NeuB is the prokaryotic N-acetylneuraminic acid synthase (Neu5Ac). 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 enzymesare not present in this family [<cite idref="PUB00007383"/>]. This family also contains SpsE spore coat polysaccharide biosynthesis proteins.