6.CarbonFixation - 6. Photosynthesis: carbon fixation by...

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Unformatted text preview: 6. Photosynthesis: carbon fixation by the 6. C3 and C4 pathways The first product of CO2 fixation in green algae is 3-phosphoglycerate 14 M. M. Calvin and coworkers exposed green algae to 14CO2 for short periods of time in the presence of light. They separated and identified the labeled products by paper chromatography. CO2 pump boiling methanol algae light C appeared first in the carboxyl carbon of 3-phosphoglycerate. 14 14 CO2- H-C-OH CH2-O- P J. Bassham, A. Benson & M. Calvin J. Biol. Chem. 185: 781 (1950). 3-phosphoglycerate is formed from ribulose-1,5-bisphosphate In In another experiment, Calvin & coworkers pre-labeled all the cellular metabolites with 14CO2 in the dark, and then looked for compounds whose concentrations decreased when they turned on a light. CH2-O- P C=O H-C-OH CH2-O- P H2O 2 H+ H-C-OH CO2- + CO2 H-C-OH CH2-O- P ribulose-1,5-bisphosphate ribulose bisphosphate ribulose carboxylase/oxygenase (“rubisco”) CO2H-C-OH CH2-O- P 3-phosphoglycerate (2 molecules) Ribulose-1,5-bisphosphate can be regenerated from 3-phosphoglycerate if ATP and NADPH are provided CO2- ATP H-C-OH CH2-O- P 3-phosphoglycerate CHO ADP O=C-O- P glyceraldehyde-3phosphate NADP+ CHO H-C-OH H-C-OH CH2-O- P CH2-O- P Pi 1,3-bisphosphoglycerate 3 ATP glyceraldehyde-3phosphate CH2-O- P 3 ADP C=O 3 H-C-OH 5 H-C-OH CH2-O- P NADPH (many steps) H-C-OH ribulose-1,5bisphosphate CH2-O- P 2 Pi 3 Stoichiometry of CO2 assimilation in the Calvin cycle CO2 + H2O 3 ribulose 1,5-bisphosphate 3 ADP 3 ATP 3 6 ribulose 5-phosphate 3-phosphoglycerate 6 ATP 6 6 ADP 1,3-bisphosphoglycerate 6 2 Pi 5 glyceraldehyde 3-phosphate 6 1 6 NADPH + H+ NADP+ + Pi glyceraldehyde 3-phosphate glyceraldehyde 3-phosphate Regeneration of pentose phosphates by the Calvin cycle CH2OH C=O CH2O- P CHO CH2O- P Pi CHO CH2OH CH2O- P CH2O- P erythrose-4-P glyceraldehyde-3-P xylulose-5-P CH2OH C=O CHO CH2O- P C=O xylulose-5-P CH2O- P fructose-1,6-bis-P CH2O- P I won’t expect you to know these reactions in detail CH2OH C=O CH2O- P C=O CHO CH2OH C=O CH2O- P CH2O- P CHO CH2O- P CH2OH C=O CH2O- P C=O Pi CH2O- P ribose-5-P CH2O- P sedoheptulose-7-P CH2O- P Four enzymes of the Calvin cycle are activated by reduction in the light glyceraldehyde-3-phosphate dehydrogenase fructose-1,6-bisphosphatase sedoheptulose-1,7-bisphosphatase ribulose-5-phosphate kinase Activation occurs on reduction of a disulfide bond between two Cys residues. The reductant is thioredoxin, a small, soluble protein that is reduced by ferredoxin. 2 2e - from PS I reduced ferredoxin 2 oxidized ferredoxin SS HS SH thioredoxin enzyme HS SH SS thioredoxin enzyme active inactive glucose-6-P dehydrogenase is switched off in the light in the same manner Structures of reduced and oxidized thioredoxin C32 C35 reduced (1trw.pdb) C32 C35 oxidized (1trs.pdb) FBPase activity (units/mg) Fructose-1,6-bisphosphatase is activated by increases of pH and [Mg2+] in the stroma 150 When chloroplasts are illuminated, the pH in the stroma rises from ~7 to ~8, and [Mg2+] increases from ~2 mM to ~5 mM. pH 8.0 100 pH 7.5 50 pH 7.0 0 0 Lehninger Fig. 20-18 5 10 15 [MgCl2] (mM) 20 Rubisco also is activated in the light, but by still another mechanism. The carboxylase reaction catalyzed by rubisco proceeds through enzyme-bound, 6-carbon intermediates CH2-O- P C=O O CH2-O- P C-OC-OH H-C-OH H-C-OH CH2-O- P H+ 3-phosphoglycerate enediolate CH2-O- P H - O-C=O H-C-OH 3-phosphoglycerate HO-C-CO2C=O CH2-O- P 2-carboxy-3-ketoarabinitol-1,5bisphosphate CH2-O- P H-C-OH H+ H2 O CH2-O- P HO-C-CO2HO-C-O - HO-C-CO2- + CH2-O- P H-C-OH CH2-O- P CH2-O- P CO2- O H-C-OH ribulose-1,5bisphosphate HO-C-H C CH2-O- P H+ A carbamoylated lysine side chain forms part of the binding site for Mg2+ and CO2 in rubisco O=C HN CH-CH2CH2CH2CH2NH-CO2- Lys 201 small subunit Asp 203 Mg Glu 204 2-carboxyarabinitol 1,5bisphosphate 2-carboxy-arabinitol bisphosphate, an inhibitor, is an analog of the normal product. Its carboxyl group probably occupies the CO2 binding site. large subunit 1bwv.pdb Carbamoylation of Lys 201 at the active site of rubisco is inhibited by bound ribulose-1,5-bisphosphate Another enzyme (rubisco activase) catalyzes ATPdependent removal of ribulose bisphosphate. CH2-O- P C=O H-C-OH ribulose-1,5bisphosphate rubisco activase CH2-O- P C=O ATP H-C-OH CH2-O- P ADP + Pi NH3+ H-C-OH This process is very sensitive to the ATP/ADP ratio. In some plants, it is stimulated in the light by reduction of rubisco activase. NH3+ nonenzymatic carbamoylation CO2 2H + CO2- NH H-C-OH CH2-O- P --- NH-CH-CO--- --- NH-CH-CO--- --- NH-CH-CO--- rubisco with bound RBP rubisco apoenzyme carbamoylated rubisco inactive inactive active Rubisco is an abundant, but relatively poor enzyme Rubisco typically accounts for more than half the protein in a leaf. It’s the world’s most abundant enzyme and probably the most abundant protein. Why do plants make so much of it? It’s a relatively sluggish enzyme, with a turnover number on the order of 5 s-1 at 20 C. Why do plants need to control its activity? Rubisco also catalyzes an oxygenase reaction that generates 2-phosphoglycolate CH2-O- P CH2-O- P C-OC-OH C=O H-C-OH H-C-OH CH2-O- P H+ O O CH2-O- P HO-C-O-OC=O H-C-OH H-C-OH CH2-O- P CH2-O- P ribulose-1,5bisphosphate CH2-O- P O=C-O 2-phosphoglycolate - O-C=O H-C-OH 3-phosphoglycerate CH2-O- P H2O OH- Phosphoglycolate is oxidized wastefully in mitochondria and peroxisomes O2 CH2-O- P CO2phosphoglycolate CO2, Pi, H2O, Gly, Ser multiple steps No ATP! ~Half ~Half of the phosphoglycolate is salvaged as glycine or serine; the rest is oxidized to CO2. This This “photorespiration” occurs during illumination, because that’s when ATP and NADPH are available for synthesis of ribulose bisphosphate. The relative rate of the oxygenase reaction leading to phosphoglycolate depends on the [O2]/[CO2] concentration ratio, and it increases with increasing temperature. At high temperatures, photorespiration can cause plants to waste as much as 1/3 of the CO2 they fix. Some plants have mechanisms to minimize photorespiration HCO3- + H+ CO2 + H2O air O- P CH2=C-CO2- - CH3-C-CO2 O oxaloacetate O2C-CH2C-CO2phosphoenolpyruvate NADPH pyruvatephosphate dikinase O Pi PEP mesophyll mesophyll cell AMP + PPi - carboxylase OH ATP + Pi pyruvate O - NADPH CH3-C-CO2(2) 3-phosphoglycerate bundle-sheath cell CO2 O2C-CH2CH-CO2- NADP+ - NADP+ OH malate O2C-CH2CH-CO2- malic enzyme ribulose-1,5-bisphosphate Typical architectures of C3 and C4 plants C3 plant bundle sheath cells have few or no chloroplasts www.dcu.ie/~parkinsm/photo.ppt mesophyll cells bundle sheath cells C4 plant bundle sheath cells have many chloroplasts Plasmodesmata connect the bundle-sheath and mesophyll cells Plasmodesmata Plasmodesmata are channels that allow water, nutrients and some other molecules to move between plant cells. They contain an actin helix surrounding a tubular membrane connected to the ER. bundle sheath cell Lehninger Fig. 20-23 plasmodesmata cell wall mesophyll cell bundle sheath cell plasma membrane mesophyll cell The plasma membrane is The continuous between the two cells. Cyanobacteria concentrate HCO3- with rubisco and carbonic anhydrase in icosahedral protein capsules in the thylakoid lumen H2O CO2 carbonic anhydrase HCO3bicarbonate transporter in plasma membrane HCO3- HCO3- rubisco carboxysome M.R. Badger & G.D.Price (2003) J. Exptl. Botany 54, 609. T.O. Yeats et al. (2008) Nature Rev. Microbiol. 6, 681. ...
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This note was uploaded on 02/17/2012 for the course CHEM 212 taught by Professor Staff during the Fall '10 term at Rutgers.

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