Metabolic Integration — MCQs

Question 1: Which amino acid is common to both the urea cycle and the TCA cycle?

• A. Aspartate (Correct Answer)
• B. Alanine
• C. Asparagine
• D. Glutamate

Explanation: ### Explanation The correct answer is **Aspartate**. **Why Aspartate is the Correct Answer:** The Urea cycle and the TCA cycle are interconnected through what is known as the **"Kreb’s Bicycle"** or the **Aspartate-Argininosuccinate Shunt**. 1. **In the Urea Cycle:** Aspartate enters the cycle by reacting with citrulline to form argininosuccinate (catalyzed by argininosuccinate synthetase). It provides the second nitrogen atom required for urea synthesis. 2. **In the TCA Cycle:** Oxaloacetate (a TCA intermediate) can be converted into Aspartate via **transamination** (catalyzed by AST/GOT). Conversely, the fumarate produced in the urea cycle can be recycled back into oxaloacetate to regenerate aspartate. **Why the Other Options are Incorrect:** * **Alanine:** Primarily involved in the **Cahill cycle** (Glucose-Alanine cycle) for transporting nitrogen from muscles to the liver. It is not a direct intermediate or substrate in the urea cycle. * **Asparagine:** While structurally related to aspartate, it must first be hydrolyzed to aspartate by asparaginase to enter these metabolic pathways. * **Glutamate:** Although glutamate provides the first nitrogen (via oxidative deamination to produce ammonia) and is the donor for transamination to form aspartate, it is not a direct component of the TCA cycle itself (its keto-acid, **α-ketoglutarate**, is). **High-Yield NEET-PG Pearls:** * **Fumarate** is the other molecule connecting the two cycles; it is produced in the urea cycle and enters the TCA cycle. * **ATP Requirement:** The synthesis of one molecule of urea consumes **4 high-energy phosphates** (3 ATP are used, but one is cleaved to AMP + PPi). * **Rate-limiting step:** Carbamoyl phosphate synthetase I (CPS-I) is the rate-limiting enzyme of the urea cycle, activated by **N-acetylglutamate (NAG)**.