Water molecules located towards the opening of the active site form a tetrahedral cluster that fills the cavity site through hydrogen bonds. Some amino acid residues are proposed to form mobile flap of the site, which gate for the substrate. Cysteine residues are common in the flap region of the enzymes, which have been determined not to be essential in catalysis, although involved in positioning other key residues in the active site appropriately. In "Sporosarcina pasteurii" urease, the flap was found in the open conformation, while its closed conformation is apparently needed for the reaction.