Amino Acid Metabolism Practice Questions

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

Phenylalanine. **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.

Q: Which enzyme is deficient in alkaptonuria?

Homogentisate oxidase. **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.

Q: What is the precursor of tyrosine?

Phenylalanine. **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.

Q: Thyroid peroxidase is not involved in which of the following processes?

Secretion of thyroglobulin into the colloid. **Explanation:** Thyroid Peroxidase (TPO) is a membrane-bound enzyme located on the apical surface of thyroid follicular cells. It is the central enzyme in thyroid hormone synthesis, but it does not participate in the transport or secretion of proteins. **Why Option C is correct:** The secretion of **thyroglobulin (Tg)** into the follicular colloid is a process of **exocytosis**. Tg is synthesized in the rough endoplasmic reticulum, packaged in the Golgi apparatus, and transported via vesicles to the apical membrane. This is a cellular transport mechanism independent of TPO enzymatic activity. **Why the other options are incorrect:** * **Option A:** TPO uses hydrogen peroxide ($H_2O_2$) to catalyze the **oxidation** of iodide ($I^-$) into an active form (atomic iodine or $I^0$), which is essential for the next steps. * **Option B:** TPO is a major **autoantigen**. Antibodies against TPO (Anti-TPO) are the hallmark of **Hashimoto’s thyroiditis** and are frequently used in clinical diagnosis. * **Option D:** TPO facilitates **organification**, where the activated iodine is added to tyrosine residues on the thyroglobulin backbone to form Monoiodotyrosine (MIT) and Diiodotyrosine (DIT). **High-Yield NEET-PG Pearls:** 1. **Wolff-Chaikoff Effect:** High levels of iodine transiently inhibit TPO, leading to a decrease in thyroid hormone synthesis. 2. **Pendred Syndrome:** Caused by a mutation in the *SLC26A4* gene (Pendrin transporter), leading to defective iodide transport into the colloid and sensorineural hearing loss. 3. **Inhibition:** Thionamides (Propylthiouracil and Methimazole) act by inhibiting TPO, making them the mainstay treatment for hyperthyroidism. 4. **Coupling:** TPO also catalyzes the coupling of MIT/DIT to form $T_3$ and $T_4$.

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

Alanine. **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).

Q: Arginosuccinate is synthesized from which substrates?

Citrulline and aspartate. **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.

Q: The estimation of 3-methyl histidine in urine is used to study:

Skeletal muscle mass. **Explanation:** **3-methylhistidine (3-MH)** is a unique amino acid derivative formed by the post-translational methylation of histidine residues specifically within the myofibrillar proteins **actin and myosin**. 1. **Why Skeletal Muscle Mass is Correct:** When skeletal muscle proteins are degraded (proteolysis), 3-methylhistidine is released into the systemic circulation. Unlike other amino acids, 3-MH cannot be reutilized for protein synthesis or further metabolized; it is excreted unchanged in the urine. Therefore, the urinary excretion rate of 3-methylhistidine serves as a reliable biochemical marker for **skeletal muscle mass** and the rate of **muscle protein breakdown**. 2. **Why Other Options are Incorrect:** * **Status of Folate:** Folate status is typically assessed via serum folate, RBC folate, or the **FIGLU (Formiminoglutamic acid) excretion test**. * **Renal Disease:** While 3-MH is excreted by the kidneys, it is a marker of muscle turnover, not a primary diagnostic tool for renal function (where Creatinine or Urea are used). * **Protein Absorption:** Protein absorption is usually evaluated through fecal nitrogen studies or D-xylose tests (for general malabsorption), not specific methylated amino acids. **High-Yield Clinical Pearls for NEET-PG:** * **Actin and Myosin:** 3-MH is found in both, but since skeletal muscle contains the largest pool of these proteins in the body, it dominates the urinary output. * **Dietary Interference:** For accurate results, patients must be on a **meat-free diet** for 3 days prior to the test, as meat consumption contains exogenous 3-MH. * **Negative Nitrogen Balance:** Increased urinary 3-MH is seen in hypercatabolic states like sepsis, major trauma, or starvation, indicating rapid muscle wasting.

Q: Which branched-chain amino acid is purely ketogenic?

Leucine. ### 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.

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

From external source. ### 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.

Question 1

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

Accepted Answer

Phenylalanine

Answer

Valine

Answer

Leucine

Answer

Isoleucine

Question 2

Which enzyme is deficient in alkaptonuria?

Accepted Answer

Homogentisate oxidase

Answer

Kynureninase

Answer

Tyrosine hydroxylase

Answer

Tyrosinase

Question 3

What is the precursor of tyrosine?

Accepted Answer

Phenylalanine

Answer

Cysteine

Answer

Histidine

Answer

Tryptophan

Question 4

Thyroid peroxidase is not involved in which of the following processes?

Accepted Answer

Secretion of thyroglobulin into the colloid

Answer

Oxidation of iodide to atomic iodine

Answer

Acting as a frequent epitope for autoantibodies

Answer

Liberation of iodine for addition onto tyrosine residues on thyroglobulin

Question 5

Which of the following is a purely glucogenic amino acid?

Accepted Answer

Alanine

Answer

Leucine

Answer

Lysine

Answer

Phenylalanine

Question 6

Arginosuccinate is synthesized from which substrates?

Accepted Answer

Citrulline and aspartate

Answer

Citrulline and arginine

Answer

Arginine and aspartate

Answer

Citrulline and fumarate

Question 7

The estimation of 3-methyl histidine in urine is used to study:

Accepted Answer

Skeletal muscle mass

Answer

Status of folate in the body

Answer

Renal disease

Answer

Protein absorption in the gut

Question 8

During vigorous strenuous exercise, which of the following amino acids is liberated from the skeletal muscles in the maximum amount into circulation?

Accepted Answer

Alanine

Answer

Glutamate

Answer

Glutamine

Answer

Branched chain amino acids

Question 9

Which branched-chain amino acid is purely ketogenic?

Accepted Answer

Leucine

Answer

Lysine

Answer

Isoleucine

Answer

Valine

Question 10

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

Accepted Answer

From external source

Answer

Synthesis in muscle

Answer

Break down from muscle

Answer

Produced in liver

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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