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Chapter 15 (part 3)

Chapter 15 (part 3). Carbon Fixation (dark reactions). Carbon Dioxide Fixation. A unique ability of plants, algae, etc. Melvin Calvin at Berkeley in 1945 showed that Chlorella could take up 14 CO 2 and produce 3-phosphoglycerate

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Chapter 15 (part 3)

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  1. Chapter 15 (part 3) Carbon Fixation (dark reactions)

  2. Carbon Dioxide Fixation • A unique ability of plants, algae, etc. • Melvin Calvin at Berkeley in 1945 showed that Chlorella could take up 14CO2 and produce 3-phosphoglycerate • What was actually happening was that CO2 was combining with a 5-C sugar to form a 6-C intermediate • This breaks down to two 3-P glycerates

  3. Reductive Pentose Phosphate Cycle 6CO2+9ATP+5H20  9ADP+8Pi+6NADP++(DHAP or G3P)

  4. Ribulose-1,5-Bisphosphate carboxylase/oxygenase (rubbisco) • Probably the world's most abundant protein • In leaves greater than 50% of the soluble protein is rubisco (stromal conc. 4 mM) • Rate Limiting step in RPP cycle • Rubisco is a slow enzyme (turnover number is 3 rxn per second) • Composed of 8 large subunits (LSU) (56,000 dal) and 8 small subunits (SSU) (14,000 dal). Active sites assocaited with LSU. • LSU encoded by chloroplast genome. SSU encoded by nuclear genome.

  5. Activation of Rubisco • Rubisco cycles between active and inactive form. • Active form requires a bound Mg2+ ion, light and high pH. • A none substrate CO2 molecule participates in Mg2+ binding to active site. • CO2 molecule binds reversibly to lysine residue forming carbamate adduct • Activation facilitated by the enzyme rubisco activase. • In the dark, carbamate adduct disassociates from active site. R 1,5-BP then binds tightly to active site and inhibits enzyme

  6. Mg2+ plays role in binding and activating R 1,6-BP to accept CO2

  7. Rubisco Rxn Mechanisms carboxylase oxygenase

  8. Reductive Pentose Phosphate Cycle

  9. Reduction Stage cytosol • Conversion of 3-phosphoglycerate to glucose is very similar to gluconeogenesis, but glyceraldehyde dehydrogenase uses NADPH not NADH. • Steps require consumption of ATP and NADPH. • 3-phosphoglycerate could also be exported to cytsol and be used in normal gluconeogenesis. • Hexoses can then be used for energy or starch synthesis F 1,6-bisphophatase aldolase Glyceraldehyde dehydrogenase Phosphoglycerate kinase

  10. Reductive Pentose Phosphate Cycle

  11. Regeneration Step • Need to regenerate ribulose 1,5-phosphate for subsequent rubisco reactions • One of the two 3-phosphoglyserates goes towards regeneration. • Need to generate 5 carbon sugar from 3 carbon and 6 carbon sugars. • Most expensive part of RPP cycle.

  12. Transketolases and Aldolases are used to make 5 carbon sugars

  13. Formation of 5 Carbon Sugars

  14. F-6-P + 2 G3P + DHAP + 3 ATP  3 R-1,5-BP + 3 ADP

  15. Regulation of RPP Cycle • Rubisco activity is regulated by pH Mg2+ • Other enzymes regulated by redox state of chloroplast • All factors are influenced by light

  16. Thioredoxin • 12 kD protein • Contains Cysteine residue that can cycle between reduced –SH and oxidized –S-S-. • Reduced thioredoxin can activate enzymes by reducing disulfides in regulatory domains.

  17. thioreodxin ties light rxns to RPP cycle regulation • In light Thioredoxin is reduced. • Reduced thioredoxin activates RPP cycle enzymes. • “dark Rxns” don’t really function well in the dark.

  18. Oxygenase Activity of Rubisco • CO2 and O2 compete for binding at active site. • Under normal conditions the rate of carboxylation is 3 to 4 times the rate of oxygenation. • Both require activation by carbamate adduct (therefore no oxygenation w/o CO2) • Oxygenase activity produces 3-phosphoglycerate (normal C3 product) and 2-phosphoglycolate (C2 product)

  19. Photorespiration (recycling of 2-phosphoglycolate) • 4 of five carbons from R 1,5-BP salvaged. • Loose one carbon as CO2 • Because O2 consumed and CO2 released the process is called photorespiration • Wasteful process, loose carbon as CO2 w/o producing ATP or NADH • Biochemist have been trying to engineer better rubisco (no luck)

  20. Mechanisms to Avoid Photorespiration • C4 Photosynthesis – Spatial separation of carbon fixation and carbon utilization • CAM (Crassulacean Acid Metabolism) Photosynthesis – temporal separation of carbon fixation and carbon utilization

  21. C4 Photosynthesis • C4 cycle is way to pump CO2 into bundle sheath cells making concentration 20 fold higher than in mesophyll cells. • Important in plants from hot climates. • Under elevated temperature rubisco favors oxygenase function causing plants to undergoe photorespiration. • By fixing CO2 in Ms Cells with PEP carboxylase and transferring it to the Bs Cells as a 4 carbon sugar can concentrate CO2 and prevent photorepsiration.

  22. CAM Photosynthesis • Found in succulent plants (Crassulacea). • Drought tolerant plants. • Gas exchange occurs by opening pores called stoma • What to import CO2 without loosing water through stoma. • CAM plants open stoma at night to fix CO2, • They then store it until daytime when it is release it to rubisco stoma

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