Assimilation of CO2 forms triose phosphate that is transported from triose phosphate-phosphate translocator and in exchange, a phosphate is transported into the chloroplast. Triose phosphate is converted to sucrose in the cytosol and inorganic phosphate is released. Some of the triose phosphate in chloroplast is converted to starch that can be used for reserve.
7.1.    Biosynthesis and degradation of starch
The ratio of 3-phosphoglycerate to phosphate concentration governs the activity of an enzyme ADP-glucose phosphorylase i.e. responsible for the conversion of carbohydrate to starch. 
The concentration of 3-phosphoglycerate is maintained high than triose phosphate by the equilibrium reactions catalyzed by phosphoglycerate kinase and glyceraldehyde phosphate dehydrogenase.
Degradation of starch is found to be stimulated by an increase in phosphate concentration of stroma. Hydrolysis of starch release maltose and glucose which are transported by respective translocators into the cytosol. Transglucosidase split maltose and transfers its one glucose molecule to α-glucan and remaining glucose molecule is released i.e. phosphorylated by ATP to glucose-6-phosphate in the presence of hexokinase. Glucose-1-phosphate formed by phosphoryl degradation of starch is converted to fructose-1, 6-bisphosphate in the presence of a fructose-6-phosphate kinase.
Fructose-1, 6-bisphosphate forms triose phosphate by aldolase and is released from chloroplast via triose phosphate is oxidized to 3-phosphoglycerate.
7.2.    Starch metabolism
7.2.1.    Synthesis of sucrose from Fructose 6 P and Glucose UDP
Synthesis of sucrose utilizes UDP-glucose.
Glucose-1-phosphate + UTP     \leftarrow \rightarrow    UDP-glucose + PP
The above reaction is catalyzed by UDP-glucose phosphorylase. The reaction is reversible because pyrophosphatase is not present in the cytosol of mesophyll cells. Glucose from UDP-glucose is transferred to fructose-6-phosphate in the presence of sucrose phosphate synthase (SPS) thus forming sucrose-6-phosphate. It is further hydrolyzed by sucrose phosphate phosphatase (SPP) thus making the reaction irreversible. Sucrose synthase is responsible for utilization of sucrose.

7.2.2.    Conversion of triose phosphate into sucrose 
Triose phosphate generated in CO2 fixation forms sucrose. The first reaction is an irreversible reaction catalyzed by cytosolic fructose-1, 6-bisphosphatase that converts fructose-1,6-bisphosphate to fructose-6-phosphate.
Fructose-2, 6-bisphosphate helps in the regulation of various metabolic processes like glycolysis and gluconeogenesis. Fructose-2, 6-bisphosphate decrease the affinity of enzyme fructose-1, 6-bisphosphatase for its substrate. It also activates an enzyme in the cytosol i.e. fructose-6-phosphate kinase which is pyrophosphate-dependent. The increase in the concentration of triose phosphate decrease the levels of fructose-2, 6-bisphosphate. A particular threshold level of triose phosphate is required for the Calvin cycle. The level above the threshold is utilized very efficiently for sucrose synthesis.

7.2.3.    Regulation of sucrose phosphate synthase (SPS)
Sucrose phosphate synthase is stimulated by glucose-6-phosphate and is inhibited by phosphate. Glucose-6-phosphate is in equilibrium with fructose-6-phosphate. Thus the enzyme activity is also regulated by the concentration of fructose-6-phosphate. Sucrose phosphate synthase is also regulated by sucrose phosphate synthase-kinase that phosphorylates the –OH group of the serine residue at 158 positions and dephosphorylated by SPS phosphatase. Dephosphorylated SPS is more active than phosphorylated SPS. Light activates SPS by increasing the activity of SPS phosphatase.
Moreover, the phosphorylation of SPS at the OH of the serine residue at 424 positions activates the enzyme.
7.2.4.    Conversion of photosynthetic products to fructan
In some plants, fructans are carbohydrate storage compounds. These are soluble polyfructoses, synthesized and stored in the vacuole. Sucrose that is synthesized in the cytosol is the precursor for fructan synthesis. It is transported into the vacuole and synthesize fructan releasing glucose which is utilized for formation of UDP-glucose in the cytosol. Fructose-6-phosphate i.e. generated by photosynthesis and UDP glucose also forms fructan by the expenditure of two ATP molecules per fructan molecule.

Next Previous