Calvin Cycle
Calvin cycle cannot be called “dark
reaction” because it is still lightdependent.
Rubisco
(1) Carbon fixation
Ribulose 1,5-bisphosphate + CO2 +
2(3-phosphoglycerate)
Catalyzed by rubisco (ribulose 1,5-bisphosphate
carboxylase;
RuBP carboxylase/oxygenase)
Rubisco is the most abundant enzyme on the
earth
In stroma
50% of chloroplast protein is rubisco.
One of the reasons why animals cannot
photosynthesize because it lacks rubisco.
Rubisco: 8 large subunits + 8 small subunits
Large subunit is the catalytic subunit.
(2). 3-phosphoglycerate  glyceraldehyde 3phosphate
(2). 3-phosphoglycerate  glyceraldehyde 3phosphate包括了兩個步驟（為glycolysis的相反）
3-phosphoglycerate
3-phosphoglycerate kinase
1,3-bisphosphoglycerate
Glyceraldehyde 3-phosphate
dehydrogenase
Glyceraldehyde 3-phosphate
Triose phosphate isomerase
Dihydroxyacetone phosphate
Carbohydrates can both
synthesized in chloroplast and
cytosol by utilizing fixed carbon
from chloroplast.
For cytosolic carbohydrate biosynthesis, triose
phosphates needed are transported by Pi-triose
phosphate antiporter (translocator)
Most of the triose
phosphates
exported from
chloroplast are
dihydroxyactone
phosphate.
On the other hand,
triose phosphates
imported into
chloroplast from
cytosol are mostly
3phosphoglycerate.
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.
(3) Regeneration of ribulose 1,5bisphosphate from triose phosphate
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
Regulation of rubisco: carbamylation of lysine residue
Covalent modification
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
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)
pH, [Mg2+] 
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
Photorespiration
•Aside from being an carboxylase,
Rubisco is also an oxygenase. The
oxygenation of ribulose 1,5-bisphosphate
produces phosphoglycolate, a
metabolically useless product.
•Although rubisco does have higher
affinity toward CO2 (9mM; O2 is
350mM), the concentration of O2 (20%)
is much higher than CO2 (0.04%).
•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.
Phosphoglycolate is
dephosphorylated in
stroma.
Similar to PDH and
a-ketoglutarate
dehydrogenase
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.