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Photosynthetic Carbon
Metabolism
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Article Contents
. C3 Photosynthesis (Calvin Cycle)
Archie R Portis, Jr, University of Illinois, Urbana, Illinois, USA
. Regulation
. C4 Photosynthesis
Photosynthetic carbon metabolism encompasses the processes by which plants and
other photosynthetic organisms use the energy captured from light in the form of NADPH
and ATP to convert atmospheric carbon dioxide into carbohydrates. Carbohydrates
produced by photosynthesis form the base for the nutrition of all other life as well as serving
as the starting materials for fibres, oils and many other natural compounds used by people.
C3 Photosynthesis (Calvin Cycle)
Although studied for over a century, the details of how
CO2 was converted into carbohydrates were not elucidated
until the 1950s. The availability of radioisotopes (14CO2)
and two-dimensional paper chromatographic techniques
allowed Melvin Calvin and co-workers to map out a cyclic
pathway involving a previously unknown pentose bisphosphate, ribulose 1,5-bisphosphate (Bassham, 1962). The
first labelled product is a three-carbon acid and therefore
the pathway is usually referred to as C3 photosynthesis to
distinguish it from adaptations involving additional
reactions (C4 and CAM photosynthesis) to be described
later. However, it is often called either the Calvin cycle to
honour Calvin, who received a Nobel prize in 1961 for the
work, or the reductive pentose phosphate cycle to
distinguish it from the oxidative pentose phosphate pathway, which uses many of the same enzymes.
The Calvin cycle involves several metabolic intermediates and consists of a complicated series of enzymatic
reactions that occur within the chloroplasts, although
some enzymes have isoforms in the cytosol. It is often
schematically simplified by consideration of only three
stages: (1) carboxylation of the five-carbon sugar ribulose
1,5-bisphosphate with CO2 to form two molecules of 3phosphoglycerate; (2) reduction of the 3-phosphoglycerate
to triose phosphate using ATP and NADPH generated
through light capture by the thylakoid membranes; (3)
regeneration of the starting 5-carbon sugar from triose
phosphate (also requiring ATP at the final step). It is easier
to grasp the cyclic nature of the process if we start with
three CO2 molecules, which are converted into one triose
phosphate with the following stoichiometry:
3 CO2 þ 9 ATP4 þ 6 NADPH þ 5 H2 O
3
! triose phosphate ðC3 H5 O3 PO2
3 Þ þ 9ADP
þ
2
þ
þ6 NADP þ 8 HOPO3 þ 3 H
The key step in C3 photosynthesis is the actual carboxylation reaction (Figure 1, Stage 1) catalysed by the enzyme
ribulose 1,5-bisphosphate carboxylase/oxygenase (often
called Rubisco) (E1), in which each of the three CO2
. CAM Plants
molecules are combined with one ribulose bisphosphate to
form a total of six molecules of 3-phosphoglycerate. The
properties of this enzyme define many of the ways that
plants have evolved to respond to their environment. For
example, Rubisco is not very efficient in catalysing the
carboxylation reaction because it has a lower maximum
activity than most enzymes and the concentration of
carbon dioxide in the atmosphere is less than that required
for even half-maximal activity. Plants compensate by
making a large amount of this one enzyme (up to 50% of
their total soluble protein). A major problem that plants
must contend with is the fact that Rubisco also allows
Calvin Cycle
Stage 2
Stage 1
(6)
3P-glycerate
(3) CO2
(6) ATP
(6) ADP
E2
E1
(6) 1,3-BP-glycerate (6) NADPH
(6) NADP
E3
& Pi
Triose P
(3) Ribulose 1,5-BP
(3) ADP
E11
(5) Glyceraldehyde 3-P
(3) ATP
(3) Ribulose 5-P
E10
E9
E4
Ribose 5-P
(2) Dihydroxyacetone P
(2) Xylulose 5-P
E5
Fructose 1,6-BP
Stage 3
E7
Sedoheptulose 7-P
Pi
E6
Pi
Fructose 6-P
E8
Sedoheptulose 1,7-BP
E5
Erythrose 4-P
E7
Xylulose 5-P
Figure 1 Intermediates and enzymatic steps of C3 photosynthesis (Calvin
cycle). The various enzymes are described in the text and have the
following EC numbers E1, 4.1.1.39; E2, 2.7.2.3; E3, 1.2.1.13; E4, 5.3.1.1;
E5, 4.1.2.13; E6, 3.1.3.11; E7, 2.2.1.1; E8, 3.1.3.37; E9, 5.1.31; E10,
5.3.1.6; E11, 2.7.1.19. Light-regulated enzymes are highlighted in red.
Green arrow, Stage 1; blue arrows, Stage 2; grey arrows, Stage 3.
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1
Photosynthetic Carbon Metabolism
oxygen to react with ribulose bisphosphate, producing the
two-carbon acid phosphoglycolate, in addition to 3phosphoglycerate. The phosphoglycolate product is recycled back into 3-phosphoglycerate in a process called
photorespiration (Ogren, 1984). In this process one CO2 is
released for every two phosphoglycolate molecules converted. Therefore, the reaction of ribulose bisphosphate
with oxygen rather than CO2 and the release of CO2 during
photorespiration reduce the productivity of C3 plants
under normal atmospheric conditions (21% O2 and
0.036% CO2). The oxygenase reaction increases faster
with temperature than does the carboxylase, but at
moderate temperatures about one O2 reacts for every three
to four CO2 molecules captured.
Reduction of six 3-phosphoglycerate molecules to triose
phosphate (Figure 1, Stage 2) requires three enzymes.
Phosphoglycerate kinase (E2) converts the 3-phosphoglycerate to 1,3-bisphosphoglycerate by phosphorylation
with six ATP. Then glyceraldehyde phosphate dehydrogenase (E3) converts the six 1,3-bisphosphoglycerate
molecules to glyceraldehyde 3-phosphate and inorganic
phosphate by reduction with six NADPH. Triose phosphate is a collective term for glyceraldehyde 3-phosphate
and dihydroxyacetone phosphate, which are rapidly interconverted by the enzyme triose phosphate isomerase (E4).
Regeneration (Figure 1, Stage 3) of the three original
ribulose bisphosphates requires only five triose phosphates, leaving one as the net product of the three initial
carboxylation reactions. The regeneration process requires
seven enzymes (E5 to E11) and proceeds in the following
manner. Two of the triose phosphates (one glyceraldehyde
3-phosphate and one dihydroxyacetone phosphate) are
joined by aldolase (E5) to form fructose 1,6-bisphosphate.
Fructose 1,6-bisphosphate is irreversibly hydrolysed to
fructose 6-phosphate and inorganic phosphate by fructose
1,6-bisphosphatase (E6). The enzyme transketolase (E7)
transfers a two-carbon group from fructose 6-phosphate to
the third triose phosphate (as glyceraldehyde 3-phosphate)
to form erythrose 4-phosphate and xylulose 5-phosphate.
Aldolase (E5) is also able to join the erthyrose 4-phosphate
with the fourth triose phosphate (as dihydroxyacetone
phosphate) to form sedoheptulose 1,7-bisphosphate,
which is irreversibly hydrolysed to sedoheptulose 7phosphate and inorganic phosphate by sedoheptulose
1,7-bisphosphatase (E8). The enzyme transketolase (E7)
can also transfer a two-carbon group from sedoheptulose
7-phosphate to the fifth and last triose phosphate (as
dihydroxyacetone phosphate) to form ribose 5-phosphate
and xylulose 5-phosphate. The three pentose phosphates
are then converted to ribulose 5-phosphate. The two
xylulose 5-phosphates are converted with ribulose phosphate epimerase (E9) and the one ribose 5-phosphate is
converted with ribose phosphate isomerase (E10). Finally
the initial three ribulose 1,5-bisphosphate molecules are
regenerated irreversibly by phosphorylation with ATP, as
catalysed by phosphoribulose kinase (E11).
2
The cyclic way in which CO2 is converted into triose
phosphate means that the cycle is autocatalytic and plants
are able to reach steady-state rates of photosynthesis after
only a short delay while the intermediates rise to the
appropriate levels. After this, the excess triose phosphate is
available for conversion into the two major end products of
photosynthesis. One pathway leads to the accumulation
during the day of starch (a glucose polymer) inside the
chloroplast. The other pathway involves export of the
triose phosphate from the chloroplast in exchange for
inorganic phosphate. Further enzymatic reactions occurring in the cytosol convert the triose phosphate into sucrose
(formed by joining glucose and fructose). In contrast to the
case in animals, sucrose rather than glucose is the major
carbohydrate transported in most plants, and starch rather
than glycogen is the major storage form. Sucrose is largely
exported from the leaf to supply energy to the roots,
developing leaves, reproductive organs and other heterotrophic tissues, but it can also be accumulated in the
vacuole during the day. Starch accumulation in the
chloroplast and sucrose storage in the vacuole allow
export to continue from the leaves during the night when
photosynthesis is not possible. However, in addition to
conversion into starch and sucrose, a significant portion of
the triose phosphate is readily converted within the
chloroplasts to amino acids and fatty acids.
Regulation
Regulation of photosynthetic carbon metabolism is
required so that key enzymes function only during the
light, when energy is available. This allows other enzymes
to participate in catabolic pathways during the night.
Regulation also allows the rate of metabolite movement
through the numerous steps of the Calvin cycle to be tightly
coupled with the rate of ATP and NADPH production by
photosynthetic electron transport in the thylakoid membranes. This reduces fluctuations in metabolites or oscillations within the cycle that might otherwise occur when
environmental conditions change. The primary signal for
the presence of ‘light’ is the availability of reducing
equivalents from photosynthetic electron transport. A
minor portion is diverted to a small protein, thioredoxin.
Thioredoxins, present in all living organisms, function as
protein disulfide oxidoreductases, reducing disulfide
bridges in target proteins to the reduced (–SH) form or
re-oxidizing them to the disulfide (S–S) form (Buchanan,
1991). The enzymes modulated by thioredoxin in C3
photosynthesis are glyceraldehyde phosphate dehydrogenase (E3), fructose bisphosphatase (E6), sedoheptulose
bisphosphatase (E8), and phosphoribulose kinase (E12)
(see Figure 1).
An increase in stromal pH from 7 to 8 and an increase in
the magnesium ion concentration due to the light-
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Photosynthetic Carbon Metabolism
dependent accumulation of protons into and release of
magnesium ions from the thylakoid membranes provide
another means of regulation. The activities of the enzymes,
Rubisco, fructose bisphosphatase and sedoheptulose bisphosphatase, are increased by these changes in the stromal
environment.
The activity of Rubisco is also modulated by light in a
unique manner by another stromal protein, Rubisco
activase (Portis, 1992). Although the details of light
modulation are not fully characterized, Rubisco activase
responds to the level of ATP in the stroma, which increases
from dark to light conditions, and the redox poise. In some
species, a specific sugar phosphate inhibitor of Rubisco,
carboxyarabinitol 1-phosphate, is also implicated in light/
dark regulation and the removal of this inhibitor is
facilitated by Rubisco activase.
C4 Photosynthesis
The oxygenase activity of Rubisco is very costly to the
productivity of plants that perform C3 photosynthesis
(Ogren, 1984). Calculations show that photosynthesis
would increase by 30–40% in C3 plants if this reaction
did not occur. Plants also face the problem of balancing
water loss with CO2 uptake through their stomata. Raising
the concentration of CO2 around Rubisco in a spatially
separate area after the CO2 has entered the leaves would
help to alleviate both problems. Plants that perform C4
photosynthesis utilize this strategy (Hatch, 1987) and
initially capture CO2 to form four-carbon acids with the
enzyme phosphoenolpyruvate (PEP) carboxylase (F1,
Figures 2, 3 and 4). PEP carboxylase has two advantages
over Rubisco for CO2 capture. The substrate is bicarbonate, which is present at 80-fold greater concentrations
than CO2, and O2 is not a substrate or competitive
inhibitor. Pyruvate phosphate dikinase (F3, Figures 2
and 3) is another key enzyme in C4 photosynthesis
(Edwards et al., 1985). It regenerates the PEP utilized for
CO2 capture using pyruvate, ATP and phosphate, and also
forms AMP and pyrophosphate in a unique double
phosphorylation mechanism. Much of the current understanding of C4 photosynthesis was pioneered by the
research of M. D. Hatch and co-workers beginning in the
1960s when studies with 14CO2 first indicated that the fourcarbon acids malate, aspartate and oxaloacetate were
labelled before phosphoglyceric acid in some species.
Hence the term C4 photosynthesis or the Hatch–Slack
pathway is used to designate these adaptations to the
Calvin cycle.
For efficient use of PEP carboxylase to concentrate CO2
around Rubisco, these two enzymes are physically
separated in two different photosynthetic cell types.
Consequently, the leaves of C4 plants typically have a
distinctive structure called Krantz anatomy. A sheath of
Mesophyll cell
Bundle sheath cell
CHLOROPLAST
CO2
Oxaloacetate
HCO3–
F1
Malate
NADP+
NADPH
+ H+
Oxaloacetate
CHLOROPLAST
Malate
NADP+
F2
CO2
Pi
Phosphoenolpyruvate
NADPH
+ H+
Phosphoenolpyruvate
AMP
PPi
Calvin
cycle
F3
ATP
Pi
Pyruvate
Pyruvate
Triose
phosphate
Figure 2 Mechanism for concentrating CO2 in C4 photosynthesis of the
NADP malic enzyme type. (Adapted from Heldt H-W (1997) Plant
Biochemistry and Molecular Biology. Oxford: Oxford University Press.)
Malate and pyruvate are the major metabolites moving between the
mesophyll and bundle sheath cells. Specific metabolite membrane
transporters are indicated by solid black circles. Key enzymes are identified
and discussed in the text (F1, EC 4.1.1.31; F2, 1.1.1.40; F3, 2.7.9.1).
Mesophyll cell
Bundle sheath cell
MITOCHONDRION
Aspartate
a-KG
Aspartate
a-KG
Glu
Oxaloacetate
Glu
Oxaloacetate
NADH
+ H+
CO2
CHLOROPLAST
CO2
NAD+
Malate
HCO3–
F1
Pi
Phosphoenolpyruvate
NAD+
CHLOROPLAST
F2a
Phosphoenolpyruvate
PPi
AMP
CO2
NADH
+ H+
F3
ATP
Pyruvate
Glu
a-KG
Alanine
Calvin
cycle
Pi
Pyruvate
Triose
phosphate
Pyruvate
Glu
a-KG
Alanine
Figure 3 Mechanism for concentrating CO2 in C4 photosynthesis of the
NAD–malic enzyme type. (Adapted from Heldt H-W (1997) Plant
Biochemistry and Molecular Biology. Oxford: Oxford University Press.)
Aspartate and alanine are the major metabolites moving between the
mesophyll and bundle sheath cells. Specific metabolite membrane
transporters are indicated by solid black circles. Key enzymes are identified
and discussed in the text (F1, EC 4.1.1.31; F2a, 1.1.1.38; F3, 2.7.9.1; a-KG,
alpha-ketoglutarate; Glu, glutamate).
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3
Photosynthetic Carbon Metabolism
Mesophyll cell
Bundle sheath cell
Aspartate
a-KG
Aspartate
a-KG
Glu
Oxaloacetate
Glu
Oxaloacetate
Pi
F1
HCO3–
ATP
F4
CO2
ADP
Phosphoenolpyruvate
Phosphoenolpyruvate
CHLOROPLAST
CO2
Malate
NADP+
Oxaloacetate
Pi
CHLOROPLAST
CO2
NADPH
Oxaloacetate
MITOCHONDRION
Malate
NAD+
CO2
Calvin
cycle
Triose
phosphate
NADH
+ H+
HCO3–
Phosphoenolpyruvate
Phospheonolpyruvate
AMP
PPi
ATP
Pyruvate
Glu
Pi
Pyruvate
a-KG
Alanine
Respiratory
chain
ADP
ATP
Pyruvate
Glu
a-KG
Alanine
Figure 4 Mechanism for concentrating CO2 in C4 photosynthesis of the phosphoenolpyruvate (PEP) carboxykinase type. (Adapted from Heldt H-W
(1997) Plant Biochemistry and Molecular Biology. Oxford: Oxford University Press.) Aspartate and PEP are the major metabolites moving between the
mesophyll and bundle sheath cells. However, some malate also is used in the bundle sheath cells for respiration by mitochondria to meet the high demand
for ATP by PEP carboxykinase. cells. Specific metabolite membrane transporters are indicated by solid black circles. Key enzymes are identified and
discussed in the text (F1, EC 4.1.1.31; F4, 4.1.1.49; a-KG, alpha-ketoglutarate; Glu, glutamate).
cells surrounds the vascular bundles (hence termed bundle
sheath cells) and they are encircled by another group of
cells, the mesophyll cells. The cells are separated by walls
containing cellulose, but remain in communication via
connections called plasmodesmata. This allows the diffusive fluxes of metabolites that cycle between the cells,
allowing release of the captured CO2 within the bundle
sheath cells and resulting in the elevation of the CO2
concentration around Rubisco.
C4 plants are subdivided into three types according to
where and how the CO2 is released from the four-carbon
acids for recapture by Rubisco (Hatch, 1987). In most C4
species, the oxaloacetate formed by PEP carboxylase in the
cytosol of the mesophyll cells is transported into the
chloroplast and is reduced to malate. The malate leaves the
chloroplasts and diffuses through the plasmodesmata to
the bundle sheath cells. There malate is decarboxylated in
the bundle sheath chloroplasts via NADP–malic enzyme
(F2, Figure 2), which must not be confused with NADP–
malate dehydrogenase in the mesophyll chloroplasts. The
pyruvate formed by this reaction is transported out of the
chloroplast, diffuses back to the mesophyll cells and is
4
transported into the chloroplasts. There, PEP is regenerated with pyruvate phosphate dikinase (F3, Figure 2) using
ATP, which serves to drive the cycle and thereby effectively
transport and concentrate the CO2 in the bundle sheath
cells. Familiar species using this type of C4 photosynthesis
are maize, sugar cane, sorghum and crabgrass.
In other C4 species, such as millet, pigweed and purslane,
the decarboxylation of malate occurs in the mitochondria
of the bundle sheath cells via NAD–malic enzyme (F2a,
Figure 3). This is accompanied by several other differences
in the metabolite pathways (which may be seen by
comparing Figures 2 and 3). Most notably, aspartate, rather
than malate, is the major metabolite diffusing from the
mesophyll cells. It is readily interconverted with oxaloacetic acid in each cell type via transamination. Nitrogen
balance is maintained by diffusion of alanine from bundle
sheath to mesophyll cells. Transamination reactions also
readily interconvert alanine and pyruvate for this purpose.
The third variation has been found in some fast-growing
tropical grasses. Aspartate is also the major metabolite
moving from the mesophyll cells, but in these species
decarboxylation occurs largely in the cytosol of the bundle
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Photosynthetic Carbon Metabolism
sheath cells with yet another enzyme, phosphoenolpyruvate (PEP) carboxykinase (F4, Figure 4), which uses ATP.
As a result, the ATP demand necessary for driving the cycle
resides largely in the bundle sheath cells rather than with
pyruvate phosphate dikinase in the mesophyll cells. To
meet this demand, some malate also diffuses to the
mitochondria in the bundle sheath cells. Oxidation of the
malate to pyruvate produces NADH, which is used by the
respiratory chain to produce ATP. The pyruvate is
converted to alanine, which returns to the mesophyll to
complete a second cycle. As shown in Figure 4, this
secondary cycle results in some additional CO2 transport.
In some species, a strict classification into only one of
these three C4 types cannot be made because decarboxylation does not occur by a single mode. Furthermore, other
species are classified as C3 –C4 intermediates. Spatial
separation of the carboxylation enzymes and the distinct
anatomy are not as marked in some of these species. In
other C3 –C4 intermediates, the C4 pathway is not fully
functional and the CO2 release occurring during photorespiration appears to be confined to the bundle sheath
cells, which also raises the CO2 concentration in these cells
and results in more efficient recycling.
C4 photosynthesis requires additional enzymes to form
the CO2 concentrating cycle and, as might be expected,
several of these enzymes are also regulated by light. Malate
is present in the cytosol and the activity of PEP carboxylase
(F1, Figures 2, 3 and 4) is increased in the light by
phosphorylation of the enzyme with a specific protein
kinase, which results in reduced malate inhibition (Chollet
et al., 1996). However, the nature of the light signal that
controls the activity of the kinase is not clear. NADP
malate dehydrogenase is modulated by thioredoxin as
described previously. Pyruvate phosphate dikinase (F3,
Figures 2 and 3) is also regulated by phosphorylation, but
with a rather unusual regulatory protein (Edwards et al.,
1985). In this case, the regulatory protein uses ADP for the
phosphorylation, resulting in an inactive enzyme. Reactivation is catalysed by the same protein and dephosphorylation occurs via phosphorolysis, converting phosphate
to pyrophosphate. Factors that control the activity of the
regulatory protein in the light and dark have not been
ascertained.
The absence of Rubisco in the mesophyll cells and the
need for additional ATP (about two molecules per CO2
incorporated) for the CO2 concentrating cycle results in
quite different energy (i.e. ATP and NADPH) requirements for reactions occurring in the mesophyll and bundle
sheath chloroplasts. This is reflected in their appearance.
Mesophyll chloroplasts contain abundant thylakoid grana
(enriched in photosystem II) and little starch, while the
bundle sheath chloroplasts contain mostly non-appressed
stroma thylakoids (enriched in photosystem I) and
numerous starch granules.
C4 photosynthesis is very advantageous over C3 photosynthesis in high-temperature environments or when water
stress is frequent. The ability to concentrate CO2 around
Rubisco means that the stomata can be more closed and
water is conserved. C4 plants typically lose only 100–300
water molecules per CO2 incorporated, whereas C3 plants
lose up to 1000. While only about 1% of the known species
perform C4 photosynthesis, plants utilizing this pathway
are predominant in those considered the most productive,
but many are also considered to be the world’s worst weeds.
CAM Plants
CAM plants utilize another strategy to avoid photorespiration and most importantly to conserve water (Osmond, 1978). The acronym CAM is derived from
Crassulacean Acid Metabolism, which in turn is derived
from the family in which this pathway was first discovered
and the prominent storage of the acid malate. However,
plants in many other families use this form of metabolism:
some familiar ones are cacti, orchids, pineapple, agave and
epiphytes. CAM plants are typically found in mostly dry
and even hot habitats such as deserts but are also found in
other water-stressed environments such as salt marshes.
In these plants there is a temporal separation in the
activities of PEP carboxylase and Rubisco (Figure 5). PEP
carboxylase (F1, Figure 5) is used to capture atmospheric
CO2 during the night when temperatures are lower and the
humidity is higher. The oxaloacetate formed by this
reaction is reduced in the cytosol by NAD malate
dehydrogenase (F5, Figure 5) to form malate, which is
stored in large vacuoles in its acidic form. The presence of
these large vacuoles is the reason many of these plants are
succulent. Most of the starch made the previous day is
metabolized during the night to supply the PEP needed for
CO2 capture.
Rubisco and the Calvin cycle are active during the day,
but the stomata can be completely closed because
decarboxylation of the stored malate via NADP–malic
enzyme (F2, Figure 5) is used to provide the CO2 for
Rubisco. Consequently, CO2 can accumulate to high levels
and suppress photorespiration. The other product, pyruvate, is converted back into starch via pyruvate phosphate dikinase (F3, Figure 5) and several other enzymatic
steps. Thus a large diurnal cycle of carbon between malic
acid accumulation during the night and starch accumulation during the day occurs in CAM plants. Similarly to the
variations observed in C4 photosynthesis, in a few CAM
plants PEP carboxykinase or NAD–malic enzyme is used
to release the CO2.
Without some form of regulation there would be
competition between Rubisco and PEP carboxylase during
the day for the released CO2. Malate normally inhibits PEP
carboxylase during the day while decarboxylation is occurring. During the night, PEP carboxylase is modified by
phosphorylation, which reduces the inhibition by malate.
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Photosynthetic Carbon Metabolism
Night
Day
VACUOLE
Malic acid
ATP
ADP
+ Pi
Malic acid
H+
H+
Malate
Malate
H+
CO2
Malate
Oxaloacetate
HCO3–
F1
F5
Malate
CHLOROPLAST
Malate
NADP+
F2
NADPH
+ H+
Pyruvate
ATP
Pi
Pi
Phosphoenolpyruvate
NADH
+ H+
3-Phosphoglycerate
NAD+
AMP
PPi
Phosphoenolpyruvate
ATP
ADP
1,3-Bisphosphoglycerate
CO2
F3
Triose
phosphate
Calvin
cycle
Triose
phosphate
Triose
phosphate
Starch
Starch
Figure 5 Mechanism for concentrating CO2 in CAM plants. Specific metabolite membrane transporters are indicated by solid black circles. Key enzymes
are identified and discussed in the text (F1, EC 4.1.1.31; F2, 1.1.1.40; F3, 2.7.9.1; F5, 1.1.1.37).
As in C4 plants, the additional metabolic reactions needed
for the temporal separation of the initial CO2 capture and
the subsequent recapture by the Calvin cycle use more
energy, but in hot dry habitats the limiting resource is
usually water. CAM plants are well adapted to these
environments and typically lose only 50 water molecules for
each CO2 gained. However, the capacity to store malic acid
in vacuoles appears to be limited, resulting in slow growth
even under favourable conditions. Some CAM plants
(described as ‘facultative’ versus ‘obligate’) have developed
the ability to switch between the C3 and CAM forms of
photosynthesis in response to their environment.
properties and mechanism of light/dark regulation. Annual Review of
Plant Physiology 36: 255–286.
Hatch MD (1987) C4 photosynthesis: a unique blend of modified
biochemistry, anatomy and ultrastructure. Biochemica et Biophysica
Acta 895: 81–106.
Ogren WL (1984) Photorespiration: pathways, regulation, and modification. Annual Review of Plant Physiology 35: 415–442.
Osmond CB (1978) Crassulacean acid metabolism: a curiosity in context.
Annual Review of Plant Physiology 29: 379–414.
Portis AR (1992) Regulation of ribulose 1,5-bisphosphate carboxylase/
oxygenase activity. Annual Review of Plant Physiology and Plant
Molecular Biology 43: 415–437.
References
Further Reading
Bassham JA (1962) The path of carbon in photosynthesis. Scientific
American 206: 88–100.
Buchanan BB (1991) Regulation of CO2 assimilation in oxygenic
photosynthesis: the ferredoxin/thioredoxin system. Archives of Biochemistry and Biophysics 288: 1–8.
Chollet R, Vidal J and O’Leary MH (1996) Phosphoenolpyruvate
carboxylase: a ubiquitous, highly regulated enzyme in plants. Annual
Review of Plant Physiology and Plant Molecular Biology 47: 273–298.
Edwards GE, Nakamoto H, Burnell JN and Slack MD (1985) Pyruvate,
Pi dikinase and NADP-malate dehydrogenase in C4 photosynthesis:
Heldt H-W (1997) Plant Biochemistry and Molecular Biology. Oxford:
Oxford University Press.
Raghavendra AS (ed.) (1998) Photosynthesis: A Comprehensive Treatise.
Cambridge: Cambridge University Press.
von Caemmerer S and Furbank RT (eds) (1997) C4 photosynthesis: 30
(or 40) years on. Australian Journal of Plant Physiology 24 (4). [Special
issue]
Winter K and Smith JAC (eds) (1996) Crassulacean acid metabolism.
Biochemistry, ecophysiology and evolution. Ecological Studies, vol.
114. Berlin: Springer-Verlag.
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