Fig. 10. Two alternative biosynthetic pathways for epimerization. (B) This type of epimerization proceeds via the 4-keto-intermediate of nucleotide-sugars. For the formation of the 4-keto-intermediate, NAD+ is utilized, and upon its reduction with NADH 4-epimerization is achieved. Such 4-keto-intermediates are variously utilized not only for epimerization of 4-OH but that of 3-OH and 5-CH2OH, as well as for the formation of deoxysaccharides, such as L- fucose, L-rhamnose, and various dideoxyaldohexoses listed in Table 3.