Which is the common amino acid between urea cycle and TCA?

Ans. (a) AsparateRef: Satyanarayan Biochemistry, pg. 339Role of Aspartate in urea cycle:* Aspartate participates in urea cycle for condensation with citrulline to form Argino succinic acid. The reaction is catalyzed by Argino succinic acid synthetase (Asparate provides one out of the two nitrogens of urea, the other nitrogen is contributed by Glutamate).* In the subsequent reaction, Argino succinic acid undergoes a lytic reaction to form Arginine and fumarate. Fumarate forms a link between TCA cycle and urea. Fumarate can be recycled to form oxaloacetate, through intermediate formation of malate that can be transmmated to form aspartate, to get reutilized in the urea cycleLink between TCA and urea cycleRole of Aspartate in TCA cycle* Oxaloacetate, the keto acid obtained from Aspartate by transamination is an intermediate of TCA cycle.* Oxaloacetate can either be utilized in TCA cycle or be channeled to the pathway of gluconeogenesis.* Hence asparatate is a common intermediate of TCA cycle (through oxaloacetate) and urea cycle (directly)