Ans. is 'c' i.e., Urea cycle o Carbamyl phosphate synthase I - Urea cycleo Carbamyl phosphate synthase II - Pyrimidine synthesisUrea cvcleo Ammonia is ultimately disposed of by formation of urea by "Kreb's Henseleit urea cycle" in the liverr. Urea cycle takes place both in mitochondria and cytosol. First two reactions of urea cycle occur in the mitochondria, and remaining reactions occurs in cytosolr.o Biosynthesis of urea occurs in five steps.Carbamoyl phosphate synthase-I (CPS-I), a mitochondrial enzyme, catalyzes the formation of carbamoyl phosphater by condensation of CO2 and ammonia. Two molecules of ATP are required for the reaction. CPS-I is the rate limiting enzyme of urea cycle0. It is an allosteric enzyme and allosterically activated by N-acetyl glutamater.Ornithine transcarbamoylaser catalyzes the formation of citruliine from carbamoyl phosphate and ornithiner.Arginosuccinate synthase6 catalyzes the formation of arginosuccinate from citruliine and aspartate. This reaction requires 1ATP, but 2 high energy phosphate bonds are consumed as ATP is converted to AMP + PPi. The amino group of aspartate provides one of the two nitrogen atoms that appear in urear (The other one is provided by ammonia NH4 r).Arginosuccinate lyase (arginosuccinase) catalyses the cleavage of arginosuccinate into arginine and fumarate. Fumarate enters in TCA cycle.Arginaser catalyses the formation of urea from argininer by hydrolytic cleavage of arginine to yield urea and ornithine. Ornithine is thus regenerated and can enter mitochondria to initiate another round of the urea cycle.o The source of two nitrogen atoms of urea one from ammoniar and one from amino group of aspartate6. Source of carbon is CO2r. Thus out of all amino acids in%rolved in urea cycle aspartate is consumed, while there is no net loss or gain of ornithine, citruliine, arginosuccinate or arginine. There is production of fumarate as by productr.o 3 molecules of ATP are consumed (2 in first reaction and 1 in third reaction). However, 4 high energy phosphate bonds are utilized as 3rd ATP is converted to AMP+PPi.