Large subunit of the arginine-specific carbamoyl phosphate synthase (CPSase). CPSase catalyzes the formation of carbamoyl phosphate from the ammonia moiety of glutamine, hydrogencarbonate, and phosphate donated by ATP, the first step of the arginine biosynthetic pathway. The large subunit (synthetase) binds the substrates ammonia (free or transferred from glutamine from the small subunit), hydrogencarbonate and ATP and carries out an ATP-coupled ligase reaction, activating hydrogencarbonate by forming carboxy phosphate which reacts with ammonia to form carbamoyl phosphate