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

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20. 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 A. Benson 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
14C appeared first in the carboxyl carbon of
3-phosphoglycerate.
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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
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Ribulose-1,5-bisphosphate can be regenerated from
3-phosphoglycerate if ATP and NADPH are provided
NADPH NADP
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
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.
glucose-6-P dehydrogenase is switched off in the
light in the same manner
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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
2-carboxy-arabinitol bisphosphate, an inhibitor,
is an analog of the normal product. Its carboxyl
group probably occupies the CO2 binding site.
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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.
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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
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
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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.