Biosynthesis of d- and L-6-Deoxyhexoses

These reactions are shown in Fig. 10.7 for the conversion of dTDP-o-glucose to dTDP-L-rhamnose [17] and for the conversion of GDP-o-mannose into GDP-D-rhamnose and GDP-L-fucose [20-22]. The initial reaction to form the 4-keto sugar has a close similarity to the reaction for the conversion of UDP-Glc into UDP-Gal. 

5 Biosynthesis of Eight- and Nine-Carbon Sugars N-Acetyl-D-Neuraminic Acid, N-Acetyl-o-Muramic Acid, and 2-Keto-3-Deoxy-D-Mannooctulosonic Acid (KDO) 

A -acetyl-D-neuraminic acid is biosynthesized from A -acetyl-D-mannosamine and phosphoenol pyruvate, catalyzed by N-acetyl-D-neuraminic acid synthase. The first step involves the addition of an electron pair from the double bond of the phosphoenol pyruvate to the aldehyde group to give an aldol-type condensation (see Fig. 10.8A). The product is the nine-carbon sugar acid, A -acetyl-D-neuraminic acid [23]. In some instances the enzyme requires A-acetyl-D-mannosamine-6-phosphate as the substrate and forms A-acetyl-D-neuraminic acid-9-phosphate. Various hydroxyl groups on C-4, -7, -8, and -9 can be acetylated by specific acetyl transferases using acetyl CoAas the donor. KDO (2-keto-3-deoxy-D-mannooctulosonic acid) is biosynthesized by a very similar condensation between D-arabinose-5-phosphate and pyruvic acid, catalyzed by KDO synthase (see Fig. 10.8B) [24]. 

B Biosynthesis of 2-keto-3-deoxy-octuiosonic acid (KDO) by KDO synthase 

2-keto-3-deoxy-octulosonic acid 8-phosphate (KDO-8-phosphate) 

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