Catalyzes the conversion of allantoin (5-ureidohydantoin) to allantoic acid by hydrolytic cleavage of the five-member hydantoin ring. (S)-allantoin + H2O = allantoate + H(+) Binds 2 Zn(2+) ions per subunit. Nitrogen metabolism; (S)-allantoin degradation; allantoate from (S)-allantoin: step 1/1. Homotetramer. Carboxylation allows a single lysine to coordinate two zinc ions. Belongs to the metallo-dependent hydrolases superfamily. Allantoinase family.