Catecholamines are synthesized from?
Which of the following amino acids is not involved in the production of creatine?
Ninhydrin test is used for?
Which enzyme catalyzes oxidative deamination?
Transamination of Aspartate forms which compound?
Which defect in the urea cycle is an X-linked disease?
What is the primary enzymatic source of ammonia production in urine?
Which amino acids accumulate in maple syrup urine disease?
What type of protein is keratin classified as?
Sweaty feet odor in urine is seen in which condition?
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 81: Catecholamines are synthesized from?
- A. Tyrosine (Correct Answer)
- B. Histidine
- C. Methionine
- D. Tryptophan
Explanation: ***Tyrosine*** - **Tyrosine** is the direct precursor amino acid for the synthesis of all **catecholamines**, including **dopamine**, **norepinephrine**, and **epinephrine**. - The synthesis pathway begins with the conversion of tyrosine to **L-DOPA** by tyrosine hydroxylase, followed by subsequent enzymatic steps. *Methionine* - **Methionine** is an essential amino acid primarily involved in **protein synthesis** and as a precursor for S-adenosylmethionine (SAM), a key methyl donor in various metabolic reactions. - It is not a direct precursor for the synthesis of **catecholamines**. *Histidine* - **Histidine** is the precursor for the synthesis of **histamine**, a neurotransmitter and inflammatory mediator. - It is not involved in the biosynthesis pathway of **catecholamines**. *Tryptophan* - **Tryptophan** is the precursor for the synthesis of **serotonin** and **melatonin**, important neurotransmitters and hormones. - It does not play a role in the synthesis of **catecholamines**.
Question 82: Which of the following amino acids is not involved in the production of creatine?
- A. Glycine
- B. Methionine
- C. Alanine (Correct Answer)
- D. Arginine
Explanation: ***Alanine*** - **Alanine** is not directly involved as a precursor for **creatine synthesis**. It can be converted to pyruvate and enter the gluconeogenic pathway. - The primary amino acids involved in **creatine synthesis** are arginine, glycine, and methionine. *Glycine* - **Glycine** is a direct precursor for creatine, reacting with arginine in the first step of its synthesis to form **guanidinoacetate**. - This reaction is catalyzed by **arginine:glycine amidinotransferase (AGAT)**. *Methionine* - **Methionine**, in the form of **S-adenosylmethionine (SAM)**, acts as the methyl donor in the second step of creatine synthesis. - It methylates guanidinoacetate to form **creatine**, a reaction catalyzed by **guanidinoacetate methyltransferase (GAMT)**. *Arginine* - **Arginine** donates its guanidino group to glycine, forming **guanidinoacetate**, the initial intermediate in creatine synthesis. - This is the first committed step in the **creatine biosynthesis pathway**.
Question 83: Ninhydrin test is used for?
- A. Bile salts
- B. Amino acids (Correct Answer)
- C. Nucleic acid
- D. Lipids
Explanation: ***Amino acids*** - The **ninhydrin test** is a chemical test used to detect the presence of **amino acids** and primary and secondary amines. - It produces a **purple-blue color** when it reacts with most amino acids, due to the formation of a colored complex called Ruhemann's purple. *Bile salts* - The detection of **bile salts** typically involves tests like Hay's test or Pettenkofer's test, which are distinct from the ninhydrin reaction. - These tests rely on the physical or chemical properties of bile salts, such as changes in surface tension or specific color reactions with sulfuric acid. *Nucleic acid* - **Nucleic acids** (DNA and RNA) are detected using specific tests like the **diphenylamine test** (for DNA) or orcinol test (for RNA). - These tests target the deoxyribose or ribose sugars present in their structures and result in different color changes compared to ninhydrin. *Lipids* - **Lipids** are typically identified using tests that exploit their nonpolar nature, such as the **emulsion test** or solubility tests in organic solvents. - Their detection does not involve ninhydrin, as they lack the primary or secondary amine groups that react with this reagent.
Question 84: Which enzyme catalyzes oxidative deamination?
- A. Glutaminase
- B. Glutamine synthase
- C. Glutamate dehydrogenase (Correct Answer)
- D. None of the options
Explanation: ***Glutamate dehydrogenase*** - This enzyme catalyzes the conversion of **glutamate** to **α-ketoglutarate** and ammonia (NH₃), which is an oxidative deamination reaction. - It utilizes **NAD⁺ or NADP⁺** as a coenzyme to remove hydrogen atoms during the oxidation process. - Plays a crucial role in both **amino acid catabolism** and anabolism. *Glutaminase* - This enzyme hydrolyzes **glutamine** to glutamate and ammonia, which is a **hydrolytic deamidation** reaction, not an oxidative deamination. - It does not involve the oxidation of the substrate or require NAD⁺/NADP⁺ as cofactors. *Glutamine synthase* - This enzyme synthesizes **glutamine** from glutamate and ammonia, using ATP, which is a **biosynthetic** reaction, not a catabolic deamination. - It is involved in **ammonia detoxification** and amino acid synthesis, functioning in the opposite direction of deamination. *None of the options* - This option is incorrect because **glutamate dehydrogenase** is a valid correct answer. - Glutamate dehydrogenase is the primary enzyme responsible for oxidative deamination in human metabolism.
Question 85: Transamination of Aspartate forms which compound?
- A. Pyruvate
- B. Acetyl-CoA
- C. Oxaloacetate (Correct Answer)
- D. Alanine
Explanation: ***Oxaloacetate*** - **Aspartate** is transaminated by **aspartate aminotransferase (AST)**, transferring its alpha-amino group to **alpha-ketoglutarate**. - This reaction converts aspartate into its corresponding alpha-keto acid, which is **oxaloacetate**. *Pyruvate* - **Pyruvate** is the alpha-keto acid corresponding to the amino acid **alanine**. - Transamination of **alanine** yields **pyruvate**, not aspartate. *Acetyl-CoA* - **Acetyl-CoA** is not a direct product of amino acid transamination; it is formed from **pyruvate** or fatty acid oxidation. - It functions as a key metabolic intermediate in energy production and biosynthesis. *Alanine* - **Alanine** is an amino acid, and therefore a reactant in transamination reactions to form pyruvate, rather than a product of aspartate transamination. - While it can be formed from pyruvate via transamination, it is not formed from aspartate.
Question 86: Which defect in the urea cycle is an X-linked disease?
- A. Ornithine transcarbamylase (Correct Answer)
- B. Arginase
- C. Argininosuccinate synthase
- D. Carbamoyl phosphate synthetase I
Explanation: ***Ornithine transcarbamylase*** - **Ornithine transcarbamylase (OTC) deficiency** is the only **X-linked recessive** disorder among the urea cycle defects. - Males are usually more severely affected, while females can be symptomatic carriers. *Carbamoyl phosphate synthetase I* - **Carbamoyl phosphate synthetase I (CPS1) deficiency** is an **autosomal recessive** disorder. - It is one of the more severe urea cycle defects, leading to profound hyperammonemia. *Arginase* - **Arginase deficiency** (hyperargininemia) is an **autosomal recessive** disorder. - It typically presents with a distinct neurological phenotype, including spasticity and developmental delay. *Argininosuccinate synthase* - **Argininosuccinate synthase deficiency**, also known as **Citrullinemia type I**, is an **autosomal recessive** disorder. - It leads to the accumulation of **citrulline** and **ammonia** in the blood.
Question 87: What is the primary enzymatic source of ammonia production in urine?
- A. Glutaminase (Correct Answer)
- B. Urease
- C. Glutamate dehydrogenase
- D. Arginase
Explanation: ***Glutaminase*** - This enzyme catalyzes the **hydrolysis of glutamine** located predominantly in the cells of the **renal tubules**, producing **ammonia** (NH3) and glutamate. - This process is crucial for **acid-base balance**, as the ammonia can bind with excess hydrogen ions to form ammonium (NH4+), which is then excreted in the urine. *Urease* - This enzyme breaks down **urea into ammonia and carbon dioxide**, primarily produced by **bacteria**, not human renal cells, and contributes to ammonia in urine in cases of **urinary tract infections**. - While it produces ammonia, it is not the primary enzymatic source within the healthy human kidney for **acid-base regulation**. *Glutamate dehydrogenase* - This enzyme converts **glutamate into alpha-ketoglutarate and ammonia**, but its contribution to urinary ammonia production is secondary to glutaminase in the kidney. - Its primary role is in **oxidative deamination** and the interconversion of glutamate and alpha-ketoglutarate, acting in both mitochondrial and cytosolic compartments. *Arginase* - This enzyme is involved in the **urea cycle**, converting **arginine into urea and ornithine** in the liver. - While it is important for the detoxification of ammonia by converting it into urea for excretion, it **does not produce ammonia** in the kidney for urinary excretion.
Question 88: Which amino acids accumulate in maple syrup urine disease?
- A. Valine
- B. Leucine
- C. Isoleucine
- D. All branched-chain amino acids (Correct Answer)
Explanation: ***All branched-chain amino acids*** - Maple syrup urine disease (MSUD) is characterized by a deficiency in the **branched-chain alpha-keto acid dehydrogenase complex**, which is responsible for the breakdown of branched-chain amino acids (BCAAs). - This deficiency leads to the accumulation of **leucine, isoleucine, and valine**, along with their corresponding alpha-keto acids, in the blood and urine. - The distinctive **maple syrup odor** in the urine is caused by the accumulation of branched-chain keto acids derived from all three BCAAs. *Leucine* - While leucine is one of the BCAAs that accumulates in MSUD, it is not the *only* amino acid involved. - The accumulation of **leucine** is particularly associated with the severe neurological symptoms seen in MSUD, as it is the most neurotoxic of the three BCAAs. *Valine* - Valine is another BCAA that accumulates due to the metabolic block in MSUD. - However, the disease involves the accumulation of all three BCAAs, not just valine in isolation. *Isoleucine* - Isoleucine is the third BCAA that accumulates in MSUD due to the defective enzyme. - Like leucine and valine, isoleucine and its corresponding keto acid accumulate in blood and urine when the branched-chain alpha-keto acid dehydrogenase complex is deficient.
Question 89: What type of protein is keratin classified as?
- A. Conjugated protein
- B. Globular protein
- C. Cylindrical protein
- D. Fibrous protein (Correct Answer)
Explanation: ***Fibrous protein*** - **Keratin** is a structural protein characterized by its **elongated, filament-like structure**, which is typical of fibrous proteins. - Fibrous proteins like keratin provide **mechanical strength** and play a significant role in the structure of tissues such as skin, hair, and nails. - Other examples of fibrous proteins include collagen, elastin, and myosin. *Globular protein* - **Globular proteins** have a **compact, spherical shape** and are often water-soluble, serving functions like enzymes, transporters, or receptors (e.g., hemoglobin or albumin). - Keratin's primary role is structural, not catalytic or transport, and its shape is not compact or spherical. *Cylindrical protein* - While some proteins might have a somewhat elongated or tube-like structure, **"cylindrical protein" is not a standard biochemical classification** of protein type. - This term does not accurately describe the characteristic fibrous nature and function of keratin. *Conjugated protein* - **Conjugated proteins** contain a non-protein component (prosthetic group) such as a carbohydrate, lipid, or metal ion attached to the protein (e.g., glycoproteins, lipoproteins, hemoglobin). - Keratin is a **simple fibrous protein** composed only of amino acids without prosthetic groups, so it is not classified as a conjugated protein.
Question 90: Sweaty feet odor in urine is seen in which condition?
- A. Phenylketonuria
- B. Isovaleric acidemia (Correct Answer)
- C. Alkaptonuria
- D. Maple syrup urine disease
Explanation: ***Isovaleric acidemia*** - This condition is characterized by a distinctive "sweaty feet" odor in body fluids, including urine, due to the accumulation of **isovaleric acid**. - It results from a deficiency in the enzyme **isovaleryl-CoA dehydrogenase**, which is crucial for leucine metabolism. *Phenylketonuria* - Patients with **phenylketonuria (PKU)** typically have a "mousy" or "musty" odor in their urine, not a sweaty feet smell. - This is due to the accumulation of **phenylalanine** and its metabolites. *Maple syrup urine disease* - This metabolic disorder is named for the characteristic sweet, maple syrup-like odor of the urine, which is distinctly different from a sweaty feet odor. - It is caused by a defect in the metabolism of **branched-chain amino acids (leucine, isoleucine, and valine)**. *Alkaptonuria* - This condition is known for urine that turns **dark brown or black** upon standing or when exposed to air, due to the oxidation of **homogentisic acid**. - It does not produce a sweaty feet odor.