218 Essays in Biochemistry 



interrupt the steps of the cycle, to dissociate the respiratory mecha- 

 nisms to which they are linked, and, finally, through enzyme isolation, 

 to examine the nature of the individual reactions. 2 7 



Phosphate Bond Energy and Formation of C-N Bonds 



The synthesis of urea is known to require the expenditure of a large 

 amount of energy, and one of the major questions concerning this 

 problem has been the source of the energy and the manner of its 

 utilization. The energy liberated in oxidative metabolism is made 

 available for synthetic needs as "high-energy" (pyrophosphate) bonds 

 largely in the form of adenosine triphosphate (ATP). It turns out 

 that the energy is utilized as ATP in the synthesis of citrulline and 

 arginine. During their formation, nitrogen becomes attached to carbon 

 with the aid of mechanisms which couple phosphate-bond energy to 

 the formation of certain types of C-N bonds. 



Conversion of Ornithine to Citrulline 



Citrulline formation involves several enzymatic steps. In reactions 

 la and lb, CO2 and NH 3 form carbamic acid; this compound then 

 reacts with ATP in the presence of G, which can be any one of several 

 N-acyl derivatives of glutamic acid (acetyl, chloroacetyl, carbamyl, 

 formyl, or propionyl), to form an unstable intermediate. Another 



NH 3 + C0 2 ^ H 2 NCOOH (la) 







II II 



H 2 NCOOH + C0 2 + ATP + G ^ H 2 NC-0— P(OH) 2 — -G + ADP 



(16) 



enzyme catalyzes the formation of citrulline from ornithine and the 

 unstable intermediate, as shown in reaction 2. Orthophosphate and 

 the glutamic acid derivative are liberated at the same time. 



O O 



II I 



H 2 NC-0-P(OH) 2 — -G + NH 2 ?± HN-C-NH 2 + H 3 P0 4 + G 



(CH 2 ) 3 (CH 2 ) 3 (2) 



NH 2 CHCOOH NH 2 CHCOOH 



Carbamyl phosphate Ornithine Citrulline Glutamic acid 



or compound X derivative 



The constitution of the intermediate is not yet known and the com- 

 pound is therefore referred to as compound X by Grisolia and Cohen 



