Amino Acid Metabolism — MCQs

Amino Acid Metabolism Indian Medical PG Practice Questions and MCQs

Question 341: Cysteine is formed from which amino acids?

A. Methionine and serine (Correct Answer)
B. Methionine and glycine
C. Alanine and glycine
D. Serine and glycine

Explanation: ### Explanation **Correct Option: A (Methionine and Serine)** Cysteine is a non-essential amino acid synthesized via the **transsulfuration pathway**. This process involves a unique "collaboration" between two amino acids: 1. **Methionine** provides the **sulfur atom**. Methionine is first converted to S-adenosylmethionine (SAM) and then to Homocysteine. 2. **Serine** provides the **carbon skeleton**. Homocysteine condenses with Serine to form Cystathionine (catalyzed by *Cystathionine $\beta$-synthase*). 3. Cystathionine is then cleaved by *Cystathionase* to release **Cysteine** and $\alpha$-ketobutyrate. **Why Incorrect Options are Wrong:** * **B, C, & D:** While Glycine and Alanine are involved in various metabolic pathways (like heme synthesis or gluconeogenesis), they do not contribute to the carbon skeleton or the sulfur group required for cysteine synthesis. Serine is specifically required for the carbon backbone, and Methionine is the only source of the thiol (-SH) group. **High-Yield Clinical Pearls for NEET-PG:** * **Essentiality:** Cysteine is considered "semi-essential." If Methionine intake is inadequate, Cysteine becomes essential. * **Rate-Limiting Enzyme:** *Cystathionine $\beta$-synthase* (CBS) is the key enzyme. It requires **Vitamin B6 (Pyridoxine)** as a cofactor. * **Homocystinuria:** A deficiency in CBS leads to Homocystinuria (Type I). Clinical features include ectopia lentis (downward dislocation), intellectual disability, and premature arterial thrombosis. * **Cystinuria:** This is a transport defect (COAL: Cystine, Ornithine, Arginine, Lysine) in the renal tubules, leading to hexagonal cysteine stones, not a metabolic synthesis defect.

Question 342: In maple syrup disease, all of the following amino acids are excreted in urine, EXCEPT:

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

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 enzyme is responsible for the oxidative decarboxylation of the alpha-keto acids derived from the three **branched-chain amino acids (BCAAs)**. 1. **Why Phenylalanine is the correct answer:** Phenylalanine is an **aromatic amino acid**, not a branched-chain amino acid. Its metabolism involves the enzyme phenylalanine hydroxylase (deficient in Phenylketonuria). Since the metabolic defect in MSUD is specific to the BCKAD complex, phenylalanine levels remain unaffected and it is not excreted in excess in the urine of these patients. 2. **Why the other options are incorrect:** * **Valine, Leucine, and Isoleucine** are the three essential branched-chain amino acids. In MSUD, the blockage of their common metabolic pathway leads to a toxic accumulation of these amino acids and their corresponding alpha-keto acids in the blood (aminoacidemia) and their subsequent excretion in the urine (aminoaciduria). **Clinical Pearls for NEET-PG:** * **Mnemonic:** Remember **"LIV"** (Leucine, Isoleucine, Valine) for MSUD. * **The Odor:** The characteristic "maple syrup" or "burnt sugar" odor of the urine is primarily due to the accumulation of **S-isoleucine** (alloisoleucine). * **Diagnosis:** Elevated levels of BCAAs in plasma and the presence of **alloisoleucine** (pathognomonic) are diagnostic. * **Cofactor:** The BCKAD complex requires five cofactors: **T**hiamine (B1), **R**iboflavin (B2), **N**iacin (B3), **P**antothenic acid (B5), and **L**ipoic acid (**Tender Loving Care For No-one**). Thiamine supplementation is effective in thiamine-responsive variants of MSUD.

Question 343: Which enzyme is deficient in alkaptonuria?

A. Kynureninase
B. Tyrosine hydroxylase
C. Homogentisate oxidase (Correct Answer)
D. Tyrosinase

Explanation: **Explanation:** **Alkaptonuria** is an autosomal recessive disorder of phenylalanine and tyrosine metabolism. The correct answer is **Homogentisate oxidase** (also known as homogentisate 1,2-dioxygenase). 1. **Why it is correct:** In the tyrosine degradation pathway, homogentisate is converted into maleylacetoacetate by the enzyme **homogentisate oxidase**. A deficiency leads to the accumulation of homogentisic acid (HGA) in the blood and tissues. HGA is excreted in the urine, where it oxidizes upon exposure to air, turning the urine black. 2. **Why the other options are incorrect:** * **Kynureninase:** This enzyme is involved in the Tryptophan metabolism pathway (converting kynurenine to anthranilate). Its deficiency is not related to alkaptonuria. * **Tyrosine hydroxylase:** This is the rate-limiting enzyme that converts Tyrosine to L-DOPA in the synthesis of catecholamines. * **Tyrosinase:** A deficiency of this copper-containing enzyme leads to **Albinism**, as it is required for melanin synthesis from tyrosine. **High-Yield Clinical Pearls for NEET-PG:** * **The Classic Triad:** (1) Blackening of urine on standing/alkalinization, (2) **Ochronosis** (bluish-black pigmentation of connective tissues like the sclera and ear cartilage), and (3) **Arthritis** (large joint involvement due to HGA deposition). * **Diagnosis:** Ferric chloride test (turns urine deep blue) and silver nitrate test. Gold standard is quantification of HGA via GC-MS. * **Dietary Management:** Restriction of Phenylalanine and Tyrosine. **Nitisinone** (inhibitor of 4-hydroxyphenylpyruvate dioxygenase) is used to reduce HGA production.

Question 344: What is the precursor of tyrosine?

A. Cysteine
B. Histidine
C. Tryptophan
D. Phenylalanine (Correct Answer)

Explanation: **Explanation:** **1. Why Phenylalanine is Correct:** Tyrosine is a **non-essential amino acid** because it can be synthesized endogenously from the essential amino acid **Phenylalanine**. This reaction is catalyzed by the enzyme **Phenylalanine Hydroxylase (PAH)**, which adds a hydroxyl (-OH) group to the para-position of the phenylalanine ring. This process requires molecular oxygen and the cofactor **Tetrahydrobiopterin (BH4)**. Because tyrosine synthesis depends entirely on phenylalanine, tyrosine becomes an "essential" amino acid if phenylalanine intake is deficient or its conversion is blocked. **2. Why Other Options are Incorrect:** * **Cysteine:** This is a sulfur-containing amino acid synthesized from Methionine (via Homocysteine). It is not involved in the aromatic pathway of tyrosine. * **Histidine:** This is an essential basic amino acid that acts as a precursor for Histamine. It has an imidazole ring, unlike the phenol ring of tyrosine. * **Tryptophan:** While also an aromatic amino acid, it is a precursor for Serotonin, Melatonin, and Niacin (Vitamin B3), not tyrosine. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Phenylketonuria (PKU):** Caused by a deficiency of Phenylalanine Hydroxylase. In PKU, Tyrosine becomes a **conditionally essential amino acid** because the body cannot produce it. * **Metabolic Fate:** Tyrosine is both **glucogenic and ketogenic**. It is the precursor for important molecules: **Catecholamines** (Dopamine, Epinephrine, Norepinephrine), **Thyroid hormones** (T3, T4), and **Melanin**. * **Rate-limiting step:** The conversion of Phenylalanine to Tyrosine is the first and rate-limiting step in phenylalanine catabolism.

Question 345: Which of the following is a purely glucogenic amino acid?

A. Leucine
B. Lysine
C. Phenylalanine
D. Alanine (Correct Answer)

Explanation: **Explanation:** Amino acids are classified based on their metabolic end-products into three categories: glucogenic, ketogenic, or both. **1. Why Alanine is Correct:** **Alanine** is a **purely glucogenic** amino acid. In the liver, alanine undergoes transamination (via ALT) to form **pyruvate**, a key substrate for gluconeogenesis. It plays a central role in the **Cahill cycle (Glucose-Alanine cycle)**, transporting nitrogen from muscles to the liver while providing a carbon skeleton for glucose synthesis during fasting. **2. Analysis of Incorrect Options:** * **Leucine (A) & Lysine (B):** These are the only two **purely ketogenic** amino acids. They are degraded into acetyl-CoA or acetoacetate and cannot be converted into glucose. * **Phenylalanine (C):** This is **both glucogenic and ketogenic**. Its breakdown yields fumarate (glucogenic) and acetoacetate (ketogenic). Other amino acids in this "both" category include Isoleucine, Tyrosine, and Tryptophan (Mnemonic: **PITTT**). **3. NEET-PG High-Yield Pearls:** * **Purely Ketogenic:** Leucine and Lysine (The "L" amino acids). * **Both Glucogenic & Ketogenic:** Phenylalanine, Isoleucine, Tryptophan, Tyrosine, and Threonine. * **Purely Glucogenic:** All remaining 13 amino acids (Alanine is the most important). * **Clinical Correlation:** In Maple Syrup Urine Disease (MSUD), there is a defect in the metabolism of Branched-Chain Amino Acids (BCAA): Leucine (Ketogenic), Isoleucine (Both), and Valine (Glucogenic).

Question 346: In Phenylketonuria, what is the first-line therapy?

A. Replacement of the defective enzyme
B. Replacement of the deficient product
C. Limiting the substrate for the deficient enzyme (Correct Answer)
D. Giving the missing amino acid by diet

Explanation: ### Explanation **1. Why Option C is Correct:** Phenylketonuria (PKU) is most commonly caused by a deficiency of the enzyme **Phenylalanine Hydroxylase (PAH)**, which converts Phenylalanine (Phe) into Tyrosine. In its absence, Phenylalanine accumulates to toxic levels in the blood and brain, leading to severe intellectual disability. The primary management strategy is **dietary restriction of Phenylalanine**. By limiting the substrate (Phe), we prevent its accumulation and the subsequent formation of toxic metabolites like phenylpyruvate. **2. Why Other Options are Incorrect:** * **Option A:** Enzyme replacement therapy (e.g., Pegvaliase) exists but is generally reserved for adults who cannot maintain control through diet. It is **not** the first-line therapy, which remains dietary management from birth. * **Option B:** While Tyrosine becomes a "conditionally essential" amino acid in PKU, simply replacing the product (Tyrosine) does not solve the primary problem of Phenylalanine toxicity. * **Option D:** Giving the "missing amino acid" (Tyrosine) is a supportive measure, but the cornerstone of therapy is the *restriction* of the toxic precursor, not just supplementation of the product. **3. High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Autosomal Recessive. * **Cofactor:** A small percentage of cases are due to a deficiency in **Tetrahydrobiopterin (BH4)**. * **Clinical Features:** "Mousy" or "Musty" body odor, hypopigmentation (fair skin/blue eyes due to low melanin), and mental retardation. * **Screening:** Guthrie Test (bacterial inhibition assay) or Tandem Mass Spectrometry. * **Safe Levels:** Phenylalanine levels should be maintained between **2–6 mg/dL**. * **Maternal PKU:** If a pregnant woman with PKU doesn't maintain a strict diet, the high Phe levels act as a **teratogen**, causing microcephaly and congenital heart defects in the fetus.

Question 347: Arginosuccinate is synthesized from which substrates?

A. Citrulline and arginine
B. Arginine and aspartate
C. Citrulline and aspartate (Correct Answer)
D. Citrulline and fumarate

Explanation: **Explanation:** The synthesis of **Argininosuccinate** is the third step of the **Urea Cycle (Ornithine Cycle)**, occurring within the cytosol of hepatocytes. **1. Why Citrulline and Aspartate is correct:** The enzyme **Argininosuccinate synthetase** catalyzes the condensation of **Citrulline** (derived from the mitochondria) and **Aspartate** (derived from the transamination of oxaloacetate). This reaction is energy-dependent, requiring the hydrolysis of **ATP to AMP and pyrophosphate (PPi)**. Crucially, this step incorporates the **second nitrogen atom** into the urea molecule; the first nitrogen comes from free ammonia (via Carbamoyl Phosphate), while the second is donated by Aspartate. **2. Analysis of Incorrect Options:** * **A & B (Arginine):** Arginine is a *product* of the urea cycle, formed downstream when argininosuccinate is cleaved by argininosuccinate lyase. It is not a substrate for synthesizing argininosuccinate. * **D (Fumarate):** Fumarate is a *byproduct* released when argininosuccinate is cleaved into arginine. It serves as a link to the TCA cycle (the "Kreb’s Bicycle"). **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Rate-limiting step:** Carbamoyl Phosphate Synthetase I (CPS-I) is the rate-limiting enzyme of the urea cycle. * **Citrullinemia (Type I):** Caused by a deficiency of Argininosuccinate synthetase, leading to hyperammonemia and elevated blood citrulline levels. * **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, equivalent to 2 phosphates). * **Compartmentalization:** The first two steps occur in the **mitochondria**, while the remaining steps occur in the **cytosol**. Argininosuccinate synthesis is the first cytosolic step.

Question 348: Which branched-chain amino acid is purely ketogenic?

A. Lysine
B. Leucine (Correct Answer)
C. Isoleucine
D. Valine

Explanation: ### Explanation **Correct Answer: B. Leucine** **Underlying Medical Concept:** Amino acids are classified based on their metabolic end-products into three categories: glucogenic, ketogenic, or both. * **Ketogenic amino acids** are degraded directly into Acetyl-CoA or Acetoacetyl-CoA, which are precursors for ketone bodies but cannot be used for gluconeogenesis. * **Leucine** is one of only two **purely ketogenic** amino acids (the other being Lysine). Among the three Branched-Chain Amino Acids (BCAAs)—Leucine, Isoleucine, and Valine—Leucine is the only one that is exclusively ketogenic. **Analysis of Incorrect Options:** * **A. Lysine:** While Lysine is indeed purely ketogenic, it is **not** a branched-chain amino acid. The question specifically asks for a BCAA. * **C. Isoleucine:** This BCAA is **both glucogenic and ketogenic**. Its catabolism yields both Acetyl-CoA (ketogenic) and Propionyl-CoA (glucogenic). * **D. Valine:** This BCAA is **purely glucogenic**. It is metabolized into Succinyl-CoA, an intermediate of the TCA cycle used for glucose synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Purely Ketogenic:** "The 2 L's" – **L**eucine and **L**ysine. * **Mnemonic for Both (Glucogenic & Ketogenic):** "PITTT" – **P**henylalanine, **I**soleucine, **T**yrosine, **T**ryptophan, **T**hreonine. * **Maple Syrup Urine Disease (MSUD):** Caused by a deficiency in the **Branched-chain α-keto acid dehydrogenase** complex. This leads to the accumulation of all three BCAAs (Leucine, Isoleucine, Valine) and their keto-acids, giving urine a characteristic burnt-sugar odor. * **Leucine's Role:** It is the most potent stimulator of muscle protein synthesis via the mTOR pathway.

Question 349: Creatinine is synthesized from which amino acids?

A. Glycine, Arginine and Methionine (Correct Answer)
B. Glycine and Methionine
C. Ornithine and Glycine
D. Thymine and Ornithine

Explanation: ### Explanation Creatine (the precursor to creatinine) is synthesized through a two-step process involving three specific amino acids: **Glycine, Arginine, and Methionine.** 1. **Step 1 (Kidney):** The enzyme *Amidinotransferase* transfers an amidine group from **Arginine** to **Glycine**, forming Guanidinoacetate (GAA). 2. **Step 2 (Liver):** GAA is methylated by *Methyltransferase*. The methyl group is donated by S-adenosylmethionine (SAM), which is derived from **Methionine**. This forms Creatine. 3. **Conversion:** Creatine is then phosphorylated to Creatine Phosphate in muscles. Spontaneous, non-enzymatic cyclization of creatine phosphate produces **Creatinine**, which is excreted by the kidneys. #### Analysis of Incorrect Options: * **Option B (Glycine and Methionine):** This is incomplete as Arginine is essential for providing the amidine group. * **Option C (Ornithine and Glycine):** Ornithine is actually a *product* of the first reaction (after Arginine loses its amidine group), not a precursor. * **Option D (Thymine and Ornithine):** Thymine is a pyrimidine nitrogenous base, not an amino acid involved in muscle metabolism. #### NEET-PG Clinical Pearls: * **Rate-limiting step:** The first step (Amidinotransferase) in the kidney is the regulatory step. * **Diagnostic Marker:** Serum creatinine is a stable indicator of **Glomerular Filtration Rate (GFR)** because its production is constant and proportional to muscle mass. * **High-Yield Mnemonic:** **"GAM"** (Glycine, Arginine, Methionine) for Creatine synthesis. * **Arginine's Versatility:** Remember that Arginine is also a precursor for Nitric Oxide (NO) and Urea.

Question 350: Which form of amino acid is derived from an external source?

A. Synthesis in muscle
B. Break down from muscle
C. From external source (Correct Answer)
D. Produced in liver

Explanation: ### Explanation The human body maintains an **amino acid pool** derived from three primary sources: dietary intake (exogenous), de novo synthesis, and protein degradation (endogenous). **1. Why the Correct Answer is Right:** The question refers to **Essential Amino Acids (EAAs)**. These are amino acids that the body cannot synthesize *de novo* at a rate sufficient to meet metabolic demands. Therefore, they must be obtained from an **external source** (dietary protein). There are 10 essential amino acids, often remembered by the mnemonic **PVT TIM HALL** (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine). **2. Why the Other Options are Incorrect:** * **Synthesis in muscle/Produced in liver:** These refer to **Non-essential Amino Acids** (e.g., Alanine, Glutamine). The liver is the primary site for the synthesis of non-essential amino acids via transamination. * **Breakdown from muscle:** This refers to the endogenous mobilization of amino acids. During fasting or starvation, muscle proteolysis releases amino acids (primarily Alanine and Glutamine) into the blood to maintain glucose levels via gluconeogenesis. While this is a "source" for the plasma pool, it is an internal redistribution, not an external acquisition. ### High-Yield Clinical Pearls for NEET-PG: * **Semi-essential Amino Acids:** Arginine and Histidine are considered semi-essential because they are required in larger quantities during periods of rapid growth (infancy) or recovery from illness. * **Purely Ketogenic Amino Acids:** Leucine and Lysine (The "L"s are purely ketogenic). * **Both Glucogenic and Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, and Isoleucine (Mnemonic: **PITT**). * **Nitrogen Balance:** A healthy adult should be in nitrogen equilibrium, where nitrogen intake (external source) equals nitrogen excretion. A negative nitrogen balance occurs in malnutrition or severe trauma.

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

Practice Questions

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