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Monosaccharide Biosynthesis

BioFiles 2007, 2.6, 21.

BioFiles 2007, 2.6, 21.

Each of the monosaccharides has its own interesting metabolism. The pathway by which a monosaccharide is made, is not the reverse of its degradation pathway (see Figure 4).

Figure 4. Monosaccharide Biosynthesis.

Glucose biosynthesis starts with the carboxylation of pyruvate (1) to oxaloacetate (2) catalyzed by pyruvate carboxylase requiring ATP and biotin as coenzymes.

The decarboxylation and phosphorylation of oxaloacetate (2) is catalyzed by phosphoenol pyruvate carboxykinase and utilizes both ATP and GTP to yield energy-rich phosphoenolpyruvate (3).

Enolase-catalyzed addition of water to the double bond of phosphoenolpyruvate (3) occurs stereospecifically and gives D-2-phosphoglycerate (4), which is converted through a sequence of phosphorylation at the primary hydroxyl group and a dephosphorylation at the secondary hydroxyl group gives D-3- phospho-glycerate (5).

Another phosphorylation of D-3-phosphoglycerate with ATP produces D-1,3-bisphospho-glycerate (6), which after reduction leads to D-glyceraldehyde-3-phosphate (7).

D-Glyceraldehyde-3-phosphate (7) tautomerizes to dihydroxyacetone phosphate (8) and the condensation of these two three-carbon units to fructose-1,6-bisphosphate (9) is catalyzed by a class I aldolase, fructose-1,6-bis-phosphate aldolase.

A hydrolysis reaction catalyzed by fructose-1,6-bisphosphatase removes the phosphate group at C1 and yields fructose-6- phosphate (10), which further tautomerizes to glucose-6- phosphate (11).

The final hydrolysis step to glucose (12) is catalyzed by phosphatase.

The pathways of other monosaccharides are also of importance to both healthy and pathological states. Sigma offers metabolites from pentose and glucuronate interconversions, fructose and mannose metabolism, galactose metabolism, ascorbate and aldarate metabolism, starch and sucrose metabolism, amino- and nucleotide sugar metabolism, inositol and other monosaccharide metabolic pathways.

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