Structure of Sucrose
Sucrose is a disaccharide found in plants as transport form of sugar. It is a major product of photosynthesis, synthesized in the cytosol from the calvin cycle product triose phosphate. It is sweet and soluble sugar. It is also called as cane sugar, table sugar. It plays roles in growth and development. It constitutes major human macronutrients and it is widely used as sweetening agent. As it is a disaccharide which constitutes two monosaccharides D-glucose and D-fructose. The chemical name of sucrose is O-α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside and as there is no free anometic hydroxy group (at carbon 2) it is non reducing sugar.
Synthesis of Sucrose
In chloroplast the triose phosphate formed from calvin cycle is transported out of the chloroplas through inorganic phosphate-triose phosphate translocator and is converted to fructose 1,6 bisphosphate catalyzed by aldolase. This fructose 1,6-bisphosphate is then converted to fructose 6-phosphate catalyzed by the enzyme fructose 1,6-bisphosphatase. Thus formed fructose 6-phosphate will then be converted to glucose 6-phosphate catalyzed by hexose phosphate isomerase. In next step glucose 6-phosphate is converted to UDP-glucose in two succeding reactions catalyzed by enzymes phosphoglucomutase and UDP-glucose pyrophosphorylase. Thus formed UDP-glucose is condensed with fructose 6-phosphate to form sucrose 6-phosphate catalyzed by sucrose phosphate synthase or more preferentially called as sucrose 6-phosphate synthase. The last step is to cleave the phosphate group to release free sucrose catalyzed by sucrose 6-phosphate phoaphatase.
Alternative Route of Sucrose Synthesis
There are some alternative pathways. These are;
- In some bacteria like Pseudomonas saccharophilla it can be synthesized by the condensation of glucose 1-phosphate and fructose to yield sucrose and inorganic phosphate catalyzed by the enzyme sucrose phosphorylase.
- In wheat germ it is synthesized by the condensation of UDP-glucose with fructose to yield sucrose and UDP catalyzed by enzyme UDP-glucose fructose transglycosidase.
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Utilization of Sucrose
sucrose has two fates and are utilized either to produce energy through glycolysis and other succiding pathways or to provide precursors for different biosynthetic pathways to yield different types of glycoproteins and polysaccharides on which there is a sucrose core unit.
- Sucrose is hydrolyzed in plants by a type of invertase called as fructofuranosidase to glucose and fructose and both of them inter to the glycolytic pathway to yield pyruvate and this pyruvate is further metabolized to produce energy.
- There are many types of polysaccharides, oligosaccharides (raffinose, stachyose, verbascose, melezitose) and some glycoproteins on which there is sucrose core unit. Biosynthesis of these types of biomolecules involve successive addition of sucrose unites catalysed by different enzymes.