Reversibly catalyzes the transfer of the carbamoyl group from carbamoyl phosphate (CP) to the N(epsilon) atom of ornithine (ORN) to produce L-citrulline, which is a substrate for argininosuccinate synthetase, the enzyme involved in the final step in arginine biosynthesis. carbamoyl phosphate + L-ornithine = H(+) + L-citrulline + phosphate Inhibited by excess of arginine and by the bisubstrate delta-N-phosphonoacetyl-L-ornithine (PALO). kcat is 15 and 766 sec(-1) at 30 degrees Celsius and pH 9 for L-ornithine and carbamoyl phosphate, respectively. kcat is 87 and 694 sec(-1) at 20 degrees Celsius and pH 9 for L-ornithine and carbamoyl phosphate, respectively. kcat is 132 and 214 sec(-1) at 5 degrees Celsius and pH 9 for L-ornithine and carbamoyl phosphate, respectively. Optimum pH is between 9 and 10. Optimum temperature is between 23 and 25 degrees Celsius. 37% of this maximal activity could still be observed at 5 degrees Celsius. Amino-acid biosynthesis; L-arginine biosynthesis; L-arginine from L-ornithine and carbamoyl phosphate: step 1/3. The enzyme is present as a mixture of trimers and dodecamers, with the relative proportions of the two forms depending on the salt concentration. In addition, the trimeric fraction could reassociate into dodecamers when the salt concentration is increased. It appears that in vivo, the main fraction is in the dodecameric form. Belongs to the aspartate/ornithine carbamoyltransferase superfamily. OTCase family. Lacks the conserved threonine and leucine residues in positions 50 and 261, respectively, which are part of the carbamoylphosphate and ornithine binding sites; they are replaced by a leucine and a glutamine residue, respectively.