Urea is formed from which substrate:

Ans. (d) AspartateRef: Harper's Biochemistry, 30th ed. pg. 294-95NH3 +HCO3 +Aspartate+3ATP-Urea+Fumarate+2ADP+AMP+4P.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.