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 Amino-sugar metabolism; N-acetylneuraminate degradation; D-fructose 6-phosphate from N-acetylneuraminate: step 1/5. Homotetramer. Belongs to the DapA family. NanA subfamily.