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... resembling that of oxaloacetate in the citric acid cycle. A molecule of ornithine combines with one molecules of ammonia and one of CO2 to form citrulline. A second amino group is added to citrullinw to form arginine, which is then hydrolysed to yield urea, with regeneration of ornithine. Five enzymatic reactions take place in the urea cycle, the first two of which take place in mitochondria, the other three in the cytosol. Firstly carbamoyl phosphate synthesase, this is not technically part of the urea cycle, it catalyses the condensation and activation of ammonia from the deamination of glutamate by glutamate dehydrogenase, and CO2 (in the form of bicarbonate, HCO3-) to form carbamoyl phosphate. The hydrolysis of two ATP molecules makes this reaction essentially irreversible. The second reaction also occurs in the mitochondria and involves the transfer of the carbamoyl group from carbamoyl phosphate to ornithine by ornithine transcarbamoylas. This reaction ...