Amino Acid Metabolism Practice Questions

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

Histidine. **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.

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

Green. **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).

Q: 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?

Increased secretion of insulin. **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.

Q: Deficiency of which enzyme leads to Tyrosinemia type II?

Tyrosine transaminase. **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).

Q: 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?

Dihydropterin reductase. **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).

Q: Which group contains non-essential amino acids?

Acidic Amino Acids. ### Explanation **1. Why Acidic Amino Acids are Correct:** Non-essential amino acids are those that the human body can synthesize de novo from metabolic intermediates (like those in the TCA cycle) and do not strictly require dietary intake. The **Acidic Amino Acids**—**Aspartic acid (Aspartate)** and **Glutamic acid (Glutamate)**—are classic examples. * **Glutamate** is synthesized via the reductive amination of $\alpha$-ketoglutarate. * **Aspartate** is synthesized via the transamination of oxaloacetate. Since both precursors are readily available in the TCA cycle, these amino acids are non-essential. **2. Analysis of Incorrect Options:** * **Branched Chain Amino Acids (BCAAs):** This group includes **Leucine, Isoleucine, and Valine**. All three are **Essential** amino acids. They must be obtained from the diet and are primarily metabolized in the muscle. * **Basic Amino Acids:** This group includes **Lysine, Arginine, and Histidine**. * **Lysine** is strictly essential. * **Arginine and Histidine** are considered semi-essential (required during periods of rapid growth or stress). * **Aromatic Amino Acids:** This group includes **Phenylalanine, Tyrosine, and Tryptophan**. * **Phenylalanine and Tryptophan** are strictly essential. * **Tyrosine** is non-essential only because it is synthesized from Phenylalanine. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Essential Amino Acids:** "PVT TIM HALL" (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine, Leucine, Lysine). * **Purely Ketogenic Amino Acids:** Leucine and Lysine (The "L"s). * **Glucogenic & Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, and Isoleucine (The "Aromatics + Isoleucine"). * **Clinical Correlation:** Defective metabolism of BCAAs leads to **Maple Syrup Urine Disease (MSUD)** due to a deficiency in the Branched-chain $\alpha$-keto acid dehydrogenase complex.

Q: Which of the following vitamins is used in the treatment of Alkaptonuria?

Vitamin C. ### Explanation **Correct Option: B (Vitamin C)** Alkaptonuria is an autosomal recessive disorder caused by a deficiency of the enzyme **Homogentisate oxidase**, leading to the accumulation of Homogentisic Acid (HGA). Vitamin C (Ascorbic Acid) is used in management for two primary reasons: 1. **Inhibition of Oxidation:** It prevents the oxidation of Homogentisic acid into **Benzoquinone acetate**, the pigmented polymer responsible for tissue damage. 2. **Reduction of Ochronosis:** By inhibiting this polymerization, it slows down the deposition of dark pigments in connective tissues (ochronosis), thereby delaying the progression of ochronotic arthritis. **Why Other Options are Incorrect:** * **Vitamin A:** Primarily involved in vision (rhodopsin synthesis) and epithelial integrity; it has no role in the tyrosine metabolic pathway. * **Vitamin D:** Essential for calcium and phosphate homeostasis; its deficiency leads to Rickets/Osteomalacia, but it does not affect HGA metabolism. * **Vitamin K:** Acts as a co-factor for the gamma-carboxylation of clotting factors (II, VII, IX, X); it does not influence the catabolism of aromatic amino acids. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Triad:** Homogentisic aciduria (urine turns black on standing/alkalinization), Ochronosis (bluish-black pigmentation of cartilage/sclera), and Arthritis (large joints). * **Diagnosis:** Ferric chloride test (transient deep blue color) and Silver nitrate test. * **Definitive Treatment:** **Nitisinone** (inhibits 4-hydroxyphenylpyruvate dioxygenase), which reduces the production of HGA. * **Dietary Advice:** Restriction of Phenylalanine and Tyrosine.

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

Skeletal muscle turnover. **Explanation:** **3-methylhistidine (3-MH)** is a unique amino acid derivative formed by the post-translational methylation of specific histidine residues in **actin and myosin**, the primary contractile proteins of skeletal muscle. **Why Option C is correct:** When muscle proteins are degraded (catabolized), 3-methylhistidine is released into the bloodstream. Unlike other amino acids, 3-MH cannot be re-utilized for protein synthesis or further metabolized; it is excreted unchanged in the urine. Therefore, the urinary excretion rate of 3-methylhistidine serves as a specific and sensitive **biochemical marker for skeletal muscle protein turnover** and myofibrillar protein breakdown. **Why other options are incorrect:** * **Option A (Folate status):** Folate status is typically assessed using serum folate levels or the **FIGLU (Formiminoglutamic acid) excretion test**, where FIGLU accumulates in the urine during folate deficiency. * **Option B (Renal disease):** While 3-MH is excreted by the kidneys, it is not a diagnostic marker for renal disease. Standard markers include Serum Creatinine, Urea, and Cystatin C. * **Option D (Protein absorption):** Protein absorption is generally assessed via fecal nitrogen studies or D-xylose tests (for general malabsorption), not by specific methylated amino acids. **High-Yield Clinical Pearls for NEET-PG:** * **Source:** 3-MH is found exclusively in actin and myosin. * **Clinical Utility:** Elevated levels are seen in hypercatabolic states like severe trauma, sepsis, starvation, and muscular dystrophy. * **Dietary Note:** To ensure accuracy, patients should follow a **meat-free diet** for 3 days prior to the test, as exogenous 3-MH from consumed animal muscle can falsely elevate results.

Q: A 7-month-old infant presents with a history of vomiting and failure to thrive. The patient improved with IV glucose. After one month, the infant returns with the same complaints. On evaluation, high glutamine and uracil levels are found. Which is the likely enzyme defect?

Ornithine transcarbamylase. ### Explanation The clinical presentation of vomiting, failure to thrive, and improvement with IV glucose (which reduces protein catabolism) suggests a **Urea Cycle Disorder (UCD)**. **Why Ornithine Transcarbamylase (OTC) Deficiency is correct:** OTC is the most common urea cycle defect and is X-linked recessive. In OTC deficiency, **Carbamoyl Phosphate** cannot be converted to Citrulline. As a result, Carbamoyl Phosphate accumulates in the mitochondria and leaks into the cytosol. Here, it enters the pyrimidine synthesis pathway, leading to the overproduction of **Orotic acid and Uracil**. High ammonia levels are shunted into **Glutamine** synthesis, explaining the elevated glutamine. **Why the other options are incorrect:** * **CPS1 Deficiency:** Like OTC deficiency, it presents with hyperammonemia and high glutamine. However, because CPS1 is upstream of Carbamoyl Phosphate production, there is **no increase** in uracil or orotic acid. * **Argininosuccinate Lyase Deficiency:** This leads to the accumulation of argininosuccinic acid. While it causes hyperammonemia, it does not typically present with the massive uracil elevation seen in OTC deficiency. * **Arginase Deficiency:** This is the least severe UCD. It presents later in childhood with spastic paraplegia rather than acute neonatal/infantile vomiting, and arginine levels would be significantly elevated. **High-Yield Clinical Pearls for NEET-PG:** * **OTC Deficiency** is the *only* X-linked Urea Cycle Disorder; all others are Autosomal Recessive. * **Orotic Aciduria + Hyperammonemia** = OTC Deficiency. * **Orotic Aciduria + Megaloblastic Anemia** (without hyperammonemia) = UMP Synthase deficiency. * **Glutamine** acts as a non-toxic storage form of ammonia; its elevation is a hallmark of most UCDs.

Question 1

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

Accepted Answer

Histidine

Answer

Glycine

Answer

Serine

Answer

Alanine

Question 2

The ferric chloride test in phenylketonuria yields what color result?

Accepted Answer

Green

Answer

Red

Answer

Yellow

Answer

Blue

Question 3

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?

Accepted Answer

Increased secretion of insulin

Answer

Increased growth hormone secretion

Answer

Enhanced rate of gluconeogenesis

Answer

Increased absorption of amino acids from the diet

Question 4

Deficiency of which enzyme leads to Tyrosinemia type II?

Accepted Answer

Tyrosine transaminase

Answer

Fumarylacetoacetate hydrolase

Answer

Phenylalanine hydroxylase

Answer

Tyrosinase

Question 5

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?

Accepted Answer

Dihydropterin reductase

Answer

Histidine decarboxylase

Answer

Phenylalanine hydroxylase

Answer

Tyrosine deficiency

Question 6

Which group contains non-essential amino acids?

Accepted Answer

Acidic Amino Acids

Answer

Branched Chain Amino Acids

Answer

Basic Amino Acids

Answer

Aromatic Amino Acids

Question 7

Which of the following vitamins is used in the treatment of Alkaptonuria?

Accepted Answer

Vitamin C

Answer

Vitamin A

Answer

Vitamin D

Answer

Vitamin K

Question 8

What is the most important amino acid transported from muscle to liver for gluconeogenesis?

Accepted Answer

Alanine

Answer

Methionine

Answer

Tryptophan

Answer

Arginine

Question 9

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

Accepted Answer

Skeletal muscle turnover

Answer

Status of folate in the body

Answer

Renal disease

Answer

Protein absorption in the gut

Question 10

A 7-month-old infant presents with a history of vomiting and failure to thrive. The patient improved with IV glucose. After one month, the infant returns with the same complaints. On evaluation, high glutamine and uracil levels are found. Which is the likely enzyme defect?

Accepted Answer

Ornithine transcarbamylase

Answer

Carbamoyl phosphate synthetase 1 (CPS1)

Answer

Arginase

Answer

Argininosuccinate lyase

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

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