Glycogen synthase is activated by?
What coenzyme is required by gulonate dehydrogenase for its activity?
What is the rate-controlling enzyme of fatty acid synthesis?
Which of the following enzymes uses citrate in fatty acid synthesis?
Which of the following is an ω-6 fatty acid?
Apo-E deficiency is seen in which of the following conditions?
Which of the following is a lipotropic factor?
Which enzyme deficiency is responsible for Hyperammonemia type-1?
Neonatal tyrosinemia is due to deficiency of which enzyme?
Which enzyme is deficient in Isovaleric acidemia?
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 151: Glycogen synthase is activated by?
- A. Insulin (Correct Answer)
- B. Glucagon
- C. AMP
- D. Epinephrine
Explanation: **Insulin** - Insulin activates **glycogen synthase** through a signaling cascade that dephosphorylates the enzyme, shifting it to its active form (glycogen synthase a). - This activation promotes **glycogen synthesis** in the liver and muscles, lowering blood glucose levels. *Glucagon* - **Glucagon** primarily acts to increase blood glucose levels by activating **glycogen phosphorylase** and inhibiting glycogen synthase. - It promotes the breakdown of glycogen (glycogenolysis) rather than its synthesis. *Epinephrine* - **Epinephrine** (adrenaline) also promotes **glycogenolysis** (glycogen breakdown) by activating glycogen phosphorylase. - Its main role is to provide rapid energy during stress, not to store glucose as glycogen. *AMP* - **AMP** (adenosine monophosphate) is a key allosteric activator of **AMP-activated protein kinase (AMPK)**, which phosphorylates and inactivates glycogen synthase. - High AMP levels signal a low energy state, prompting ATP-generating pathways like glycogenolysis, not glycogen synthesis.
Question 152: What coenzyme is required by gulonate dehydrogenase for its activity?
- A. FAD
- B. FMN
- C. NADP
- D. NAD (Correct Answer)
Explanation: ***NAD*** - **Gulonate dehydrogenase** is an enzyme involved in the **uronic acid pathway**, specifically in the conversion of **L-gulonate to D-xylulose**. - This reaction is an **NAD-dependent oxidation**, meaning **NAD** acts as the electron acceptor, being reduced to **NADH**. *NADP* - **NADP** (nicotinamide adenine dinucleotide phosphate) is primarily involved in **anabolic pathways** like **fatty acid synthesis** and the **pentose phosphate pathway**, often in reduction reactions where it is converted to **NADPH**. - While structurally similar to NAD, it is generally not the direct coenzyme for gulonate dehydrogenase. *FAD* - **FAD** (flavin adenine dinucleotide) is a coenzyme derived from **riboflavin** (vitamin B2) and is typically involved in **redox reactions** where it repeatedly accepts and donates electrons, often in dehydrogenase reactions involving **carbon-carbon double bonds**. - Enzymes like **succinate dehydrogenase** (in the citric acid cycle) or acyl-CoA dehydrogenase (in fatty acid oxidation) utilize FAD, but not gulonate dehydrogenase. *FMN* - **FMN** (flavin mononucleotide) is another coenzyme derived from **riboflavin** and serves as a prosthetic group in various **flavoproteins**, often facilitating **single-electron transfers**. - It is frequently found in complexes like **NADH dehydrogenase** (Complex I of the electron transport chain) but is not the required coenzyme for gulonate dehydrogenase activity.
Question 153: What is the rate-controlling enzyme of fatty acid synthesis?
- A. Thioesterase
- B. Transacetylase
- C. Acetyl-CoA carboxylase (Correct Answer)
- D. Ketoacyl synthase
Explanation: ***Acetyl-CoA carboxylase*** - **Acetyl-CoA carboxylase (ACC)** catalyzes the committed step in fatty acid synthesis, converting **acetyl-CoA** to **malonyl-CoA**. - This enzyme is subject to both allosteric regulation (e.g., activation by **citrate** and inhibition by **long-chain fatty acyl-CoA**) and hormonal regulation (e.g., phosphorylation by glucagon and dephosphorylation by insulin). *Thioesterase* - **Thioesterase** is the enzyme responsible for releasing the completed fatty acid chain from the **fatty acid synthase complex**. - While essential for the termination of synthesis, it does not regulate the initiation or overall rate of the pathway. *Transacetylase* - **Transacetylase** (specifically, acetyl-CoA-ACP transacetylase and malonyl-CoA-ACP transacetylase) is involved in transferring acetyl and malonyl groups to the **acyl carrier protein (ACP)** within the fatty acid synthesis complex. - This is an intermediary step, but not the primary **rate-controlling** or committed step. *Ketoacyl synthase* - **Ketoacyl synthase (or β-ketoacyl-ACP synthase)** is responsible for condensing the growing acyl chain with malonyl-ACP, leading to the formation of a **β-ketoacyl-ACP**. - This is a crucial chain elongation step within the fatty acid synthase complex, but not the enzyme that controls the overall commitment to fatty acid synthesis.
Question 154: Which of the following enzymes uses citrate in fatty acid synthesis?
- A. Aconitase
- B. ATP citrate lyase (Correct Answer)
- C. Malic enzyme
- D. Citrate synthase
Explanation: ***ATP citrate lyase*** - This enzyme is crucial for fatty acid synthesis, as it cleaves **citrate** in the cytoplasm to generate **acetyl-CoA** and oxaloacetate. - The acetyl-CoA produced is then used as the primary building block for **fatty acid synthesis**. *Aconitase* - This enzyme isomerizes **citrate** to isocitrate within the **Krebs cycle** (TCA cycle) in the mitochondria. - It does not directly participate in the cytosolic pathway of fatty acid synthesis. *Citrate synthase* - This enzyme synthesizes **citrate** from acetyl-CoA and oxaloacetate, initiating the **Krebs cycle** in the mitochondrial matrix. - It is involved in citrate formation, not its utilization for fatty acid synthesis in the cytoplasm. *Malic enzyme* - This enzyme converts **malate** to pyruvate, generating **NADPH** in the cytoplasm. - While NADPH is essential for fatty acid synthesis, malic enzyme does not directly use citrate.
Question 155: Which of the following is an ω-6 fatty acid?
- A. Cervonic acid
- B. Linoleic acid (Correct Answer)
- C. Alpha linolenic acid
- D. Elaidic acid
Explanation: ***Linoleic acid*** - **Linoleic acid** (LA), an 18-carbon fatty acid with two double bonds (18:2), is classified as an **ω-6 fatty acid** because its first double bond is located at the sixth carbon atom from the methyl end of the fatty acid chain. - It is an **essential fatty acid** that must be obtained through diet, serving as a precursor for other ω-6 fatty acids like arachidonic acid. *Cervonic acid* - **Cervonic acid** is another name for **docosahexaenoic acid (DHA)**, which is an **ω-3 fatty acid** (22:6). - Its first double bond is located at the third carbon from the methyl end. *Alpha linolenic acid* - **Alpha-linolenic acid** (ALA) is an **ω-3 fatty acid** (18:3). - Its first double bond is located at the third carbon atom from the methyl end. *Elaidic acid* - **Elaidic acid** is a **trans fatty acid** (18:1 trans-9). - It is classified as an **ω-9 fatty acid** due to the position of its double bond, but its trans configuration is the primary distinguishing feature.
Question 156: Apo-E deficiency is seen in which of the following conditions?
- A. Type II hyperlipoproteinemia
- B. Type III hyperlipoproteinemia (Correct Answer)
- C. Type I hyperlipoproteinemia
- D. Type IV hyperlipoproteinemia
Explanation: ***Type III hyperlipoproteinemia*** - This condition, also known as **familial dysbetalipoproteinemia** or **broad beta disease**, is characterized by a deficiency or abnormal function of **apolipoprotein E (apoE)**. - The deficiency in functional apoE impairs the clearance of **chylomicron remnants** and **intermediate-density lipoproteins (IDLs)** from the blood. *Type II hyperlipoproteinemia* - This condition primarily involves elevated **LDL cholesterol** and is often due to defects in the **LDL receptor** or mutations in **apoB-100**, not apoE deficiency. - It does not directly involve the impaired clearance of chylomicron remnants or IDLs. *Type I hyperlipoproteinemia* - Also known as **familial chylomicronemia syndrome**, this condition is characterized by severe elevation of **chylomicrons** and **triglycerides**. - It is caused by a deficiency of **lipoprotein lipase (LPL)** or its cofactor **apoC-II**, not apoE. *Type IV hyperlipoproteinemia* - This condition, also known as **familial hypertriglyceridemia**, is characterized by abnormally high levels of **very-low-density lipoproteins (VLDL)** and **triglycerides**. - It is typically caused by increased VLDL production or impaired VLDL clearance, but not directly due to an apoE deficiency.
Question 157: Which of the following is a lipotropic factor?
- A. Sphingomyelin
- B. Histidine
- C. Bilirubin
- D. Methionine (Correct Answer)
Explanation: ***Methionine*** - **Methionine** is an essential amino acid that serves as a precursor for **choline** and **creatine**, both of which play crucial roles in lipid metabolism and transport. - Lipotropic factors prevent or reverse the accumulation of **fat in the liver** by promoting the synthesis of **lipoproteins**, which package and transport fats from the liver to other tissues. *Sphingomyelin* - **Sphingomyelin** is a type of **sphingolipid**, a component of cell membranes and myelin sheaths, but it does not directly act as a lipotropic factor to prevent fatty liver. - While it's involved in cellular signaling and membrane structure, it does not directly facilitate the metabolism or transport of **hepatic triglycerides** in the same way as lipotropic agents. *Histidine* - **Histidine** is an essential amino acid involved in protein synthesis and the production of **histamine**, but it is not considered a primary lipotropic factor. - Its main roles are in **immune response** and **neurotransmission**, not in preventing fat accumulation in the liver. *Bilirubin* - **Bilirubin** is a waste product from the breakdown of **heme**, primarily from red blood cells. It is excreted by the liver. - It is known for its **antioxidant properties** but does not play a direct role as a lipotropic factor in lipid metabolism or in preventing **fatty liver**.
Question 158: Which enzyme deficiency is responsible for Hyperammonemia type-1?
- A. Arginase deficiency
- B. Arginosuccinate lyase deficiency
- C. Arginosuccinate synthase deficiency
- D. Carbamoyl phosphate synthetase I (CPS-1) deficiency (Correct Answer)
Explanation: ***Carbamoyl phosphate synthetase I (CPS-1) deficiency*** - This enzyme deficiency is classified as **Hyperammonemia type-1**, or **CPS1 deficiency**, and results in the inability to initiate the urea cycle. - **CPS-1** catalyzes the first committed step of the urea cycle, combining ammonia and bicarbonate to form carbamoyl phosphate. *Arginase deficiency* - This deficiency causes **Hyperargininemia**, which is a disorder of the urea cycle distinct from Hyperammonemia type-1. - Arginase is involved in the final step of the urea cycle, converting arginine to urea and ornithine. *Arginosuccinate lyase deficiency* - This deficiency leads to **Argininosuccinic aciduria**, another urea cycle disorder. - **Arginosuccinate lyase** is responsible for breaking down argininosuccinate into arginine and fumarate. *Arginosuccinate synthase deficiency* - This deficiency causes **Citrullinemia type 1**, a metabolic disorder characterized by high levels of citrulline and ammonia. - **Arginosuccinate synthase** catalyzes the condensation of citrulline and aspartate to form argininosuccinate.
Question 159: Neonatal tyrosinemia is due to deficiency of which enzyme?
- A. Tyrosine transaminase
- B. Hydroxyphenyl pyruvate hydroxylase (Correct Answer)
- C. Fumarylacetoacetate hydroxylase
- D. Tyrosinase
Explanation: ***Hydroxyphenyl pyruvate hydroxylase*** - **Neonatal (transient) tyrosinemia** is caused by delayed maturation or deficiency of **hydroxyphenylpyruvate hydroxylase** (also called 4-hydroxyphenylpyruvate dioxygenase or HPPD). - This enzyme converts 4-hydroxyphenylpyruvate to homogentisic acid in tyrosine catabolism. - Common in **premature infants** and newborns, leading to elevated tyrosine levels in blood. - The condition is **benign and self-limiting**, usually resolving with **vitamin C supplementation** or as the enzyme matures. - Note: Severe hereditary deficiency of this enzyme causes **tyrosinemia type III**, a distinct and rare disorder. *Fumarylacetoacetate hydroxylase* - Deficiency of **fumarylacetoacetate hydroxylase (FAH)** causes **tyrosinemia type I** (hepatorenal tyrosinemia), NOT neonatal tyrosinemia. - This is a severe hereditary disorder with liver failure, renal tubular dysfunction, and accumulation of toxic metabolites like succinylacetone. - Distinct from the benign transient neonatal form. *Tyrosine transaminase* - Deficiency of **tyrosine transaminase** (tyrosine aminotransferase) causes **tyrosinemia type II** (Richner-Hanhart syndrome). - Presents with corneal ulcers, palmoplantar hyperkeratosis, and sometimes intellectual disability. *Tyrosinase* - Deficiency of **tyrosinase** causes **albinism**, characterized by lack of melanin pigment in skin, hair, and eyes. - Not involved in tyrosine catabolism but in melanin synthesis.
Question 160: Which enzyme is deficient in Isovaleric acidemia?
- A. Isovaleryl CoA dehydrogenase (Correct Answer)
- B. Phenylalanine hydroxylase
- C. Arginase
- D. Methylmalonyl CoA mutase
Explanation: ***Isovaleryl CoA dehydrogenase*** - **Isovaleric acidemia** is an **autosomal recessive** metabolic disorder caused by a deficiency in the enzyme **isovaleryl-CoA dehydrogenase** - This enzyme is crucial for the metabolism of **leucine**, a branched-chain amino acid, leading to the accumulation of toxic byproducts like **isovaleryl-CoA** and **isovaleric acid** - Characteristic **sweaty feet odor** due to isovaleric acid accumulation *Phenylalanine hydroxylase* - A deficiency in **phenylalanine hydroxylase** is responsible for **phenylketonuria (PKU)**, a different metabolic disorder involving the metabolism of **phenylalanine** - This enzyme converts **phenylalanine to tyrosine**, and its deficiency leads to the accumulation of phenylalanine and its metabolites, causing neurological damage if untreated *Arginase* - A deficiency in **arginase** causes **argininemia (hyperargininemia)**, which is a disorder of the **urea cycle** - This enzyme converts **arginine into urea and ornithine**, and its deficiency leads to the buildup of arginine and ammonia in the blood, causing neurological symptoms and developmental delay *Methylmalonyl CoA mutase* - A deficiency in **methylmalonyl CoA mutase** causes **methylmalonic acidemia**, another organic acidemia distinct from isovaleric acidemia - This disorder involves **propionate metabolism** and can present with metabolic acidosis, but affects a different metabolic pathway than leucine catabolism