<prompt> Urea is formed from which substrate:ArginineOrginineCitrullineAspartate  <response>: Ans. d AspartateRef: Harpers Biochemistry, 30th ed. pg. 294-95NH3 HCO3 Aspartate3ATP-UreaFumarate2ADPAMP4P.UREA CYCLE Urea Is the Major End Product of Nitrogen Catabolism in Humans Some reactions of urea synthesis occur in the matrix of the mitochondrion, other reactions in the cytosol Synthesis of 1 mol of urea requires 3 mol of ATP plus 1 mol each of ammonium ion and of the amino nitrogen of aspartate.STEPS OF UREA METABOLISMStep -1: Site: Mitochondria Carbamoyl Phosphate Synthase I Initiates Urea Biosynthesis Condensation of CO2 , ammonia, and ATP to form carbamoyl phosphate is catalyzed by mitochondrial carbamoyl phosphate synthase I. Carbamoyl phosphate synthase I, the rate-limiting enzyme of the urea cycle, is active only in the presence of its allosteric activator N-acetyl glutamate The step requires 2 ATPsStep - 2: Site: Mitochondria Carbamoyl Phosphate Plus Ornithine Forms Citrulline L-Ornithine transcarbamoylase catalyzes the reaction Next 3 steps take place in cytosolStep -3: Site: Cytosol Citrulline plus Aspartate forms Argininosuccinate Argininosuccinate synthase links aspartate and Citrulline via the amino group of aspartate and provides the second nitrogen of urea. The reaction requires ATPStep: 4: Site: Cytosol Cleavage of Argininosuccinate Forms Arginine Fumarate catalyzed by argininosuccinase.Step: 5: Site: Cytosol Cleavage of Arginine Releases Urea Re-Forms Ornithine Hydrolytic cleavage of the guanidino group of arginine, catalyzed by liver arginase, releases urea The other product, ornithine, reenters liver mitochondria for additional rounds of urea synthesis. 