AGE domain; N-acyl-D-glucosamine 2-epimerase domain; Responsible for intermediate epimerization during biosynthesis of N-acetylneuraminic acid. Catalytic mechanism is believed to be via nucleotide elimination and readdition and is ATP modulated. AGE is structurally and mechanistically distinct from the other four types of epimerases. The AGE domain monomer is composed of an alpha(6)/alpha(6)-barrel, the structure of which is also found in glucoamylase and cellulase. The active form is a homodimer. The alignment also contains subtype III mannose 6-phosphate isomerases.