Catalyzes the hydrolytic deamination of guanine, producing xanthine and ammonia. guanine + H(+) + H2O = NH4(+) + xanthine Binds 1 zinc ion per subunit. Purine metabolism; guanine degradation; xanthine from guanine: step 1/1. Homodimer. Belongs to the metallo-dependent hydrolases superfamily. ATZ/TRZ family.