacial separation • Crassulacean Acid Metabolism (CAM): temporal separation

  18. The C4 carbon cycle: Spatial separation • Different Cells: Bundle Sheath cells/ Kranz anatomy • Within one cell

  19. Kranz (=Wreath) Anatomy Bundle sheath cells

  20. (V Single Cell C4 Photosynthesis Borszczowia

  21. CAM: temporal separation Minimizing water loss H20 loss/CO2 gained (g) CAM 50-100g C4 250-300g C3 400-500g

  22. CAM: Day/Night switch

  23. Topics: • Regulation of the Calvin Cycle • Photorespiration • CO2 concentrating mechanisms • Sucrose and starch synthesis

  24. UDP-Glucose

  25. Saccharide Synthesis: Overview Saccharides Pi Triose-P Glc-1-P Glc-NtDP NTP (ATP/UTP) PPi

  26. Plastids: Starch Synthesis

  27. Starch is a branched polymer Remember: Cellulose = b-D-1,4-glucosyl

  28. Regulation of starch and sucrose biosynthesis

  29. Pi Sucrose Suc-6-P UDP-Glc PPi UTP SPS Phosphate is generated in the cytosol during sucrose synthesis Glc-1-P Glc-6-P Fru-6-P ATP Pi Pi PPi ADP Fru-1,6-bisP Triose-P

  30. Balance: Starch vs Sucrose Synthesis Plastid Cytosol Sucrose Synthesis Triose-P Triose-P Pi Pi Starch Synthesis

  31. Regulation of Starch and Sucrose Synthesis Glc-1-P UDP-Glc + Fru-6-P Suc-6P SPS Pi ATP Glc-6-P Pi 3PGA 2xPi PPi Ferredoxin Red. SPS-P ADP-Glc Sucrose-P Synthase (SPS) ADP-Glc Pyrophosphorylase (AGPase)

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