Biosynthesis of Different Monosaccharides by Epimerization, Oxidation, and Decarboxylation

3 Biosynthesis of Different Monosaccharides by Epimerization, Oxidation, and Decarboxylation 

In section 10.1, we saw that the first hexose to be formed in the fixation of CO2 was D-fructose-l,6-bisphosphate. This sugar phosphate is converted to D-fruc-tose-6-phosphate by a specific phosphatase (reaction 10.3). The keto group can be epimerized by two different enzymes, D-fructose-6-phosphate/D-glucose epimer-ase or D-fructose-6-phosphate/D-mannose epimerase, to form D-glucose-6-phosphate and D-mannose-6-phosphate, respectively (see Fig. 10.5). Both of these sugar phosphates can be converted into their 1-phosphates by reaction with specific mutase enzymes. a-D-Glucose-1-phosphate and a-D-mannose-1-phosphate can then react with UTP and GTP to form UDP-Glc and GDP-Man (see Fig. 10.5). 

A-acetyl-mannosamine is an important precursor for the biosynthesis of the nine carbon sugar acid, iV-acetyl-D-neuraminic acid (see section 10.5 and Fig. 10.8A) that is frequently found as a monosaccharide in glycoproteins and glyco-lipids (see Chapter 9). 

UDP-Glc is converted into UDP-Gal by a reaction catalyzed by UDP-Gal-4-epimerase. The enzyme is NAD dependent and the first reaction that it catalyzes is the oxidation of C-4 of UDP-Glc to give the 4-keto intermediate and NADH. The complex undergoes a conformational change and the NADH-product stereospecifically reduces the keto group to give the D-galactose structure [13-15] (see Fig. 10.6). 

UDP-Glc is also converted into UDP-glucuronic acid (UDP-GlcUA) by a reaction catalyzed by an enzyme requiring two moles of NAD per mole of UDP-Glc [16] (see Fig. 10.6). In plants, there is an enzyme, UDP-GlcUA decarboxy- 

---