Involved in the catabolism of purine nucleotides. Can use allantoate as substrate, but not Nalpha-carbamoyl-L-Asp, Nalpha-carbamoyl-L-Ala or Nalpha-carbamoyl-Gly. The sequential activity of AAH, UGLYAH and UAH allows a complete purine breakdown without the intermediate generation of urea. allantoate + 2 H(+) + H2O = (S)-2-ureidoglycine + CO2 + NH4(+) Inhibited by borate, fluoride, L-Asn and L-Asp, but not by phenylphosphorodiamidate. kcat is 27.2 sec(-1) for allantoate. Homodimer. Expressed in seedlings, roots, stems, leaves, flowers, siliques and seeds. No visible phenotype under normal growth conditions, but mutant plants are unable to grow on a medium containing allantoin as the sole nitrogen source and accumulate allantoate. Belongs to the peptidase M20A family.