6. Photosynthesis: carbon fixation by the C3 and C4 pathways

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6. Photosynthesis carbon fixation by the C3 and
C4 pathways
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The first product of CO2 fixation in green algae
is 3-phosphoglycerate
14CO2
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.
pump
boiling methanol
algae
light
J. Bassham, A. Benson M. Calvin J. Biol. Chem.
185 781 (1950).
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3-phosphoglycerate is formed from
ribulose-1,5-bisphosphate
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.
CO2
ribulose bisphosphate carboxylase/oxygenase
(rubisco)
ribulose-1,5-bisphosphate
3-phosphoglycerate (2 molecules)
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Ribulose-1,5-bisphosphate can be regenerated from
3-phosphoglycerate if ATP and NADPH are provided
NADPH NADP
ATP ADP
Pi
glyceraldehyde-3-phosphate
3-phosphoglycerate
1,3-bisphosphoglycerate
3 ATP 3 ADP
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(many steps)
ribulose-1,5-bisphosphate
glyceraldehyde-3-phosphate
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Stoichiometry of CO2 assimilation in the Calvin
cycle
ATP
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Regeneration of pentose phosphates by the Calvin
cycle
xylulose-5-P
Pi
fructose-1,6-bis-P
erythrose-4-P
glyceraldehyde-3-P
xylulose-5-P
I wont expect you to know these reactions in
detail
ribose-5-P
sedoheptulose-7-P
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Four enzymes of the Calvin cycle are activated by
reduction in the light
glyceraldehyde-3-phosphate dehydrogenase fructose-
1,6-bisphosphatase sedoheptulose-1,7-bisphosphatas
e 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.
reduced ferredoxin
oxidized ferredoxin
glucose-6-P dehydrogenase is switched off in the
light in the same manner
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Structures of reduced and oxidized thioredoxin
C32
C32
C35
C35
reduced (1trw.pdb)
oxidized (1trs.pdb)
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Fructose-1,6-bisphosphatase is activated by
increases of pH and Mg2 in the stroma
When chloroplasts are illuminated, the pH in the
stroma rises from 7 to 8, and Mg2 increases
from 2 mM to 5 mM.
Rubisco also is activated in the light, but by
still another mechanism.
Lehninger Fig. 20-18
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The carboxylase reaction catalyzed by rubisco
proceeds through enzyme-bound, 6-carbon
intermediates
2-carboxy-3-keto-arabinitol-1,5-bisphosphate
enediolate
ribulose-1,5-bisphosphate
H
H
3-phosphoglycerate
3-phosphoglycerate
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A carbamoylated lysine side chain forms part of
the binding site for Mg2 and CO2 in rubisco
small subunit
large subunit
2-carboxy-arabinitol bisphosphate, an inhibitor,
is an analog of the normal product. Its carboxyl
group probably occupies the CO2 binding site.
1bwv.pdb
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Carbamoylation of Lys 201 at the active site of
rubisco is inhibited by bound ribulose-1,5-bisphos
phate
This process is very sensitive to the ATP/ADP
ratio. In some plants, it is stimulated in the
light by reduction of rubisco activase.
Another enzyme (rubisco activase) catalyzes
ATP-dependent removal of ribulose bisphosphate.
ribulose-1,5-bisphosphate
rubisco activase
nonenzymatic carbamoylation
ATP ADP Pi
CO2
2 H
rubisco with bound RBP
rubisco apoenzyme
carbamoylated rubisco
active
inactive
inactive
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Rubisco is an abundant, but relatively poor enzyme
Rubisco typically accounts for more than half the
protein in a leaf. Its the worlds most
abundant enzyme and probably the most abundant
protein. Why do plants make so much of it? Its
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?
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Rubisco also catalyzes an oxygenase reaction that
generates 2-phosphoglycolate
OH-
ribulose-1,5-bisphosphate
2-phosphoglycolate
H2O
3-phosphoglycerate
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Phosphoglycolate is oxidized wastefully in
mitochondria and peroxisomes
Half of the phosphoglycolate is salvaged as
glycine or serine the rest is oxidized to CO2.
This photorespiration occurs during
illumination, because thats 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.
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Some plants have mechanisms to minimize
photorespiration
phosphoenolpyruvate carboxylase
malate
malic enzyme
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Typical architectures of C3 and C4 plants
mesophyll cells
C4 plant
C3 plant
bundle sheath cells
bundle sheath cells have few or no chloroplasts
bundle sheath cells have many chloroplasts
www.dcu.ie/parkinsm/photo.ppt
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Plasmodesmata connect the bundle-sheath and
mesophyll cells
Lehninger Fig. 20-23
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.
The plasma membrane is continuous between the two
cells.
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Cyanobacteria concentrate HCO3- with rubisco and
carbonic anhydrase in icosahedral protein
capsules in the thylakoid lumen
bicarbonate transporter in plasma membrane
M.R. Badger G.D.Price (2003) J. Exptl. Botany
54, 609. T.O. Yeats et al. (2008) Nature Rev.
Microbiol. 6, 681.