Biochemistry
10 questionsWhy is the citric acid cycle called an amphibolic pathway?
Which enzyme is deficient in Isovaleric acidemia?
Neonatal tyrosinemia is due to deficiency of which enzyme?
Which enzyme deficiency is responsible for Hyperammonemia type-1?
Which of the following is a lipotropic factor?
Apo-E deficiency is seen in which of the following conditions?
Which of the following is an ω-6 fatty acid?
Which of the following enzymes uses citrate in fatty acid synthesis?
What is the rate-controlling enzyme of fatty acid synthesis?
What coenzyme is required by gulonate dehydrogenase for its activity?
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 361: Why is the citric acid cycle called an amphibolic pathway?
- A. Both exergonic and endergonic reactions take place
- B. Metabolites are utilized in other pathways. (Correct Answer)
- C. It can proceed in both forward and backward directions.
- D. The same enzymes can be used in reverse directions.
Explanation: ***Metabolites are utilized in other pathways.*** - The citric acid cycle is termed **amphibolic** because it serves both catabolic (breakdown) and anabolic (synthetic) functions. - Its intermediates are constantly drawn off for biosynthesis of molecules like **amino acids**, **heme**, and **glucose**, meaning it's not solely degradative. *Both exergonic and endergonic reactions take place* - While both types of reactions do occur in many metabolic pathways, this is a general characteristic of metabolism and not specific to the definition of an **amphibolic pathway**. - The amphibolic nature specifically refers to the dual role in both **catabolism** and **anabolism**. *It can proceed in both forward and backward directions.* - This statement typically describes a **reversible pathway** or individual reversible reactions, not necessarily an amphibolic pathway. - The citric acid cycle is primarily an oxidative cycle that proceeds in a forward, cyclic direction under aerobic conditions. *The same enzymes can be used in reverse directions.* - While some individual enzymes within metabolic pathways can catalyze reversible reactions, this is not the defining characteristic of an **amphibolic pathway**. - The amphibolic designation refers to the overall pathway's contribution to both breakdown and synthesis of molecules.
Question 362: 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
Question 363: 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 364: 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 365: 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 366: 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 367: 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 368: 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 369: 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 370: 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.