conditions are right (i.e., if enough carbon exists in the PCR cycle to allow sufficient regeneration of ribu-
lose-1,5-bisphosphate to maintain the photosynthetic CO 2 fixation rate), some carbon may be diverted out
of the PCR cycle for starch synthesis. Fru-6-P is converted to glucose-6-phosphate (Glu-6-P) via a chloro-
plastic form of the enzyme hexose phosphate isomerase (reaction 1 in Figure 1), and Glu-6-P is converted
to glucose 1-phosphate (Glu-1-P) by chloroplastic phosphoglucomutase (reaction 2). Glu-1-P is then con-
verted to a sugar nucleotide, adenosine diphosphoglucose (ADPG), via the enzyme ADPG pyrophos-
phatase (ADPGPPase, reaction 3). The sugar nucleotide ADPG then acts as the glucose donor for the re-
action catalyzed by starch synthase (reaction 4), which lengthens the glucan chain by one -1,4-linkage.
A further enzyme, the branching enzyme (not shown), is responsible for creation of the -1,6 linkages of
amylopectin. There appear to be multiple enzyme forms of both starch synthase and branching enzyme,
which may be related to the structural asymmetries associated with the starch molecule [3–5].
- Regulation of Starch Synthesis in Leaves
Regulation of starch synthesis in leaves is at the level of the enzyme ADPGPPase [4,5,7]. This enzyme is
allosterically controlled by levels of 3-phosphoglyceric acid (3-PGA), the initial product of CO 2 fixation,
by the PCR cycle, which activates the enzyme, and inorganic phosphate (Pi), which inactivates it. The ra-
tio of 3-PGA to Piin the chloroplast thus determines the activity of the ADPGPPase enzyme. Conse-
quently, starch synthesis is promoted during periods of high photosynthetic rate, during which high lev-
els of 3-PGA are formed and Piis rapidly incorporated into ATP and other phosphorylated intermediates
of the PCR cycle. Details of the allosteric control mechanism of this enzyme are not fully understood.
There is evidence that at least two lysine sites, Lys404 and Lys441, on the small subunit of the protein
might be involved in the regulation of the enzyme. Site-directed mutagenesis of these lysines results in a
decrease in affinity for both its activator, 3-PGA, and its inhibitor, Pi, and thus results in decreased en-
zyme activity [8].
In addition, the starch synthesis rate is coupled to the sucrose synthesis rate through the export of
triose-P out of the chloroplast. As indicated in Figure 1, this export occurs in strict exchange with the im-
port of Pivia operation of the phosphate translocator of the chloroplast membrane [9]. Thus, conditions
that favor triose-P export out of the chloroplast (i.e., high rates of cytosolic sucrose synthesis) result in a
low PGA/Piratio inside the chloroplast and inhibit the formation of starch through inhibition of ADPGP-
Pase activity [4,5,7]. Conversely, under conditions of reduced sucrose synthesis, cytosolic levels of Pi, a
product of the sucrose synthetic pathway (Figure 1), are low, preventing the export of triose-P from the
chloroplast. The resulting reduction in import of Picoupled with reduced export of triose-P raise the
PGA/Piratio and activates the ADPGPPase [4,5,7].
B. Sucrose
As already indicated, the synthesis of sucrose and starch in photosynthesizing leaves is coupled with the
operation of the phosphate translocator of the chloroplast membrane. Unlike starch synthesis, which oc-
curs in the chloroplast, synthesis of the disaccharide sucrose (-D-glucose-1,2--D-fructofuranoside, Fig-
ure 2) occurs in the cytosol of the photosynthetic cell from triose-P that is exported to this compartment
via the phosphate translocator [9–11].
As indicated in Figure 3, once in the cytosol, triose-P is converted to fructose 1,6-bisphosphate (Fru-
1,6-bis-P), which is dephosphorylated to fructose-6-phosphate (Fru-6-P) via a specific fructose 1,6-bis-
phosphatase (FBPase, reaction 1 in Figure 3). In a series of reactions paralleling that seen in the chloro-
plast for starch synthesis, Fru-6-P can then be converted to glucose 1-phosphate (Glu-1-P) and then to a
sugar nucleotide, in this case uridine diphosphoglucose (UDPG), via the enzyme uridine diphosphoglu-
cose pyrophosphorylase (UDPGPPase, reaction 2). This glucose residue of this sugar nucleotide is then
transferred to Fru-6-P in the reaction catalyzed by sucrose-phosphate synthase (SPS; reaction 3). The su-
crose phosphate produced in this reaction is finally converted to sucrose by sucrose phosphate phos-
phatase (reaction 4), resulting in the release of Pito the cytosol.
- Regulation of Sucrose Formation in Leaves
To prevent inhibition of photosynthetic carbon fixation during sucrose synthesis, the export of triose-P,
which is also required to run the PCR cycle, must be controlled [10]. Specifically, to maintain optimal
CO 2 fixation rates, no more than one triose-P molecule out of six produced photosynthetically can leave
CARBOHYDRATE SYNTHESIS AND CROP METABOLISM 469