NEET-PG 2013 - Biochemistry Practice Questions

Q: Serotonin is derived from -

Tryptophan. ***Tryptophan*** - **Serotonin**, also known as 5-hydroxytryptamine (5-HT), is synthesized from the essential amino acid **tryptophan** through a two-step enzymatic pathway. - Tryptophan is first hydroxylated by tryptophan hydroxylase to 5-hydroxytryptophan (5-HTP), which is then decarboxylated by L-amino acid decarboxylase to form serotonin. *Tyrosine* - **Tyrosine** is a precursor for the synthesis of **catecholamines** (dopamine, norepinephrine, and epinephrine) and thyroid hormones. - It is not involved in the synthesis pathway for serotonin. *Phenylalanine* - **Phenylalanine** is an essential amino acid that is hydroxylated to form **tyrosine**. - Therefore, it is indirectly involved in catecholamine synthesis but not in serotonin synthesis. *Methionine* - **Methionine** is an essential amino acid primarily known for its role in protein synthesis and as a precursor for **S-adenosylmethionine (SAM)**, a methyl group donor in many biological reactions. - It does not serve as a direct precursor for serotonin.

Q: Catecholamines are synthesized from?

Tyrosine. ***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**.

Q: Which of the following amino acids is not involved in the production of creatine?

Alanine. ***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**.

Q: Ninhydrin test is used for?

Amino acids. ***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.

Q: Which enzyme catalyzes oxidative deamination?

Glutamate dehydrogenase. ***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.

Q: Transamination of Aspartate forms which compound?

Oxaloacetate. ***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.

Q: Which defect in the urea cycle is an X-linked disease?

Ornithine transcarbamylase. ***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.

Q: What is the primary enzymatic source of ammonia production in urine?

Glutaminase. ***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.

Q: Which amino acids accumulate in maple syrup urine disease?

All branched-chain amino acids. ***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.

Q: What type of protein is keratin classified as?

Fibrous protein. ***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 1

Serotonin is derived from -

Accepted Answer

Tryptophan

Answer

Tyrosine

Answer

Phenylalanine

Answer

Methionine

Question 2

Catecholamines are synthesized from?

Accepted Answer

Tyrosine

Answer

Histidine

Answer

Methionine

Answer

Tryptophan

Question 3

Which of the following amino acids is not involved in the production of creatine?

Accepted Answer

Alanine

Answer

Glycine

Answer

Methionine

Answer

Arginine

Question 4

Ninhydrin test is used for?

Accepted Answer

Amino acids

Answer

Bile salts

Answer

Nucleic acid

Answer

Lipids

Question 5

Which enzyme catalyzes oxidative deamination?

Accepted Answer

Glutamate dehydrogenase

Answer

Glutaminase

Answer

Glutamine synthase

Answer

None of the options

Question 6

Transamination of Aspartate forms which compound?

Accepted Answer

Oxaloacetate

Answer

Pyruvate

Answer

Acetyl-CoA

Answer

Alanine

Question 7

Which defect in the urea cycle is an X-linked disease?

Accepted Answer

Ornithine transcarbamylase

Answer

Arginase

Answer

Argininosuccinate synthase

Answer

Carbamoyl phosphate synthetase I

Question 8

What is the primary enzymatic source of ammonia production in urine?

Accepted Answer

Glutaminase

Answer

Urease

Answer

Glutamate dehydrogenase

Answer

Arginase

Question 9

Which amino acids accumulate in maple syrup urine disease?

Accepted Answer

All branched-chain amino acids

Answer

Valine

Answer

Leucine

Answer

Isoleucine

Question 10

What type of protein is keratin classified as?

Accepted Answer

Fibrous protein

Answer

Conjugated protein

Answer

Globular protein

Answer

Cylindrical protein

NEET-PG 2013 — Biochemistry