Catalyzes the synthesis of carbamoyl phosphate from ATP, ammonium and bicarbonate. Proceeds via a three-step mechanism, i.e. the phosphorylation of hydrogencarbonate to carboxyphosphate, a nucleophilic attack of ammonia on carboxyphosphate yielding carbamate, and the phosphorylation of carbamate forming carbamoyl phosphate. In M.smithii, the predominant archaeon in the human gut, one function of this enzyme may be to sequester ammonia, a scarce nutrient in the intestine which is the major source of nitrogen in M.smithii for the biosynthesis of nucleotides, amino acids, and many other metabolites. 2 ATP + hydrogencarbonate + NH4(+) = 2 ADP + carbamoyl phosphate + 2 H(+) + phosphate Binds 2 magnesium or manganese ions per subunit. kcat is 0.043 sec(-1) towards ATP (at 38 degrees Celsius and pH 8.0). kcat is 0.052 sec(-1) towards NH4(+) (at 38 degrees Celsius and pH 8.0). kcat is 0.044 sec(-1) towards hydrogencarbonate (at 38 degrees Celsius and pH 8.0). Forms homodimers and homotetramers (dimers of dimers). Lacks the domain involved in allosteric regulation present in CarB CPSases. Several lines of evidence suggest that obesity is associated with elevated levels of intestinal methanogenic archaea. Consequently, this enzyme may be an attractive drug target in the treatment of human obesity. Belongs to the small carbamoyl-phosphate synthase family.