provide energy for the formation of 5 ATP (cytosolic NADH provides 2.5 ATP with the malate-aspartate shuttle in human liver cell), a net production of two high-energy phosphate bond for the urea cycle. However, if gluconeogenesis is underway in the cytosol, the latter reducing equivalent is used to drive the reversal of the GAPDH step instead of generating ATP. The fate of oxaloacetate is either to produce aspartate via transamination or to be converted to phosphoenolpyruvate, which is a substrate for gluconeogenesis. Products of the urea cycle As stated above many vertebrates use the urea cycle to create urea out of ammonium so that the ammonium does not damage the body. Though this is helpful, there are other effects of the urea cycle. For example: consumption of two ATP, production of urea, generation of H+, the combining of and to forms where it can be regenerated, and finally the consumption of . Regulation N-Acetylglutamic acid The synthesis of carbamoyl phosphate and the urea cycle are dependent on the presence of N-acetylglutamic acid (NAcGlu), which allosterically activates CPS1. NAcGlu is an obligate activator of carbamoyl phosphate synthetase. Synthesis of NAcGlu by N-acetylglutamate synthase (NAGS) is stimulated by both Arg, allosteric stimulator of NAGS, and Glu, a product in the transamination reactions and one of NAGS's substrates, both of which are elevated when free amino acids are elevated. So Glu not only is a substrate for NAGS but also serves as an activator for the urea cycle. Substrate concentrations The remaining enzymes of the cycle are controlled by the concentrations of their substrates. Thus, inherited deficiencies in cycle enzymes other than ARG1 do not result in significant decreases in urea production (if any cycle enzyme is entirely missing, death occurs shortly after birth). Rather, the deficient enzyme's substrate builds up, increasing the rate of the deficient reaction to normal. The anomalous substrate buildup is not without cost, however. The substrate concentrations become elevated all the way back up the cycle to , resulting in hyperammonemia (elevated []P). Although the root cause of toxicity is not completely understood, a high [] puts an enormous strain on the -clearing system, especially in the brain (symptoms of urea cycle enzyme deficiencies include intellectual disability and lethargy). This clearing system involves GLUD1 and GLUL, which decrease the 2-oxoglutarate (2OG) and Glu pools. The brain is most sensitive to the depletion of these pools. Depletion of 2OG