CHAPTER 20 Carbohydrate Biosynthesis in Plants

1. CHAPTER 20Carbohydrate Biosynthesis in Plants 20.1 Photosynthetic Carbohydrate Synthesis • - Plastids • - CO2 => CH2O (carbohydrates) 3 stages Carbon-fixation CO2 + ribulose 1,5 bisphosphate => 2 3-phosphoglycerate 3-PG => triose phosphates Pentose Phosphate Pathway 20.2 Photorespiration and the C4 and CAM Pathways 20.3 Biosynthesis of Starch and Sucrose 20.4 Synthesis of Cell Wall Polysaccharides: 20.5 Integration of Carbohydrate Metabolism in the Plant Cell

2. Introduction to Anabolic Pathways • The previous chapters were mainly concerned with catabolism: how to extract energy from biomolecules • This and the following chapters are concerned with anabolism: how to build biomolecules • Plants are extremely versatile in biosynthesis • Can build organic compounds from CO2 • Can use energy of sunlight to support biosynthesis • Can adopt to a variety of environmental situations

3. Assimilation of CO2 by Plants

4. CO2 Assimilation Occurs in Plastids • Organelles found in plants and algae • Enclosed by a double membrane • Have their own small genome • The inner membrane is impermeable to ions such as H+, and to polar and charged molecules

5. Origin and Differentiation of Plastids • Plastids were acquired during evolution by early eukaryotes via endosymbiosis of photosynthetic cyanobacteria • Plastids reproduce asexually via binary fission • The undifferentiated protoplastids in plants can differentiate into several types, each with a distinct function • Chloroplasts for photosynthesis • Amyloplasts for starch storage • Chromoplasts for pigment storage • Elaioplasts for lipid storage • Proteinoplasts for protein storage

6. CO2 Assimilation • The assimilation of carbon dioxide occurs in the stroma of chloroplasts via a cyclic process known as the Calvin cycle • The key intermediate, ribulose 1,5-bisphosphate is constantly regenerated using energy of ATP • The key enzyme, ribulose 1,5-bisphosphate carboxylase / oxygenase (Rubisco), is probably the most abundant protein on Earth • The net result is the reduction of CO2 with NADPH that was generated in the light reactions of photosynthesis

7. The Calvin Cycle • CO2 fixation • Net reaction 3CO2 + 9ATP + 6NADPH –> GAP + 9ADP + 6Pi + 6NADP+ 3 5C sugars + 3 CO2 –> 6 3C sugars 5 3C sugars –> 3 5C sugars

8. Rubisco (ribulose 1,5-bisphosphate carboxylase) • Rubisco is a large Mg++-containing enzyme that makes a new carbon-carbon bond using CO2 as a substrate • Inefficient (kcat~ 3s-1) • Ru1,5P2 + CO2 –> 2 3-phosphoglycerate (3PG)

9. Synthesis of Glyceraldehyde-3 Phosphate (First Stage) • Three rounds of the Calvin cycle fixes three CO2 molecules and produces one molecule of 3-phosphoglycerate

10. Glyceraldehyde 3-phosphate • Converted to starch in the chloroplast • Converted to sucrose for export • Recycled to ribulose 1,5-bisphosphate

11. Stage 3: C3, C4, C5, C7 Rxns • Isomerization • Phosphatase/Kinase • Carbon backbone rearrangements • Regenerates ribulose 1,5-bisphosphate

12. Calvin Cycle - Isomerization reactions • Triose phosphate isomerase • GAP <=> DHAP • Ribose phosphate isomerase • R5P <=> Ru5P • Phosphopentose epimerase • Xu5P <=> Ru5P

13. Calvin Cycle - CC bond rearrangements • Aldolase • GAP + DHAP —> F1,6P2 • E4P + DHAP —> S1,7P2 • Transketolase • F6P + GAP —> E4P + Xu5P • S7P + GAP —> R5P + Xu5P

14. Calvin Cycle - Phosphatase reactions • Fructose bisphosphatase • F1,6P2 —> F6P + Pi • Sedoheptulose bisphosphatase • S1,7P2 —> S7P + Pi

17. Stoichiometry and Energetics • Fixation of three CO2 molecules yields one glyceraldehyde 3-phosphate • Nine ATP molecules and six NADPH molecules are consumed

19. Photosynthesis: From Light and CO2 to Glyceraldehyde 3-phosphate • The photosynthesis of one molecule of glyceraldehyde 3-phosphate requires the capture of roughly 24 photons

20. Enzymes in the Calvin Cycle are Regulated by Light • Target enzymes are active when reduced • ribulose 5-phosphate kinase, • fructose 1,6-bisphosphatase, • seduloheptose 1,7-bisphosphatase, and • glyceraldehyde 3-phosphate dehydrogenase

21. Photorespiration • So far, we saw that plants oxidize water to O2 and reduce CO2 to carbohydrates during the photosynthesis • Plants also have mitochondria where usual respiration with consumption of O2 occurs in the dark • In addition, a wasteful side reaction catalyzed by Rubisco occurs in mitochondria • This reaction consumes oxygen and is called photorespiration; unlike mitochondrial respiration, this process does not yield energy

22. Oxygenase Activity of Rubisco • The reactive nucleophile in the Rubisco reaction is the electron-rich enediol form of ribulose 1,5-bisphosphate • The active site meant for CO2also accommodates O2 • Mg++ also stabilizes the hydroperoxy anion that forms by electron transfer from the enediol to oxygen

23. Rubisco in C3 Plants Cannot Avoid Oxygen • Plants that assimilate dissolved CO2 in themesophyll of the leaf into three-carbon 3-phosphoglycerate are called the C3 plants • Our atmosphere contains about 21% of oxygen and 0.038% of carbon dioxide • The dissolved concentrations in pure water are about 260 M O2 and 11 M CO2(at the equilibrium and room temperature) • The Km of Rubisco for oxygen is about 350 M

24. C4 plants concentrate CO2 • Many tropical plants avoid wasteful photorespiration by a physical separation of CO2 capture and Rubisco activity • CO2 is captured into oxaloacetate (C4) in mesophyll cells • CO2 is transported to bundle-sheath cells where Rubisco is located • The local concentration of CO2 in bundle-sheath cells is much higher than the concentration of O2

25. Conversion of stored fatty acids to sucrose in germinating seeds

27. Chapter 20: Summary In this chapter, we learned that: • ATP and NADPH from light reactions are needed in order to assimilate CO2 into carbohydrates • Assimilations of three CO2 molecules via the Calvin cycle leads to the formation of one molecule of 3-phosphoglycerate • 3-Phosphoglycerate is a precursor for the synthesis of larger carbohydrates such as fructose and starch • The key enzyme of the Calvin cycle, Rubisco, fixes carbon dioxide into carbohydrates • Low selectivity of Rubisco causes a wasteful incorporation of molecular oxygen in C3 plants; this is avoided in C4 plants by increasing the concentration of CO2 near Rubisco