Mechanism of activation of bicarbonate ion by mitochondrial carbamoyl-phosphate synthetase: formation of enzyme-bound adenosine diphosphate from the adenosine triphosphate that yields inorganic phosphate.

The mechanism of the reaction catalyzed by rat liver mitochondrial carbamoyl-phosphate synthetase has been studied by using [beta-18O2]ATP and HC18O-3, monitoring the isotopic composition of adenosine triphosphate (ATP) and inorganic phosphate (Pi) by high-resolution 31P NMR spectroscopy. In the presence of both HCO3- and acetylglutamate, the enzyme catalyzes the exchange of oxygen atoms between the beta, gamma bridging and the beta nonbridging positions of ATP. Addition of NH3 stops the exchange, Pi released by the ATPase activity of the enzyme in the absence of NH3 contains one oxygen atom from HC18O3- but there is no incorporation of 18O into ATP. There is no significant incorporation of [14C]ADP or 32Pi into ATP. It is concluded that in the enzyme-ATPA.HCO30.ATPB complex formed in the presence of ATP and HCO3- there is reversible transfer of the gamma-PO3 group of ATPA (the molecule that yields Pi) to HCO3- without dissociation of products. The beta-PO3 of the enzyme-bound ADP that is formed can rotate. Virtually all of the complex appears to be in the form in which ATPA is cleaved, but in the absence of NH3, ATP is reconstituted and dissociates from the complex on at least 75% of the occasions. On the remainder, the carbonyl phosphate is cleaved in an irreversible process that yields Pi and a low-energy form of carbonic acid (probably HCO3-). NH3 reacts rapidly and irreversibly with the complex, and at saturation the rate (greater than 10 times the rate of Pi release in the absence of NH3) is sufficient to prevent dissociation of ATPA. In the absence of HCO3- an enzyme-ATPA.ATPB complex is formed, but cleavage of the bond between beta, gamma bridging oxygen and P gamma of ATPA does not occur.