GDP-D-mannose 3 ,5 -epimerase

Higher plants make large amounts of L-ascorbate, which in leaves may account for 10% of the soluble carbohydrate content.28 However, the pathway of synthesis differs from that in Fig. 20-2. Both D-mannose and L-galactose are efficient precursors. The pathway in Eq. 20-4, which starts with GDP-d-mannose and utilizes known enzymatic processes, has been suggested.28 29 The GDP-D-mannose-3, 5-epimerase is a well documented but poorly understood enzyme. Multistep mechanisms related to that of UDP-glucose 4-epimerase (Eqs. 20-1,15-14) can be envisioned. 

A separate enzyme, a 3,5-epimerase catalyzes inversion at both C-3 and C-5 (step d).59e Finally, a third enzyme is needed for a second reduction (step e) using NADPH.59f Tire biosynthesis of GDP-L-fiicose from GDP-D-mannose occurs by a parallel sequence.60 61b... 

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). 

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