Download Rubisco

Calvin Cycle
Calvin cycle cannot be called “dark
reaction” because it is still lightdependent.
Rubisco
1. Carbon fixation
Ribulose 1,5-bisphosphate
Ribulose 1,5-bisphosphate
carboxylase/oxygenase
The most abundant enzyme
on the earth (50% of
stromal protein)
CO2
Rubisco
3-phosphoglycerate
Rubisco: 8 large subunits + 8 small subunits
Large subunit is the catalytic subunit.
2. reduction
3-phosphoglycerate
ATP
1,3-bisphosphoglycerate
ADP
3-phosphoglycerate
kinase
NADPH
glyceraldehyde
Pi
NADP+
Glyceraldehyde 3-phosphate
3-phosphate
dehydrogenase
Dihydroxyacetone
Triose
phosphate
phosphate
isomerase
3-PGA
ATP
G 3-P
NADP+
GPi3-P
DH
3-PGA
ADP
1,3-BPG
kinase
NADPH
Triose
DHAP
phosphate
isomerase
TransF 1,6-BP
aldolase
starch
F 6-P
FBPase-1
Carbohydrates can both
synthesized in chloroplast and
cytosol by utilizing fixed carbon
from chloroplast.
G 3-P
DHAP
Pi-triose
phosphate
antiporter
Pi
FSucrose
F1,6-BP
6-P
DHAP
G 3-P
For cytosolic carbohydrate biosynthesis, triose
phosphates needed are transported by Pi-triose
phosphate antiporter (translocator)
Pi-Triose phosphate antiporter
This antiporter removes dihydroxyacetone
phosphate from stroma into cytosol, importing
Pi into stroma to ensure continuous supply of
inorganic phosphate for photophosphorylation
ATP synthesis.
It will also move NADPH synthesized by
photorespiration into cytosol. NADPH will be
converted to NADH during this process.
Most of the triose
phosphates
exported from
chloroplast are
dihydroxyacetone
phosphate.
On the other hand,
triose phosphates
imported into
chloroplast from
cytosol are mostly
3phosphoglycerate.
3. Regeneration of RuBP
Fructose 6-phosphate is an important branchpoint.
Cell can choose to synthesize starch or regenerate
ribulose 1,5-bisphosphate from F-6-P.
Animals do not have these following enzymes so
they can not perform photosynthesis:
Sedoheptulose 1,7-bisphosphatase
ribulose 5-phosphate kinase
rubisco
For every
triose
phosphate
synthesized, 9
ATP and 6
NADPH are
consumed.
One phosphate
from ATP is
exported with
glyceraldehyde
3-phosphate
Regulation of Carbon metabolism in
plants is more complex than animals
because of photosynthesis
1.thioredoxin system (photosystem I)
2.variation of H+ and Mg2+ concentration due
to light exposure
3.Allosteric regulation by intermediates
4.covalent modification
Covalent modification
rubisco
rubisco
Lys
CO2
RuBP
Rubisco activase
-COO
Lys -COO
Although carbamylation at lysine residue of
rubisco will activate it, ribulose 1,5bisphosphate will inhibit this carbamylation
at physiological pH.
Rubisco activase will promote the ATPdependent release of ribulose 1,5bisphosphate, exposing lysine residue for
carbamylation.
Rubisco is activated after carbamylation
activated rubisco will not be inhibited by
ribulose 1,5-bisphosphate.
Nocturnal inhibitor also regulate
photosynthesis
• Some plants
synthesize 2carboxyarabinitol 1phosphate in the dark,
which is a potent
inhibitor of
carbamolyated
rubisco.
• It will be break down
by rubisco activase or
by light.
Nocturnal inhibitor is similar to
the b-keto acid intermediate of
rubisco reaction
p. 766
Light reaction result in H+ transport into
stroma. This causes stromal pH to rise.
Mg2+ exported from thylakoid into
stroma to balance charges.
pH, [Mg2+] activate
rubisco
fructose 1,6-bisphosphatase
(FBPase-1)
Mg2+
NADPH
NADP+
H+
Calvin rubisco
cycle
G 3-P
FBPase-1
F 6-P
Light will cause disulfide bond reduction of these following
enzymes through thioredoxin system :
Ribulose 5-phosphate kinase
Fructose 1,6-bisphosphatase
Sedoheptulose 1,7-bisphosphatase
Glyceraldehyde 3-phosphate dehydrogenase
In the dark, these enzymes will slowly re-oxidize.
Thioredoxin
/\
Thioredoxin
Thioredoxin
Enzyme
S-S
Fd/ox
\
red
/\
PSI HS SH
S-S
HS
SH
Enzyme
/\
p.765
S-S
Enzyme
O2
/\
S-S
Photorespiration
•Aside from being an carboxylase,
Rubisco is also an oxygenase. The
oxygenation of ribulose 1,5-bisphosphate
produces phosphoglycolate, a
CO2
metabolically useless product.
•Although rubisco does have higher
RuBP
affinity toward CO2 (9mM; O2 is
350mM), the concentration of O2 (20%)
is much higher than CO2 (0.04%).
O2
•Rubisco requires aqueous solution of
CO2. However, the solubility of CO2
decrease abruptly at higher temperature.
•On average, one photorespiration
happens for every three
photophosphorylation.
2-phosphoglycolate
rubisco
3-PGA
Peroxisome
Chloroplast
-NH2
O2
2-phosphoRuBP
3-PGA
glycolate
glycolate
Pi
O2
glyoxylate
H2O2
glycine
Mitochondria
NAD+
NADH
NH
Serine
CO32
Glycine
ADP
3-PGA
glycerate
NAD+
ATP
hydroxypyruvate
-NH2
NADH
p.767
C4 metabolism
C4 metabolism
• Because the initial trapping of CO2 in C4
metabolism involved PEP carboxylase and
the production of oxaloacetate (a four
carbon compound), it is called C4
metabolism.
• PEP carboxylase utilizes HCO3-, which is
structurally distinct from CO2 and O2.
• Moving Calvin cycle to bundle sheath cell
will shield rubisco from any possible
exposure to oxygen.
Bundle sheath cell is located in
the center of the leaf
Crassulacean acid metabolism
(CAM)
• Succulent plants such as cactus and
pineapple grow in hot and very dry area
have evolved a different strategy for carbon
assimilation.
• They also separate the initial trapping of
CO2 from Calvin cycle like C4 plants, but
the difference between CAM and C4 is
CAM separated CO2 trapping and Calvin
cycle over time, not space.