(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.