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2pp_6.Photosynthesis - 6 Photosynthesis carbon fixation by...

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Unformatted text preview: 1/13/2010 6. Photosynthesis: carbon fixation by the C.3 and (34 pathways The first product of CO2 fixation in green algae is 3-phosphoglycerate M. Calvin and coworkers exposed green algae to “CO: for short periods of time in the presence of light. They separated and identified the labeled products by paper ch romatography. boiling methanol ' 1“C appeared first in “co _ L 2 the carboxyl carbon of I 3-phosphoglycerate. H .C'OH cle2-0® J. Bassham. A. Benson & M. Calvin J. Biol. Chem. 135: 781 (1950). 3-phosphoglycerate is formed from ribulose-1,5-bi5phosphate ln another experiment. Calvin & coworkers pre-labeled all the cellular metabolites with 1“CO, in the dark. and then looked for compounds whose concentrations decreased when they turned on alight. CH -0( :) CH2-0® I 2 (|:_O H20 H-C-OH I I + cc.2 AL, coz- H-{IJ-OH ribulose bisphosphate H'C'OH carboxylaseloxygenase €01 I H ' H CH2_0® { rublsco ) “.00” l ribulose-1,5-bisphosphate CH2'0® 3-phosphoglycerate (2 molecules) Ribulose-1,S-bisphosphate can be regenerated from 3-phosphoglycerate if ATP and NADPH are provided 002- ATP ADP o=|c.o® NADPH NADP+ CH0 I I H-(|:-OH 5 4”. H-(lz-OH 5. H-{IZ-OH CH2~O® CH2-0® v CH2-0® P glyceraldehyde-s- phosphate 3-p hosphoglyce rate 1,3-bisphosphoglyce rate $H2-°® CHO C=0 I I HuC-OH —p —p H-C-Ot-l 2 I éHZ'OG) (many steps) H_C_OH | glyceraldehyde-S- ribulose-1,5- CH2'0® phosphate bisphosphate ATP 3 ADP 1/13/2010 3 A ribulose 1,5-bisphosphate ADP ATP ribulose 5-phosphate PI Stoichiometry of (:02 assimilation in the Calvin cycle 3-phosphoglycerate 5 ATP 5 ADP 1,3-bisphosphoglycerate 5 NADPH + H’ 6 NADP+ + P. glyceraldehyde 3-phosphate 6 glyceraldehyde 3-phosphate \r/ 1 glyceraldehyde 3-phosphate Regeneration of pentose phosphates by the Calvin cycle CHon fructose- 1 ,6-bis-P glyceraldehyde- 3.? (IJHZOH xylulose- 6:0 5P I won't expect you to know these reactions in detail ribose-S—P sedcheptulose-T-P 1/13/2010 Four enzymes of the Calvin cycle are activated by reduction in the light glyceraldehyde-3-phosphate dehydrogenase fructose-1,S-bisphosphatase sedoheptulose-1,T-bisphosphatase ribulose-S-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. HS SH S—S reduced ,' ' _ t. 2 imam ac XHSSHXs—s from PS I glucose-B-P dehydrogenase is switched Qfl in the light in the same manner Structures of reduced and oxidized thioredoxin reduced (1trw.pdb) oxidized (1trs.pdb) 1/13/2010 Fructose-1,B-bisphosphatase is activated by increases of pH and [M92*] in the stroma _L 0| Q When chloroplasts are illuminated, the pH in the stroma rises from -T to ~8‘ and [Mg2*] increases from ~2 mM to ~5 mM. FBPase activity (unitslmg) O 10 15 [M902] {mill} 0 Rubisco also is activated in the light, but by still another Lehnlnger Fig. 20-18 meChal'IlSlTL The carbcxylase reaction catalyzed by rubiscc proceeds through enzyme-bound, 6-carbon intermediates HO-C-COz' CH2-0® l C=O I 2-carboxy-3-keto- I " H-(II-OH fl: ' c=o arabinitol-1,5- H-C-OH H+ : H.c.0H bisphosphate I | I : CH2'0® ; CH2'°® enediolate 3 OH' .000.00000000000000.0000... bisphosphate — — CH2-0® | Ho-c-co; '0-(|3=O 3-phosphoglycerate H'C'OH on-0® 3-phosphoglycerate 1/13/2010 A carbamoylated lysine side chain forms part of the binding site for Mg2+ and (:02 in rubisco | o=c :CH-CHZCHZCHZCHZN H-002- small "it __. x‘subunit t_-' x . _ . 2—carboxy- arabinitol1,5- bisphosphate 2-carboxy-arabinitol bisphosphate, an inhibitor, is an analog of the normal product. Its carboxyl group probably occupies the CO2 binding site. ‘Ihvadb Carbamoylation of Lys 201 at the active site of rubisco is inhibited by bound ribulose-1,5-bisphosphate This process is very sensitive to the Hro® ATPIADP ratlo. In some =0 . . plants, it is stimulated "bumse DISphOSphate‘ ('3 in the light by reduction H.c.o|.| of rubisco activase. ribulose-1.5- l ' H-C-OH blsphosphate mbisco éH O® nonenzymatic activase 2' $H2'o® carbamoylation =0 ('3 ATP ADP + P, co 2 H1 902‘ 2 NH H.c.o|-| NH,‘ Nl-l3t l 4.4, H-C-OH Another enzyme (rubisco activase} catalyzes ATP- dependent removal of (I: cHz-o® —- NH-CH-CO-n —- NH-CH-CO-n ‘ -- NH-CH-CO-n rubisco with bound RBP rubisco apoenzyme carbamoylated rubisco inactive inactive active 1/13/2010 1/13/2010 Rubisco is an abundant, but relatively poor enzyme Rubisco typically accounts for more than half the protein. it’s the world's most abundant enzyme Why do plants make so much of it? it’s a relatively poor enzyme with a turnover number on the order of 5 s-1 at 20 0. Why do plants need to control its activity? Rubisco also catalyzes an oxygenase reaction that generates 2-phosphoglycolate ribulose-1.5- bisphosphate 2-phosphoglycolate '0-C=0 | H-C-OH l 3-phosphoglycerate CH2.0® Phosphoglycolate is oxidized wastefully in mitochondria and peroxisomes 02 Ema-'06) t cog, P., H20, co: Gly, Ser multiple steps No ATP! phosphoglycolate ~Half of the phosphoglycolate is salvaged as glycine or serine; the rest is oxidized to 002. 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 [OEMCOR] concentration ratio, and it increases with increasing temperature. At high temperatures, photorespiration can cause plants to waste as much as 1l3 of the coz they fix. Some plants have mechanisms to minimize photorespiration a" (:02 + H20 2 HCO3' + W mesophyuce" PI 9 oxaloacetate pyruvate- “OZC-CHzc-COZ- phosphate NADPH dikinase h “ADE” (PH 'OZC-CHZCH-COZ‘ l I NADPH NADP+ " (IJH '020£H26H-C02‘ co2 malate {2} 3-phosphoglycerate Y bundle_sheath ca" ribulose-1,5-bisphosphate 1/13/2010 Typical architectures of (:3 and (24 plants mesophyll cells bundle sheath C,s plant bundle sheath cells have cells few or no chloroplasts wwwdcu. ief~parldnsmfpholoppt bundle sheath cells have many chloroplasts Plasmodesmata connect the bundle-sheath and mesophyll cells 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 ce|| plasmodesmata / plasma mesophyll cell membrane The plasma membrane is continuous between the two cells. h .r mes-ophle cell 1/13/2010 ...
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