Binding of N-acetyl-L-glutamate to rat liver carbamoyl phosphate synthetase (ammonia)

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Abstract
The binding of N-acetyl-L-glutamate, the physiological allosteric activator, to rat liver carbamoyl-phosphate synthetase (ammonia) was studied by techniques of rate of dialysis and of ultracentrifugation in the Airfuge. There is 1 binding site for acetylglutamate per enzyme monomer (MW 165,000). K+. Mg2+ (free) and ATP were required to demonstrate binding. The concentrations of ATP required indicate that binding of ATPA (the ATP molecule that yields Pi) is needed. HCO-3 was not essential, but it enhanced binding of acetylglutamate. Glycerol also favored binding. Plots of kd values vs. the reciprocal of free Mg2+ and ATP concentrations are linear and indicate that ATPA, K+ and Mg2+ bind before acetylglutamate. In the presence of these ligands and HCO-3, ammonia increased drastically the Kd value for acetylglutamate, whereas in absence of HCO-3 ammonia had little effect. This suggests that acetylglutamate dissociates with the products and explains the higher Km for acetylglutamate in the synthetase (overall) reaction than in the ATPase (partial) reaction. In the absence of ATP acetylglutamate was bound with high affinity if ADP and carbamoyl phosphate were present. ADP or carbamoyl phosphate alone did not promote substantial binding. Binding of acetylglutamate at low concentration was slow; it was accelerated at higher concentrations of the activator. Exchange of bound acetylglutamate with acetylglutamate in solution was fast. A scheme proposed earlier for allosteric activation of the enzyme is refined to incorporate the new information. Binding of ATPA, K+ and Mg2+ and formation of ''active CO2'' (the central complex) are greatly favored by acetylglutamate.