(a) How many ATP equivalents are consumed by the reactions of the urea cycle? (b) Operation of the urea cycle actually generates more ATP than it consumes. Explain.

The urea cycle is irreversible and consumes total of3 ATP molecules. Two ATP are utilized for the synthesis of carbamoyl phosphate in converting 2 ATP into 2ADP. One ATP is converted to AMP and PP_i to produce arginosuccinate which equals to 2 ATP.

Hence this process consumes equivalent of 4 ATP

Urea cyclecauses the production of 2 NADH, so the overall reaction releases slightly more energy than it consumes. The NADH is produced in two ways:

1 NADH molecule is produced by the enzymeglutamate dehydrogenase in the conversion of glutamate to ammonium andα-ketoglutarate.

The fumarate released in the cytosol is hydrated tomalate by cytosolicfumarase. This malate is then oxidized tooxaloacetate by cytosolicmalate dehydrogenase, generating a reduced NADH in the cytosol.

The two NADH produced can provide energy for the formation of 5ATP