Q.48 Enzymes involved in urea biosynthesis from ammonia are listed below.
Arrange these in order of their actions:
A. Arginase
B. Ornithine carbamoyl transferase
C. Argininosuccinate lyase
D. Carbamoyl phosphate synthetase I
E. Argininosuccinate synthetase
Choose the correct answer from the options given below.
- E, D, C, B, A
- D, A, B, C, E
- A, B, C, D, E
- D, B, E, C, A
The correct sequence of enzymes in urea biosynthesis is D, B, E, C, A. This follows the standard urea cycle pathway where carbamoyl phosphate is first synthesized, leading sequentially to citrulline, argininosuccinate, arginine, and finally urea.
Urea Cycle Overview
The urea cycle detoxifies ammonia in hepatocytes via five enzymatic steps across mitochondria and cytosol. It consumes 4 ATP equivalents per urea molecule and links to the citric acid cycle via fumarate.
Correct Sequence: D → B → E → C → A
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D. Carbamoyl phosphate synthetase I (CPS I): Mitochondria; NH₄⁺ + HCO₃⁻ + 2ATP → carbamoyl phosphate (rate-limiting, NAG-activated).
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B. Ornithine carbamoyl transferase (OTC): Mitochondria; carbamoyl phosphate + ornithine → citrulline.
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E. Argininosuccinate synthetase: Cytosol; citrulline + aspartate + ATP → argininosuccinate.
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C. Argininosuccinate lyase: Cytosol; argininosuccinate → arginine + fumarate.
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A. Arginase: Cytosol; arginine → urea + ornithine (cycle regenerates).
Option Analysis
Option Sequence Explanation E, D, C, B, A Wrong order Starts with synthetase (step 3); skips proper initiation . D, A, B, C, E Wrong order Places arginase (last) too early . A, B, C, D, E Wrong order Begins with final arginase; reverses cycle . D, B, E, C, A Correct Matches mitochondrial start (D-B), cytosolic continuation (E-C-A) . Urea biosynthesis enzymes order is crucial for NEET/GATE Life Sciences, detailing the five-step cycle converting toxic ammonia to urea in liver cells.
Step-by-Step Enzyme Actions
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Carbamoyl phosphate synthetase I (D): Forms carbamoyl phosphate from ammonia; rate-limiting mitochondrial step.
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Ornithine carbamoyl transferase (B): Yields citrulline; also mitochondrial.
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Argininosuccinate synthetase (E): Adds aspartate nitrogen; cytosol, ATP-dependent.
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Argininosuccinate lyase (C): Produces arginine + fumarate; cytosol.
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Arginase (A): Hydrolyzes arginine to urea + ornithine; completes cycle.
Exam Relevance
This sequence tests pathway recall; disruptions cause hyperammonemia. Mnemonics like “CPS-OTC-ASS-ASL-Arginase” aid memory.
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