Arginosuccinate, from aspartate

Another reaction which leads to the formation of PP from ATP is the synthesis of arginosuccinate from citrulline and aspartate. 

The transfer of an amino group from aspartate to citrulline involves the participation of ATP, which simultaneously is converted to ADP with formation of inorganic phosphate. The intermediary complex has been shown to have the composition of arginosuccinic acid with the (former) amino N of aspartate now linked to both partners of the complex. This complex is then hydrolyzed with liberation of fumarate and arginine. 

The term Krebs bicycle refers to two interlocking cyclic reaction pathways. The aspartate-arginosuccinate shunt of the citric acid cycle is responsible for regenerating the aspartate needed for the urea cycle from fumarate. The molecule that the two cycles have in common is arginosuccinate. 

The primary step in the urea cycle is the synthesis of carbamyl phosphate from ammonia and carbon dioxide (11.76). This first stage, and the later stage of synthesis of arginosuccinic acid from citrulline and aspartic acid, both require the transfer of energy from ATP hydrolysis. The pyrophosphate formed in the latter reaction is itself hydrolysed which, together with the former reaction. 

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