Q.32 During the biosynthesis of urea in the urea cycle, the two nitrogen atoms are derived from
(A) Two free ammonium groups
(B) Free ammonium group and aspartate
(C) Both nitrogen atoms are derived from arginine
(D) One nitrogen atom is derived from citrulline and one from glutamate.
Urea Cycle Nitrogen Entry
The urea cycle detoxifies ammonia in liver mitochondria/cytosol, producing urea for kidney excretion.
First nitrogen enters as NH₄⁺ forming carbamoyl phosphate (via CPS I), which combines with ornithine → citrulline.
Second nitrogen from aspartate (carrying NH₂ from glutamate transamination) joins in argininosuccinate synthetase step.
Correct Answer: NH₄⁺ + Aspartate
Option (B) Free ammonium group and aspartate accurately describes the direct donors.
NH₄⁺ → carbamoyl-P → urea’s carbon-attached N; aspartate → urea’s terminal N via fumarate release.
This links amino acid catabolism (glutamate → NH₄⁺/aspartate) to waste nitrogen disposal.
Options Analysis Table
| Option | Nitrogen Sources | Explanation |
|---|---|---|
| (A) Two free ammonium groups | NH₄⁺ only | Incorrect; second N comes from aspartate, not second NH₄⁺ |
| (B) Free ammonium + aspartate | NH₄⁺ & Asp | Correct; matches carbamoyl-P and argininosuccinate steps |
| (C) Both from arginine | Arg only | Wrong; arginine is cycle product, cleaved last to release urea |
| (D) Citrulline + glutamate | Cit & Glu | Incorrect; citrulline carries first N, glutamate feeds aspartate indirectly |
Common errors confuse direct donors with precursors or products.
Exam Relevance
Tests nitrogen metabolism integration in NEET/CSIR, linking TCA cycle (aspartate shuttle) to urea synthesis.
CPS I regulation by N-acetylglutamate responds to high protein intake.
