Catalyzes the conversion of allantoin (5-ureidohydantoin) to allantoate by hydrolytic cleavage of the five-member hydantoin ring. Catalyzes the first step of the ureide allantoin degradation followed by the sequential activity of AAH, UGLYAH and UAH which allows a complete purine breakdown without the intermediate generation of urea. (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. No visible phenotype under normal growth conditions, but mutant plants are unable to grow on a medium containing allantoin as the sole nitrogen source, accumulate allantoin and have increased tolerance to drought and osmotic stresses. In Arabidopsis the intermediary metabolite allantoin plays a role in abiotic stress tolerance via activation of abscisic acid (ABA) metabolism. Belongs to the metallo-dependent hydrolases superfamily. Allantoinase family.