Produces N-acetylneuraminic acid (Neu5Ac) and 2-keto-3-deoxy-D-glycero-D-galacto-nononic acid (KDN) (PubMed:11886840, PubMed:21919466). Can also use N-acetylmannosamine 6-phosphate and mannose 6-phosphate as substrates to generate phosphorylated forms of Neu5Ac and KDN, respectively (PubMed:11886840). Essential for biosynthesis of sialic acids in neurons of the central nervous system (PubMed:21919466). H2O + N-acetyl-D-mannosamine + phosphoenolpyruvate = N-acetylneuraminate + phosphate an N-acyl-D-mannosamine 6-phosphate + H2O + phosphoenolpyruvate = an N-acylneuraminate 9-phosphate + phosphate Expressed at all stages of development. Extended N-terminus.