Catalyzes the reversible aldol cleavage of N-acetylneuraminic acid (sialic acid; Neu5Ac) to form pyruvate and N-acetylmannosamine (ManNAc) via a Schiff base intermediate. aceneuramate = N-acetyl-D-mannosamine + pyruvate kcat is 250 min(-1) for the cleavage of Neu5Ac at pH 7.4, and is 260 min(-1) at pH 6.8. Optimum pH is 7.4. Amino-sugar metabolism; N-acetylneuraminate degradation; D-fructose 6-phosphate from N-acetylneuraminate: step 1/5. Homotetramer. When the catalytic Lys-165 residue is replaced by the unnatural amino acid gamma-thialysine, the enzyme regains significant activity compared with the Cys-165 mutant; regains of approximately 17% of the wild-type catalytic efficiency. Belongs to the DapA family. NanA subfamily.