Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 361: Which amino acid does not participate in one-carbon transfer reactions?

A. Glycine
B. Serine
C. Alanine
D. Histidine (Correct Answer)

Explanation: **Explanation:** One-carbon (1C) metabolism involves the transfer of single carbon units (such as methyl, methylene, or formyl groups) via carriers like **Tetrahydrofolate (THF)** and **S-adenosylmethionine (SAM)**. These units are essential for the synthesis of DNA, RNA, and amino acids. **Why Histidine is the Correct Answer:** Actually, there is a nuance in this question. In classical biochemistry, **Histidine** *does* participate in 1C metabolism; it is catabolized to Formiminoglutamate (FIGLU), which donates a formimino group to THF to become **N5-formimino-THF**. However, in the context of many standardized exams, **Alanine** is typically the amino acid that has **no role** in 1C transfer. Alanine undergoes simple transamination to Pyruvate. *Note: If the provided key marks Histidine as the answer, it may be based on a specific textbook interpretation or a focus on "direct" methyl/methylene donation. However, biochemically, Alanine is the most common "distractor" as it never enters the 1C pool.* **Analysis of Options:** * **Glycine:** A major 1C donor. The Glycine Cleavage System breaks it down into CO₂, NH₄⁺, and a **methylene** group (-CH₂-) which is transferred to THF. * **Serine:** The **most important** source of 1C units. It reacts with THF via *Serine Hydroxymethyltransferase* to form Glycine and **N5, N10-methylene-THF**. * **Alanine:** Does not donate or carry 1C units; it is purely glucogenic via Pyruvate. **High-Yield Clinical Pearls for NEET-PG:** 1. **FIGLU Excretion Test:** In **Folic acid deficiency**, FIGLU cannot donate its 1C unit to THF and is excreted in the urine after a Histidine load. 2. **The "Methyl Trap":** Vitamin B12 deficiency leads to a functional folate deficiency because 5-methyl-THF cannot transfer its methyl group to Homocysteine. 3. **Key Donors:** Remember the mnemonic **"SHG"** (Serine, Histidine, Glycine) as the primary donors to the THF pool.

Question 362: In "Maple Syrup Urine Disease", which amino acids are excreted in the urine?

A. Valine, Leucine, Isoleucine (Correct Answer)
B. Phenylalanine, Leucine, Isoleucine
C. Tyrosine, Leucine, Isoleucine
D. Histidine, Leucine, Valine

Explanation: **Explanation:** Maple Syrup Urine Disease (MSUD) is an autosomal recessive metabolic disorder caused by a deficiency in the **Branched-Chain Alpha-Keto Acid Dehydrogenase (BCKAD) complex**. This multi-enzyme complex is responsible for the oxidative decarboxylation of the keto-acid derivatives of the three **branched-chain amino acids (BCAAs)**: **Valine, Leucine, and Isoleucine**. 1. **Why Option A is correct:** When the BCKAD complex is defective, the BCAAs and their corresponding alpha-keto acids (alpha-ketoisovalerate, alpha-ketoisocaproate, and alpha-keto-beta-methylvalerate) accumulate in the blood and spill over into the urine. The characteristic "maple syrup" or burnt sugar odor of the urine is specifically due to the accumulation of **isoleucine** derivative (alpha-keto-beta-methylvalerate). 2. **Why other options are incorrect:** * **Phenylalanine (Option B):** Accumulates in Phenylketonuria (PKU), not MSUD. * **Tyrosine (Option C):** Accumulates in Tyrosinemia. * **Histidine (Option D):** Accumulates in Histidinemia. While Leucine and Valine are present in these options, the inclusion of non-branched-chain amino acids makes them incorrect. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** Remember **"LIV"** (Leucine, Isoleucine, Valine) for MSUD. * **Cofactors:** The BCKAD complex requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (Mnemonic: **T**ender **R**eves **N**icely **P**lays **L**otto). * **Treatment:** Dietary restriction of BCAAs. Some patients respond to high doses of **Thiamine** (Thiamine-responsive MSUD). * **Diagnosis:** Elevated levels of BCAAs in plasma and presence of **alloisoleucine** (pathognomonic).

Question 363: Carbamoyl phosphate synthetase I is stimulated by which of the following?

A. ATP
B. Arginosuccinate
C. Carbamoyl phosphate
D. N-acetyl glutamate (Correct Answer)

Explanation: **Explanation:** **Carbamoyl Phosphate Synthetase I (CPS-I)** is the rate-limiting and first committed enzyme of the **Urea Cycle**, occurring within the mitochondria of hepatocytes. Its primary role is to convert ammonia, bicarbonate, and ATP into carbamoyl phosphate. **Why N-acetyl glutamate (NAG) is correct:** CPS-I is an allosteric enzyme that is **obligatorily activated** by N-acetyl glutamate. Without NAG, CPS-I is virtually inactive. NAG acts as a signal of high amino acid availability; it is synthesized from glutamate and acetyl-CoA by the enzyme *N-acetylglutamate synthase (NAGS)*, which is itself stimulated by **Arginine**. Thus, high protein intake leads to increased NAG, which "switches on" the urea cycle to dispose of excess nitrogen. **Why other options are incorrect:** * **A. ATP:** While ATP is a required **substrate** for the reaction (2 molecules are consumed), it is not the specific allosteric activator that regulates the enzyme's flux. * **B. Arginosuccinate:** This is an intermediate later in the urea cycle. While it is part of the pathway, it does not exert feedback or feed-forward control on CPS-I. * **C. Carbamoyl phosphate:** This is the **product** of the reaction. High levels would typically inhibit a reaction via the law of mass action, not stimulate it. **Clinical Pearls & High-Yield Facts:** * **CPS-I vs. CPS-II:** Do not confuse them. CPS-I is mitochondrial and involved in the **Urea Cycle** (activated by NAG). CPS-II is cytosolic and involved in **Pyrimidine Synthesis** (inhibited by UTP). * **Hyperammonemia Type I:** Caused by a deficiency of CPS-I. It is the most severe urea cycle disorder, presenting with lethargy, seizures, and coma in neonates. * **NAGS Deficiency:** Mimics CPS-I deficiency clinically because NAG is absent, leaving CPS-I inactive. It can be treated with **Carglumic acid**, a functional analog of NAG.

Question 364: Which of the following is a non-essential amino acid?

A. Tryptophan
B. Tyrosine (Correct Answer)
C. Arginine
D. Histidine

Explanation: **Explanation:** Amino acids are classified based on their dietary requirement into **Essential** (must be obtained from diet) and **Non-essential** (can be synthesized by the body). **Why Tyrosine is the Correct Answer:** **Tyrosine** is a non-essential amino acid because it is synthesized in the human body from the essential amino acid **Phenylalanine** via the enzyme *phenylalanine hydroxylase*. Since the body can produce it endogenously, it does not strictly need to be consumed in the diet, provided there is an adequate supply of phenylalanine. **Analysis of Incorrect Options:** * **Tryptophan (A):** This is a strictly **essential** amino acid. It is a precursor for Serotonin, Melatonin, and Niacin (Vitamin B3). * **Arginine (C) & Histidine (D):** These are classified as **Semi-essential (or Conditionally Essential)** amino acids. They are synthesized in the body, but the rate of synthesis is insufficient to meet demands during periods of rapid growth (infancy) or severe physiological stress. In the context of standard MCQ classification, they are grouped with essential amino acids. **NEET-PG High-Yield Pearls:** 1. **Mnemonic for Essential Amino Acids:** "PVT TIM HALL" (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine). 2. **Clinical Correlation:** In **Phenylketonuria (PKU)**, the enzyme phenylalanine hydroxylase is deficient. Consequently, Tyrosine cannot be synthesized and becomes a **conditionally essential** amino acid for these patients. 3. **Purely Ketogenic Amino Acids:** Leucine and Lysine (The "L"s). 4. **Both Glucogenic and Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, Isoleucine (The "Aromatics" + Isoleucine).

Question 365: Which of the following amino acids is essential for the synthesis of nitric oxide?

A. Arginine (Correct Answer)
B. Aspartate
C. Alanine
D. Glycine

Explanation: ### Explanation **1. Why Arginine is Correct:** Arginine is the sole precursor for the synthesis of **Nitric Oxide (NO)**, a potent vasodilator and signaling molecule. This reaction is catalyzed by the enzyme **Nitric Oxide Synthase (NOS)**. In this process, L-arginine is converted into **L-citrulline** and NO in the presence of oxygen and several cofactors (NADPH, FAD, FMN, and Tetrahydrobiopterin/BH4). Nitric oxide plays a critical role in vascular homeostasis, neurotransmission, and immune response. **2. Why the Other Options are Incorrect:** * **Aspartate:** While it participates in the Urea cycle (condensing with citrulline to form argininosuccinate) and the Malate-aspartate shuttle, it is not a substrate for NO synthesis. * **Alanine:** Primarily functions as a major carrier of nitrogen from muscles to the liver via the Cahill cycle (Glucose-Alanine cycle). * **Glycine:** A precursor for heme, purines, creatine, and glutathione, but it does not contribute to the production of nitric oxide. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cofactor Alert:** **BH4 (Tetrahydrobiopterin)** is a frequently tested cofactor required for NOS; its deficiency can impair NO production. * **Isoforms of NOS:** There are three types: **eNOS** (endothelial - vasodilation), **nNOS** (neuronal - neurotransmission), and **iNOS** (inducible - macrophage bactericidal activity). * **The "Arginine Paradox":** Even though intracellular arginine levels are usually high, exogenous administration of arginine can still increase NO production in certain clinical states. * **Vascular Effect:** NO acts by stimulating **Guanylyl Cyclase**, increasing **cGMP**, which leads to smooth muscle relaxation. This is the mechanism behind drugs like Sildenafil and Nitroglycerin.

Question 366: The ferric chloride test in phenylketonuria yields what color result?

A. Green (Correct Answer)
B. Red
C. Yellow
D. Blue

Explanation: **Explanation:** **1. Why Green is Correct:** In Phenylketonuria (PKU), a deficiency of the enzyme **phenylalanine hydroxylase** leads to the accumulation of phenylalanine. This excess phenylalanine is alternatively metabolized into **phenylpyruvate** (a phenylketone), which is excreted in the urine. When **Ferric Chloride ($FeCl_3$)** is added to the urine of a PKU patient, the ferric ions react with the enol group of phenylpyruvate to form a characteristic **transient olive-green** color. This serves as a classic, though non-specific, bedside screening test. **2. Why Other Options are Incorrect:** * **Red:** A wine-red or reddish-brown color with ferric chloride is seen in the presence of **Acetoacetate** (ketoacidosis) or salicylates. * **Yellow:** This is generally the color of the reagent itself or a negative result; it does not indicate the presence of phenylpyruvic acid. * **Blue:** A blue-green color can sometimes be seen in **Tyrosinemia**, but the classic "green" description is specifically associated with PKU in medical examinations. **3. Clinical Pearls for NEET-PG:** * **Guthrie Test:** The definitive neonatal screening test for PKU; it is a semi-quantitative bacterial inhibition assay. * **Mousy Odor:** The urine and sweat of PKU patients have a characteristic "mousy" or "musty" odor due to phenylacetic acid. * **Associated Findings:** Intellectual disability, hypopigmentation (due to decreased melanin), and "fair hair/blue eyes." * **Ferric Chloride in Alkaptonuria:** Produces a **transient deep blue/green** color (due to homogentisic acid).

Question 367: In a seriously ill patient, the addition of amino acids to the diet results in a positive nitrogen balance. What is the mechanism for this effect?

A. Increased growth hormone secretion
B. Enhanced rate of gluconeogenesis
C. Increased absorption of amino acids from the diet
D. Increased secretion of insulin (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Increased secretion of insulin.** **Mechanism:** Nitrogen balance is the difference between nitrogen intake (protein) and nitrogen excretion. A **positive nitrogen balance** occurs when the body retains more nitrogen than it excretes, indicating a state of net protein synthesis (anabolism). When amino acids (especially branched-chain amino acids like Leucine) are added to the diet, they act as potent secretagogues for **insulin** from the pancreatic beta cells. Insulin is the body’s primary anabolic hormone. It promotes a positive nitrogen balance by: 1. Increasing the cellular uptake of amino acids. 2. Stimulating ribosomal protein synthesis. 3. Inhibiting protein degradation (proteolysis). **Analysis of Incorrect Options:** * **A. Growth Hormone (GH):** While GH is anabolic, its secretion is primarily stimulated by hypoglycemia or specific amino acids (like Arginine) under fasting conditions, not simply by general dietary supplementation in a clinical setting. * **B. Enhanced Gluconeogenesis:** This is a catabolic process where amino acids are broken down to produce glucose. This would lead to increased urea production and a **negative** nitrogen balance. * **C. Increased Absorption:** While absorption is necessary, it is a transport mechanism, not the metabolic *driver* of protein synthesis or nitrogen retention. **NEET-PG High-Yield Pearls:** * **Positive Nitrogen Balance:** Seen in growing children, pregnancy, and recovery from illness/trauma. * **Negative Nitrogen Balance:** Seen in starvation, severe burns, uncontrolled diabetes, and cancer cachexia. * **Insulin vs. Glucagon:** Insulin promotes protein synthesis (anabolic), whereas Glucagon promotes amino acid breakdown for gluconeogenesis (catabolic). * **Arginine:** Often used in clinical tests to stimulate Growth Hormone release.

Question 368: Deficiency of which enzyme leads to Tyrosinemia type II?

A. Fumarylacetoacetate hydrolase
B. Phenylalanine hydroxylase
C. Tyrosine transaminase (Correct Answer)
D. Tyrosinase

Explanation: **Explanation:** Tyrosinemia Type II (also known as **Richner-Hanhart Syndrome**) is caused by a deficiency of the enzyme **Tyrosine transaminase** (specifically, hepatic tyrosine aminotransferase). This enzyme is responsible for the first step of tyrosine catabolism, converting tyrosine into p-hydroxyphenylpyruvate. Its deficiency leads to significant hypertyrosinemia, which results in characteristic clinical findings: **palmoplantar keratosis** (painful skin lesions) and **corneal ulcers/dystrophy**. **Analysis of Incorrect Options:** * **A. Fumarylacetoacetate hydrolase:** Deficiency of this enzyme leads to **Tyrosinemia Type I** (Hepatorenal Tyrosinemia). This is the most severe form, characterized by cabbage-like odor, liver failure, and renal tubular acidosis (Fanconi syndrome). * **B. Phenylalanine hydroxylase:** Deficiency leads to **Phenylketonuria (PKU)**. This enzyme normally converts phenylalanine to tyrosine; its absence causes intellectual disability and a "mousy" body odor. * **D. Tyrosinase:** Deficiency of this copper-containing enzyme leads to **Oculocutaneous Albinism**, as it is essential for converting tyrosine into melanin. **High-Yield Clinical Pearls for NEET-PG:** * **Tyrosinemia Type III:** Caused by a deficiency of **p-hydroxyphenylpyruvate dioxygenase**. * **Alkaptonuria:** Caused by a deficiency of **Homogentisate oxidase** (look for black urine on standing and ochronosis). * **Dietary Management:** For Tyrosinemia Type II, the treatment involves a diet low in both **Phenylalanine and Tyrosine**. * **Diagnostic Marker:** Elevated serum tyrosine levels without the presence of succinylacetone (which is specific to Type I).

Question 369: Phenylketonuria (PKU) is a congenital amino acid metabolic disorder. In one of the rare variants of PKU, dihydropterin synthesis is affected. Which enzyme is deficient in this variant?

A. Histidine decarboxylase
B. Phenylalanine hydroxylase
C. Dihydropterin reductase (Correct Answer)
D. Tyrosine deficiency

Explanation: **Explanation:** **Phenylketonuria (PKU)** is primarily caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)**, which converts Phenylalanine to Tyrosine. However, this reaction requires a vital co-factor: **Tetrahydrobiopterin (BH4)**. **Why Option C is correct:** In the "classic" form of PKU, the PAH enzyme itself is defective. However, in **rare/malignant variants**, the defect lies in the metabolism of the co-factor BH4. BH4 must be regenerated from Dihydrobiopterin (BH2) by the enzyme **Dihydropterin reductase**. If this enzyme is deficient, BH4 levels fall, leading to a secondary failure of Phenylalanine Hydroxylase activity. These variants are clinically more severe because BH4 is also a required co-factor for the synthesis of neurotransmitters like Dopamine and Serotonin. **Why other options are incorrect:** * **Option A (Histidine decarboxylase):** This enzyme converts Histidine to Histamine; it is unrelated to Phenylalanine metabolism. * **Option B (Phenylalanine hydroxylase):** Deficiency of this enzyme causes "Classic PKU," not the rare variant involving dihydropterin synthesis. * **Option D (Tyrosine deficiency):** While Tyrosine becomes an essential amino acid in PKU patients, its deficiency is a *consequence* of the metabolic block, not the *enzymatic cause* of the disorder. **High-Yield Clinical Pearls for NEET-PG:** * **Mousiness/Musty Odor:** Characteristic of PKU due to Phenylacetate in sweat and urine. * **Guthrie Test:** A bacterial inhibition assay used for neonatal screening of PKU. * **Malignant PKU:** Unlike classic PKU, these variants do not respond to simple dietary restriction of Phenylalanine because neurotransmitter synthesis remains impaired. Treatment requires BH4 supplementation and neurotransmitter precursors (L-Dopa, 5-HTP).

Question 370: The acidic property of an amino acid's alpha-carbon is primarily due to which functional group?

A. Carboxyl group (Correct Answer)
B. Amino group
C. Hydrogen atom
D. None of the above

Explanation: **Explanation:** The acidity of an amino acid is fundamentally determined by its ability to donate a proton ($H^+$). In the structure of an alpha-amino acid, the **carboxyl group (-COOH)** acts as the primary acidic functional group. At physiological pH (~7.4), the carboxyl group dissociates into a negatively charged carboxylate ion ($-COO^-$) and a proton ($H^+$). This dissociation is what defines its acidic property. Furthermore, the presence of the nearby electronegative amino group exerts an inductive effect, which stabilizes the carboxylate ion and makes the carboxyl group even more acidic than in simple organic acids. **Analysis of Options:** * **Amino group (Option B):** This is a **basic** functional group. It acts as a proton acceptor, becoming positively charged ($-NH_3^+$) at physiological pH. * **Hydrogen atom (Option C):** The alpha-hydrogen is relatively inert and does not contribute to the acid-base properties of the molecule under biological conditions. * **Carboxyl group (Option A):** Correct. It has a low $pK_a$ (typically around 2.0), meaning it readily loses a proton. **High-Yield NEET-PG Pearls:** * **Zwitterion:** At a specific pH (isoelectric point or pI), an amino acid exists as a dipolar ion with a net charge of zero. * **Amphoteric Nature:** Amino acids can act as both acids and bases because they possess both carboxyl and amino groups. * **Buffering Action:** Amino acids can act as buffers; however, in proteins, the most significant buffering capacity at physiological pH comes from the **imidazole group of Histidine**. * **Titration Curves:** The $pK_1$ always refers to the dissociation of the alpha-carboxyl group, while $pK_2$ refers to the alpha-amino group.

Practice by Chapter

Protein Digestion and Absorption

Practice Questions

Transamination and Deamination

Practice Questions

Urea Cycle

Practice Questions

Disorders of Urea Cycle

Practice Questions

Metabolism of Individual Amino Acids

Practice Questions

Inborn Errors of Amino Acid Metabolism

Practice Questions

Phenylketonuria and Alkaptonuria

Practice Questions

Homocystinuria and Methionine Metabolism

Practice Questions

Synthesis of Biologically Important Compounds from Amino Acids

Practice Questions

Nitrogen Balance

Practice Questions

Ammonia Metabolism and Toxicity

Practice Questions

One-Carbon Transfer Reactions

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