NeuB is the prokaryotic N-acetylneuraminic acid (Neu5Ac) synthase. It catalyzes 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 synthesize the 9-phosphate form, Neu5Ac-9-P, and utilize ManNAc-6-P instead of ManNAc. Such eukaryotic enzymes are not present in this family. This family also contains SpsE spore coat polysaccharide biosynthesis proteins.