Amino Acid Metabolism Practice Questions

Q: Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is due to deficiency of which enzyme?

Ornithine translocase. **Explanation:** **HHH Syndrome** is an autosomal recessive urea cycle disorder caused by a mutation in the **SLC25A15 gene**, which encodes the **Ornithine Translocase (ORNT1)** protein. 1. **Why Ornithine Translocase is correct:** The urea cycle occurs in both the mitochondria and the cytosol. Ornithine is produced in the cytosol but must be transported into the mitochondria to react with carbamoyl phosphate (via OTC). Ornithine translocase is the specific transporter responsible for this shuttle. When deficient, ornithine cannot enter the mitochondria, leading to: * **Hyperornithinemia:** Accumulation of ornithine in the cytosol/blood. * **Hyperammonemia:** The urea cycle is "stalled" due to lack of mitochondrial ornithine, leading to ammonia buildup. * **Homocitrullinuria:** In the absence of ornithine, the enzyme OTC uses **lysine** as an alternative substrate, producing homocitrulline, which is excreted in the urine. 2. **Why the other options are incorrect:** * **Ornithine permease:** This is a general term for transport proteins in bacteria/lower organisms; it is not the specific human transporter involved in HHH syndrome. * **Ornithine decarboxylase:** This enzyme converts ornithine to putrescine (polyamine synthesis); its deficiency does not cause the HHH triad. * **Ornithine transcarbamoylase (OTC):** Deficiency causes the most common urea cycle disorder (X-linked), characterized by hyperammonemia and orotic aciduria, but *not* hyperornithinemia. **High-Yield Clinical Pearls for NEET-PG:** * **The Triad:** Hyperornithinemia, Hyperammonemia, and Homocitrullinuria. * **Genetics:** Autosomal Recessive (unlike OTC deficiency, which is X-linked). * **Clinical Presentation:** Post-prandial irritability, lethargy, seizures, and characteristic **spastic paraplegia** in chronic cases. * **Diagnosis:** Elevated plasma ornithine and urinary homocitrulline.

Q: All of the following vitamins are involved in sulfur-containing amino acid metabolism except?

Vitamin B3 (Niacin). ### Explanation The metabolism of sulfur-containing amino acids (Methionine and Cysteine) relies heavily on specific B-complex vitamins acting as cofactors. **Vitamin B3 (Niacin)** is the correct answer because it is not directly involved in the primary metabolic pathways of these amino acids (transmethylation and transsulfuration). #### Why Vitamin B3 is the Exception: Niacin (as NAD+/NADP+) is primarily involved in redox reactions and the TCA cycle. While it plays a role in general energy metabolism, it does not serve as a specific cofactor for the enzymes converting methionine to homocysteine or homocysteine to cysteine. #### Role of Other Vitamins: * **Vitamin B6 (Pyridoxine):** Essential for the **Transsulfuration pathway**. It is a cofactor for *Cystathionine β-synthase* (CBS) and *Cystathionase*, which convert Homocysteine to Cysteine. * **Vitamin B9 (Folate) & Vitamin B12 (Cobalamin):** Both are critical for the **Remethylation pathway**. Vitamin B12 is a cofactor for *Methionine Synthase*, while B9 (as N5-methyl THF) provides the methyl group to convert Homocysteine back into Methionine. #### High-Yield Clinical Pearls for NEET-PG: 1. **Hyperhomocysteinemia:** Deficiencies in B6, B9, or B12 lead to elevated homocysteine levels, a significant risk factor for coronary artery disease and thrombosis. 2. **Homocystinuria:** The most common cause is a deficiency of the B6-dependent enzyme **Cystathionine β-synthase**. 3. **Methylfolate Trap:** A B12 deficiency "traps" folate in the N5-methyl THF form, leading to functional folate deficiency and megaloblastic anemia. 4. **Cysteine** becomes an **essential amino acid** if there is a block in the transsulfuration pathway.

Q: Glycine is present in all except?

Glutamine. ### Explanation The question tests your knowledge of the metabolic precursors and derivatives of **Glycine**, the simplest amino acid. **Why Glutamine is the correct answer:** **Glutamine** is a distinct non-essential amino acid synthesized from **Glutamate** and ammonia via the enzyme *Glutamine synthetase*. It is not derived from Glycine. While both are involved in nitrogen metabolism, Glycine does not contribute to the carbon skeleton or the amino group of Glutamine. **Analysis of incorrect options (Derivatives of Glycine):** * **Creatine:** Glycine is the first precursor in creatine synthesis. It reacts with Arginine to form Guanidinoacetate (GAA) in the kidney, which is later methylated in the liver to form Creatine. * **Purine:** Glycine is a major contributor to the purine ring (Adenine and Guanine). Specifically, it provides the **C4, C5, and N7** atoms of the purine skeleton. * **Glutathione:** This is a tripeptide (γ-glutamyl-cysteinyl-**glycine**). Glycine is the third amino acid added during its synthesis, playing a crucial role in antioxidant defense. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Glycine Products:** "Glycine makes **H**appy **P**eople **G**et **C**reative" (**H**eme, **P**urines, **G**lutathione, **C**reatine). * **Heme Synthesis:** Glycine + Succinyl CoA are the starting materials for Heme synthesis (catalyzed by ALA synthase). * **Conjugation:** Glycine is used for the conjugation of bile acids (Glycocholic acid) and the detoxification of Benzoic acid to form **Hippuric acid**. * **Inhibitory Neurotransmitter:** Glycine acts as an inhibitory neurotransmitter in the spinal cord and brainstem.

Q: Transamination of aspartate directly leads to which intermediate in the citric acid cycle?

Oxaloacetate. ### Explanation **1. Why Oxaloacetate is Correct:** Transamination is the process where an amino group is transferred from an amino acid to a keto acid, catalyzed by **aminotransferases (transaminases)**. This reaction requires **Pyridoxal Phosphate (Vitamin B6)** as a cofactor. * **Aspartate** is a 4-carbon amino acid. * When aspartate undergoes transamination (typically transferring its amino group to $\alpha$-ketoglutarate), it is converted into its corresponding keto acid, **Oxaloacetate (OAA)**. * The enzyme involved is **Aspartate Aminotransferase (AST)**, also known as Serum Glutamate Oxaloacetate Transaminase (SGOT). **2. Why Other Options are Incorrect:** * **A. Citrate:** This is formed by the condensation of Acetyl-CoA and Oxaloacetate. It is not a direct product of amino acid transamination. * **C. $\alpha$-ketoglutarate:** This is the keto acid corresponding to **Glutamate**. While $\alpha$-ketoglutarate is often the *acceptor* of the amino group in transamination, it is not the product derived from the carbon skeleton of aspartate. * **D. Fumarate:** This is an intermediate in the TCA cycle and the Urea cycle. While aspartate enters the Urea cycle and eventually contributes to fumarate formation, this occurs via the **Argininosuccinate** pathway, not directly through transamination. **3. High-Yield Clinical Pearls for NEET-PG:** * **AST (SGOT):** Found in the liver, heart, and skeletal muscle. It is a sensitive marker for hepatocellular injury and was historically used as a cardiac marker. * **The "Pairs" to Remember:** * Alanine $\leftrightarrow$ Pyruvate (via ALT) * Aspartate $\leftrightarrow$ Oxaloacetate (via AST) * Glutamate $\leftrightarrow$ $\alpha$-ketoglutarate * **Cofactor:** Always remember **PLP (B6)** for any transamination, deamination, or decarboxylation of amino acids.

Q: Transamination of pyruvate and glutamic acid leads to the formation of which compound?

alpha-ketoglutarate. ### Explanation **Concept Overview:** Transamination is the process where an amino group ($\text{-NH}_2$) is transferred from an amino acid to a keto acid, resulting in the formation of a new amino acid and a new keto acid. This reaction is catalyzed by **aminotransferases (transaminases)** and requires **Pyridoxal Phosphate (Vitamin B6)** as a mandatory cofactor. **Why the Correct Answer is Right:** In this specific reaction, **Glutamic acid** (the amino donor) transfers its amino group to **Pyruvate** (the keto acid acceptor). 1. When Glutamic acid loses its amino group, it is converted into its corresponding keto acid: **$\alpha$-ketoglutarate**. 2. When Pyruvate accepts the amino group, it is converted into its corresponding amino acid: **Alanine**. This reaction is catalyzed by **Alanine Aminotransferase (ALT)**, also known as Serum Glutamic Pyruvic Transaminase (SGPT). **Analysis of Incorrect Options:** * **A. Oxaloacetate:** This is the keto acid produced when **Aspartate** undergoes transamination. * **C. Aspartate:** This is the amino acid formed when Oxaloacetate accepts an amino group (catalyzed by AST). * **D. Malate:** This is an intermediate in the TCA cycle formed by the hydration of fumarate or the reduction of oxaloacetate; it is not a direct product of transamination. **NEET-PG High-Yield Pearls:** * **Cofactor:** All transaminases require **Pyridoxal Phosphate (PLP)**, a derivative of Vitamin B6. * **Exceptions:** Lysine, Threonine, Proline, and Hydroxyproline **do not** undergo transamination. * **Clinical Marker:** ALT (SGPT) is more specific for **liver cell damage** than AST (SGOT), as AST is also found in cardiac and skeletal muscle. * **Equilibrium:** Transamination reactions are freely reversible and play a crucial role in both amino acid synthesis and catabolism.

Q: Enzymes for urea synthesis are present in which cellular compartment?

Both cytoplasm and mitochondrial matrix. ### Explanation The Urea Cycle (Ornithine cycle) is the primary mechanism for detoxifying ammonia into urea. It is unique because it is a **compartmentalized metabolic pathway**, meaning its enzymes are distributed between two different parts of the cell. **1. Why Option C is Correct:** The synthesis of urea requires five key enzymatic steps. The first two steps occur within the **mitochondrial matrix**, while the remaining three steps occur in the **cytosol (cytoplasm)**. * **Mitochondria:** Carbamoyl Phosphate Synthetase I (CPS-I) and Ornithine Transcarbamoylase (OTC) convert ammonia and bicarbonate into Citrulline. * **Cytoplasm:** Citrulline is transported out of the mitochondria, where Argininosuccinate synthetase, Argininosuccinate lyase, and Arginase complete the cycle to produce Urea and regenerate Ornithine. **2. Why Other Options are Incorrect:** * **Option A:** While the majority of the steps (3 out of 5) occur here, the cycle cannot initiate without the mitochondrial enzymes. * **Option B:** Only the initial "priming" steps occur here. If the cycle were restricted to the mitochondria, the final cleavage of Arginine to Urea could not take place. **3. High-Yield Clinical Pearls for NEET-PG:** * **Rate-Limiting Step:** CPS-I is the rate-limiting enzyme of the urea cycle and requires **N-acetylglutamate (NAG)** as an essential allosteric activator. * **Site of Urea Cycle:** It occurs exclusively in the **Liver**. * **Mnemonic for Dual-Compartment Pathways:** "**H**ot **U****M**ans" — **H**eme synthesis, **U**rea cycle, and **G**luconeogenesis occur in both the mitochondria and cytosol. * **Most Common Deficiency:** Ornithine Transcarbamoylase (OTC) deficiency is the most common urea cycle disorder and is the only one that is **X-linked recessive** (others are autosomal recessive).

Q: Pellagra is caused by which of the following conditions?

Hartnup's disease. **Explanation:** **Pellagra** is a clinical syndrome caused by a deficiency of **Niacin (Vitamin B3)**. While it is commonly associated with dietary deficiency, it can also occur due to **Hartnup’s disease** (Option A). 1. **Why Hartnup’s Disease is Correct:** Hartnup’s disease is an autosomal recessive disorder characterized by a defect in the **SLC6A19 transporter**, which is responsible for the absorption of neutral amino acids (especially **Tryptophan**) in the small intestine and renal tubules. Since Tryptophan is a metabolic precursor for Niacin (60 mg Tryptophan = 1 mg Niacin), its malabsorption leads to a secondary Niacin deficiency, manifesting as the "3 Ds" of Pellagra: **Dermatitis** (Casal’s necklace), **Diarrhea**, and **Dementia**. 2. **Why Other Options are Incorrect:** * **Cystinuria (B):** A defect in the transport of COAL (Cystine, Ornithine, Arginine, Lysine). It leads to renal stones (hexagonal crystals) but does not affect Niacin levels. * **Cystinosis (C):** A lysosomal storage disorder where cystine accumulates within lysosomes, leading to Fanconi syndrome and renal failure, not pellagra. * **Type I Tyrosinemia (D):** Caused by a deficiency of Fumarylacetoacetate hydrolase. It presents with liver failure, cabbage-like odor, and renal rickets. **High-Yield Clinical Pearls for NEET-PG:** * **The 4th D:** If untreated, Pellagra leads to **Death**. * **Hartnup Diagnosis:** Look for **Neutral Aminoaciduria** in the urine. * **Other causes of Pellagra:** Carcinoid syndrome (Tryptophan is diverted to Serotonin synthesis) and prolonged Isoniazid (INH) therapy (depletes Vitamin B6, a cofactor for Niacin synthesis).

Q: The reactions of the urea cycle occur in which one of the following locations?

The mitochondrial matrix and the cytosol. The urea cycle (Ornithine cycle) is a critical metabolic pathway that converts toxic ammonia into non-toxic urea for excretion. It is unique because it is **compartmentalized**, requiring enzymes located in both the mitochondria and the cytoplasm. ### **Detailed Explanation** The cycle consists of five main steps, split between two cellular locations: 1. **Mitochondrial Matrix:** The first two reactions occur here. * Ammonia and bicarbonate are converted to **Carbamoyl Phosphate** by *Carbamoyl Phosphate Synthetase I (CPS-I)*—the rate-limiting step. * Carbamoyl phosphate combines with Ornithine to form **Citrulline** via *Ornithine Transcarbamoylase (OTC)*. 2. **Cytosol:** Citrulline is transported out of the mitochondria. The remaining three reactions (formation of Argininosuccinate, cleavage into Arginine and Fumarate, and finally the release of Urea) occur in the cytosol. ### **Analysis of Options** * **A & C (The cytosol / The mitochondrial matrix):** These are incorrect because neither compartment contains the full complement of enzymes required to complete the cycle. * **B (Lysosomes):** Lysosomes are involved in macromolecule degradation (proteolysis), not the metabolic processing of nitrogenous waste. ### **High-Yield NEET-PG Pearls** * **Rate-Limiting Enzyme:** CPS-I (requires **N-acetylglutamate/NAG** as an essential allosteric activator). * **Mnemonic for Location:** *"**M**other **C**ooking"*: **M**itochondria (first 2 steps) and **C**ytosol (remaining steps). * **Link to TCA Cycle:** Fumarate produced in the cytosol (by Argininosuccinate lyase) links the Urea cycle to the TCA cycle (the "Krebs Bicycle"). * **Most Common Deficiency:** **OTC deficiency** (the only X-linked urea cycle disorder; all others are Autosomal Recessive).

Q: In Maple syrup urine disease, which of the following compounds is accumulated?

a-keto acid decarboxylase. **Explanation:** **Maple Syrup Urine Disease (MSUD)** is an autosomal recessive metabolic disorder caused by a deficiency in the **Branched-Chain α-keto acid dehydrogenase (BCKDH) complex**. 1. **Why Option C is Correct:** The BCKDH complex is a multi-enzyme system responsible for the **oxidative decarboxylation** of α-keto acids derived from the branched-chain amino acids (BCAAs): **Leucine, Isoleucine, and Valine**. A deficiency in the **α-keto acid decarboxylase** component leads to the accumulation of these α-keto acids in the blood and urine, giving the urine a characteristic burnt-sugar or maple syrup odor. 2. **Why Other Options are Incorrect:** * **Option A (Homogentisic acid oxidase):** Deficiency of this enzyme leads to **Alkaptonuria**, characterized by blackening of urine upon standing and ochronosis. * **Option B (Methyl malonyl CoA):** Accumulation occurs in **Methylmalonic Acidemia**, typically due to a deficiency of Methylmalonyl-CoA mutase or Vitamin B12. * **Option D (Transaminase):** Transamination is the first step of BCAA catabolism (converting amino acids to α-keto acids). In MSUD, transamination is functional; the block occurs at the subsequent decarboxylation step. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** "I Love Vermont maple syrup" (**I**soleucine, **L**eucine, **V**aline). * **Diagnostic Marker:** Elevated levels of **Alloisoleucine** are pathognomonic for MSUD. * **Cofactors:** The BCKDH 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**oving **N**ights **P**lease **L**uck). * **Treatment:** Dietary restriction of BCAAs and high-dose **Thiamine** supplementation (in thiamine-responsive variants).

Question 1

Hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is due to deficiency of which enzyme?

Accepted Answer

Ornithine translocase

Answer

Ornithine permease

Answer

Ornithine decarboxylase

Answer

Ornithine transcarbamoylase

Question 2

All of the following vitamins are involved in sulfur-containing amino acid metabolism except?

Accepted Answer

Vitamin B3 (Niacin)

Answer

Vitamin B6 (Pyridoxine)

Answer

Vitamin B9 (Folate)

Answer

Vitamin B12 (Cobalamin)

Question 3

Mental retardation is not a feature in which of the following aminoacidurias?

Accepted Answer

Albinism

Answer

Phenylketonuria

Answer

Homocystinuria

Answer

Maple syrup urine disease

Question 4

Glycine is present in all except?

Accepted Answer

Glutamine

Answer

Creatine

Answer

Purine

Answer

Glutathione

Question 5

Transamination of aspartate directly leads to which intermediate in the citric acid cycle?

Accepted Answer

Oxaloacetate

Answer

Citrate

Answer

alpha-ketoglutarate

Answer

Fumarate

Question 6

Transamination of pyruvate and glutamic acid leads to the formation of which compound?

Accepted Answer

alpha-ketoglutarate

Answer

Oxaloacetate

Answer

Aspartate

Answer

Malate

Question 7

Enzymes for urea synthesis are present in which cellular compartment?

Accepted Answer

Both cytoplasm and mitochondrial matrix

Answer

Cytoplasm

Answer

Mitochondrial matrix

Answer

None of the above

Question 8

Pellagra is caused by which of the following conditions?

Accepted Answer

Hartnup's disease

Answer

Cystinuria

Answer

Cystinosis

Answer

Type I Tyrosinemia

Question 9

The reactions of the urea cycle occur in which one of the following locations?

Accepted Answer

The mitochondrial matrix and the cytosol

Answer

The cytosol

Answer

Lysosomes

Answer

The mitochondrial matrix

Question 10

In Maple syrup urine disease, which of the following compounds is accumulated?

Accepted Answer

a-keto acid decarboxylase

Answer

Homogentisic acid oxidase

Answer

Methyl malonyl CoA

Answer

Transaminase

Amino Acid Metabolism — MCQs

Amino Acid Metabolism — MCQs

On this page

Practice by Chapter

Want unlimited practice?