NeuB is the prokaryotic N-acetylneuraminic acid (Neu5Ac) synthase. It catalyses the direct formation of Neu5Ac (the most common sialic acid) by condensation of phosphoenolpyruvate (PEP) and N-acetylmannosamine (ManNAc). This reaction has only been observed in prokaryotes; eukaryotes synthesise the 9-phosphate form, Neu5Ac-9-P, and utilise ManNAc-6-P instead of ManNAc. Such eukaryotic enzymes are not present in this family [1]. This family also contains SpsE spore coat polysaccharide biosynthesis proteins.

1: Molecular cloning and expression of the mouse N-acetylneuraminic acid 9-phosphate synthase which does not have deaminoneuraminic acid (KDN) 9-phosphate synthase activity. Nakata D, Close BE, Colley KJ, Matsuda T, Kitajima K; Biochem Biophys Res Commun 2000;273:642-648. PMID:10873658