Photosynthesis: The Carbon Reactions 207
1,5-bisphosphate (Figure 8.4). The addition of CO2 to the
unstable rubisco-bound enediol intermediate drives the
second partial reaction to the irreversible formation of
2-carboxy-3-ketoarabinitol 1,5-bisphosphate. The hydration of 2-carboxy-3-ketoarabinitol 1,5-bisphosphate yields
two molecules of 3-phosphoglycerate.
In the reduction phase of the Calvin–Benson cycle, two
successive reactions reduce the carbon of the 3-phosphoglycerate produced by the carboxylation phase (see Figure
8.3 and Table 8.1, reactions 2 and 3):
dehyde 3-phosphate, in a reaction catalyzed by the
chloroplast enzyme NADP–glyceraldehyde-3-phosphate dehydrogenase.
The operation of three carboxylation and reduction phases
yields six molecules of glyceraldehyde 3-phosphate (6
molecules × 3 carbons/molecule = 18 carbons total) when
three molecules of ribulose 1,5-bisphosphate (3 molecules
× 5 carbons/molecule = 15 carbons total) react with three
molecules of CO2 (3 carbons total) and the six molecules
of 3-phosphoglycerate are reduced (see Figure 8.3).
1. First, ATP formed by the light reactions phosphorylates 3-phosphoglycerate at the carboxyl group,
yielding a mixed anhydride, 1,3-bisphosphoglycerate, in a reaction catalyzed by 3-phosphoglycerate
kinase.
The regeneration of ribulose 1,5-bisphosphate
ensures the continuous assimilation of CO2
In the regeneration phase, the Calvin–Benson cycle facilitates the continuous uptake of atmospheric CO2 by restoring the CO2 acceptor ribulose 1,5-bisphosphate. To this
end, three molecules of ribulose 1,5-bisphosphate (3 mol-
2. Next, NADPH, also generated by the light reactions, reduces 1,3-bisphosphoglycerate to glyceral-
Condensation
Keto-enol
isomerization
Hydration/
protonation
H
C
OP
O
C
O–
C
OH
C
OH
H
C
OH
H
C
OH
H
C
OP
H
C
OP
H
C
OP
C
H
H
H+
H
H
Ribulose 1,5bisphosphate
Enediol
C
OP
H
C
OP
HO
C
C
OO–
O H 2O
H
C
OH
H
C
OH
H
C
OP
H
C
OP
COOH
Photosynthesis
H
3-Phosphoglycerate
2-Hydroperoxy-3ketoarabinitol
1,5-bisphosphate
CO2
Photorespiration
2-Phosphoglycolate
H
O2
H
COOH
H
H
H
H
H
C
OP
HO
C
COO–
C
O
H
C
OH
H
C
OP
H
C
OP
HO
C
COOH
Photosynthesis
H
3-Phosphoglycerate
H2O
H
2-Carboxy-3ketoarabinitol
1,5-bisphosphate
Figure 8.4 Carboxylation and oxygenation of ribulose
1,5-bisphosphate catalyzed by rubisco. The binding of
ribulose 1,5-bisphosphate to rubisco facilitates the formation of an enzyme-bound enediol intermediate that can be
attacked by CO2 or O2 at carbon 2. With CO2, the product
is a six-carbon intermediate (2-carboxy-3-ketoarabinitol
1,5-bisphosphate); with O2, the product is a five-carbon
reactive intermediate (2-hydroperoxy-3-ketoarabinitol
H
COOH
H
C
OH
H
C
OP
Photosynthesis
H
3-Phosphoglycerate
1,5-bisphosphate). The hydration of these intermediates
at carbon 3 triggers the cleavage of the carbon–carbon
bond between carbons 2 and 3, yielding two molecules of
3-phosphoglycerate (carboxylase activity) or one molecule
of 2-phosphoglycolate and one molecule of 3-phosphoglyerate (oxygenase activity). The important physiological
effect of the oxygenase activity is described in the section
The C2 Oxidative Photosynthetic Carbon Cycle.
© 2014 Sinauer Associates, Inc. This material cannot be copied, reproduced,
manufactured or disseminated in any form without express written permission from the publisher.