Carbamoyl phosphate synthase - 1 acts in -

Ans. is 'c' i.e., Urea cycle o Biosynthesis of urea occurs in five steps.Carbamoyl phosphate synthase-I (CPS-I), a mitochondrial enzyme, catalyzes the formation of carbamoyl phosphate by condensation of CO, and ammonia. Two molecules of ATP are required for the reaction. CPS- I is the rate limiting enzyme of urea cycle, It is an allosteric enzyme and allosterically activated by N- acetyl glutamate.o There is one cytosolic carbamyl phosphate synthase-II (CPS-II) which uses glutamine rather than ammonia as the nitrogen donor and functions in pyrimidine synthesis.Ornithine transcarbamoylase catalyzes the formation of citrulline from carbamoyl phosphate and ornithine.Arginosuccinate synthase catalyzes the formation of arginosuccinate from citrulline and aspartate. This reaction requires I ATP, 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 urea (The other one is provided by ammonia NH4).Arginosuccinate lyase (arginosuccinase) catalyses the cleavage of arginosuccinate into arginine and fumarate. Fumarate enters in TCA cycle.Arginase catalyses the formation of urea from arginine 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.