Urea hydrolase involved in nitrogen recycling from ureide, purine, and arginine catabolism. 2 H(+) + H2O + urea = CO2 + 2 NH4(+) Binds 2 nickel ions per subunit. Requires the three urease accessory proteins URED, UREF AND UREG for its activation. Nitrogen metabolism; urea degradation; CO(2) and NH(3) from urea (urease route): step 1/1. Homohexamer. Carboxylation allows a single lysine to coordinate two nickel ions. No visible phenotype under normal growth conditions, but mutant plants cannot grow on medium with urea as the sole source of nitrogen. In the C-terminal section; belongs to the metallo-dependent hydrolases superfamily. Urease alpha subunit family.