the urea cycle (also known as the ornithine cycle) is a cycle of biochemical reactions occurring in many animals that produces urea ((nh2)2co) from ammonia (nh3). this cycle was the first metabolic cycle discovered (hans krebs and kurt henseleit, 1932), five years before the discovery of the tca cycle. in mammals, the urea cycle takes place primarily in the liver, and to a lesser extent in the kidney. ==function== organisms that cannot easily and quickly remove ammonia usually have to convert it to some other substance, like urea or uric acid, which are much less toxic. insufficiency of the urea cycle occurs in some genetic disorders (inborn errors of metabolism), and in liver failure. the result of liver failure is accumulation of nitrogenous waste, mainly ammonia, which leads to hepatic encephalopathy. ==reactions== the urea cycle consists of five reactions: two mitochondrial and three cytosolic. the cycle converts two amino groups, one from nh4+ and one from asp, and a carbon atom from hco3â, to the relatively nontoxic excretion product urea at the cost of four "high-energy" phosphate bonds (3 atp hydrolyzed to 2 adp and one amp). ornithine is the carrier of these carbon and nitrogen atoms. {| class="wikitable" |+ reactions of the urea cycle ! step || reactants || products || catalyzed by || location |- ! 1 | nh3 + hco3â + 2atp || carbamoyl phosphate + 2adp + pi || cps1 || mitochondria |- ! 2 | carbamoyl phosphate + ornithine || citrulline + pi || otc || mitochondria |- ! 3 | citrulline + aspartate + atp || argininosuccinate + amp + ppi || ass || cytosol |- ! 4 | argininosuccinate || arg + fumarate || asl || cytosol |- ! 5 | arg + h2o || ornithine + urea || arg1 || cytosol |} ::the reactions of the urea cycle thumb|480px|left| 1 l-ornithine2 carbamoyl phosphate3 l-citrulline4 argininosuccinate5 fumarate6 l-arginine7 urea l-asp l-aspartatecps-1 carbamoyl phosphate synthetase iotc ornithine transcarbamoylaseass argininosuccinate synthetaseasl argininosuccinate lyasearg1 arginase 1  in the first reaction, nh4+ + hco3â is equivalent to nh3 + co2 + h2o. thus, the overall equation of the urea cycle is: * nh3 + co2 + aspartate + 3 atp 
Nitrogen_metabolism Urea_cycle