Biochemistry
10 questionsTaurine is biosynthesized from which amino acid?
Selenocysteine is associated with ?
Which cofactor is primarily associated with the activity of glutamate dehydrogenase?
What is the primary biochemical defect in alkaptonuria?
What metabolic changes occur during overnight fasting?
Which vitamin is considered the most potent antioxidant?
ATP is consumed at which of the following steps of glycolysis?
All are activated by insulin except?
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 81: Taurine is biosynthesized from which amino acid?
- A. Cysteine (Correct Answer)
- B. Valine
- C. Arginine
- D. Leucine
Explanation: ***Cysteine*** - **Taurine** is primarily synthesized from the amino acid **cysteine** through a pathway involving **cysteine sulfinic acid** and **hypotaurine**. - This pathway utilizes enzymes like **cysteine dioxygenase** and **cysteine sulfinic acid decarboxylase**. - The biosynthetic pathway: Cysteine → Cysteine sulfinic acid → Hypotaurine → Taurine. *Arginine* - **Arginine** is a precursor for **nitric oxide**, **urea**, and **creatine**, not taurine. - It is involved in various metabolic pathways, including the **urea cycle** and protein synthesis. *Valine* - **Valine** is a **branched-chain amino acid (BCAA)** involved in protein synthesis and energy production. - It is not a direct precursor for taurine biosynthesis. *Leucine* - **Leucine** is also a **branched-chain amino acid (BCAA)** crucial for protein synthesis and muscle metabolism. - It does not participate in the synthesis of taurine.
Question 82: Selenocysteine is associated with ?
- A. Carbonic anhydrase
- B. Catalase
- C. Transferase
- D. Deiodinase (Correct Answer)
Explanation: ***Deiodinase*** - Selenocysteine is a critical component of **iodothyronine deiodinases**, a family of enzymes that regulate **thyroid hormone metabolism**. - These enzymes catalyze the removal of iodine from thyroid hormones, converting **thyroxine (T4)** into the more active **triiodothyronine (T3)** or inactive forms. *Carbonic anhydrase* - This enzyme contains **zinc** as its essential metal cofactor and is involved in the interconversion of **carbon dioxide** and **bicarbonate**. - Its primary role is in pH regulation and CO2 transport, without any direct association with selenocysteine. *Catalase* - Catalase is an enzyme primarily found in **peroxisomes** and contains **iron-porphyrin** groups as its prosthetic group. - Its function is to convert **hydrogen peroxide** into water and oxygen, protecting cells from oxidative damage. *Transferase* - Transferases are a broad class of enzymes that catalyze the transfer of **functional groups** (e.g., methyl, glucose) from one molecule to another. - While essential for many metabolic processes, there is no inherent association of the general class of transferases with selenocysteine.
Question 83: Which cofactor is primarily associated with the activity of glutamate dehydrogenase?
- A. NAD+ (Correct Answer)
- B. FAD
- C. FMN
- D. FADH2
Explanation: ***NAD+*** - Glutamate dehydrogenase catalyzes the oxidative deamination of **glutamate** to **α-ketoglutarate** and ammonia, and this reaction primarily uses **NAD+** as an electron acceptor. - In some organisms and contexts, it can also use **NADP+**, but **NAD+** is the more common and significant cofactor for its catabolic role. *FAD* - **FAD (flavin adenine dinucleotide)** is typically associated with **flavoproteins** and enzymes involved in oxidation-reduction reactions, such as those in the **electron transport chain** and the **Krebs cycle**. - Enzymes like **succinate dehydrogenase** use FAD, not glutamate dehydrogenase. *FMN* - **FMN (flavin mononucleotide)** is another flavin coenzyme, similar to FAD, and is found in various **flavoproteins** and enzymes of the **electron transport chain**, such as **NADH dehydrogenase (Complex I)**. - It does not serve as a primary cofactor for **glutamate dehydrogenase** activity. *FADH2* - **FADH2** is the reduced form of **FAD**, carrying high-energy electrons to the **electron transport chain** for ATP synthesis. - It's a product or reactant of various metabolic pathways, but not a direct cofactor for **glutamate dehydrogenase**.
Question 84: What is the primary biochemical defect in alkaptonuria?
- A. FeCl3 test is negative
- B. Urine turns black immediately upon voiding
- C. Benedict's test is diagnostic for alkaptonuria
- D. Deficiency of homogentisate 1,2-dioxygenase (Correct Answer)
Explanation: ***Deficiency of homogentisate 1,2-dioxygenase*** - **Alkaptonuria** is an autosomal recessive disorder caused by the deficiency of **homogentisate 1,2-dioxygenase**, an enzyme in the **tyrosine degradation pathway**. - This deficiency leads to the accumulation of **homogentisic acid** in the body, which is excreted in urine and deposited in connective tissues. *Urine turns black immediately upon voiding* - While urine in alkaptonuria does **turn black**, it typically darkens upon **standing** and exposure to air, not immediately upon voiding. - The darkening is due to the oxidation of accumulated **homogentisic acid**. *FeCl3 test is negative* - The **ferric chloride (FeCl3) test** typically yields a **positive result** (transient green color) in the presence of homogentisic acid in the urine. - Therefore, a negative result would argue against a diagnosis of alkaptonuria. *Benedict's test is diagnostic for alkaptonuria* - **Benedict's test** is used to detect reducing sugars like glucose in urine and would not be used to diagnose alkaptonuria. - A positive Benedict's test in alkaptonuria is due to the reducing properties of homogentisic acid, but it is not specific or diagnostic.
Question 85: What metabolic changes occur during overnight fasting?
- A. Blood glucose decreases slightly
- B. Fat breakdown increases
- C. Glucose production increases (Correct Answer)
- D. Ketone levels rise slightly
Explanation: ***Glucose production increases*** - During overnight fasting (typically 8-12 hours), the body's **primary metabolic priority** is to maintain **blood glucose homeostasis** to fuel the brain and other glucose-dependent tissues. - As **hepatic glycogen stores** become depleted, the liver significantly increases **gluconeogenesis** (glucose production from non-carbohydrate sources like amino acids, lactate, and glycerol) to supply glucose. - This represents the **most critical metabolic adaptation** during overnight fasting, as the brain requires a constant glucose supply (~120g/day) and cannot initially use alternative fuels. *Blood glucose decreases slightly* - During a normal overnight fast, blood glucose levels remain **relatively stable** (70-100 mg/dL) due to compensatory mechanisms. - The body's homeostatic mechanisms (increased glucose production, decreased glucose utilization by muscles) prevent any significant drop in blood glucose. - A significant decrease would indicate **hypoglycemia**, which is prevented by the metabolic changes described above. *Fat breakdown increases* - **Lipolysis** (fat breakdown) does indeed increase significantly during overnight fasting to provide **fatty acids** as an alternative fuel source for skeletal muscle, cardiac muscle, and liver. - This is an important metabolic change, but is **secondary to glucose production** in terms of priority, as it serves to spare glucose for the brain rather than directly maintaining glucose levels. - Increased fatty acid oxidation provides acetyl-CoA for **ketone body synthesis** and reduces glucose consumption by peripheral tissues (glucose-sparing effect). *Ketone levels rise slightly* - **Ketone body production** (acetoacetate, β-hydroxybutyrate) does begin to increase as fasting progresses beyond 8-12 hours. - However, during an *overnight* fast, ketone levels rise only **modestly** (typically <1 mM); clinically significant ketosis develops during **prolonged fasting** (24-72 hours), when ketone bodies become a major fuel source for the brain. - The overnight period represents the **transition phase** where glucose production remains the dominant metabolic response.
Question 86: Which vitamin is considered the most potent antioxidant?
- A. Vit A
- B. Vit K
- C. Vit E (Correct Answer)
- D. Vit C
Explanation: ***Vit E*** - **Vitamin E** is a **lipid-soluble antioxidant** that primarily protects cell membranes from **oxidative damage** by scavenging free radicals. - Its ability to interrupt **lipid peroxidation** makes it highly effective in protecting tissues rich in polyunsaturated fatty acids, such as cell membranes. *Vit A* - **Vitamin A**, particularly in its carotenoid forms like **beta-carotene**, is an antioxidant, but its primary role is in **vision** and **immune function**. - While it can quench **singlet oxygen** and trap free radicals, it is generally considered less potent than vitamin E in protecting against lipid peroxidation. *Vit K* - **Vitamin K** is crucial for **blood coagulation** and **bone metabolism**, but it does not have significant antioxidant properties. - Its primary biological functions are unrelated to scavenging **free radicals** or preventing oxidative stress. *Vit C* - **Vitamin C** is a potent **water-soluble antioxidant** that works in aqueous environments, such as the cytoplasm and extracellular fluid. - While it can neutralize **reactive oxygen species** and regenerate other antioxidants like vitamin E, its solubility limits its direct activity in protecting lipid membranes, making vitamin E more potent in that specific context.
Question 87: ATP is consumed at which of the following steps of glycolysis?
- A. Pyruvate kinase
- B. Isomerase
- C. Hexokinase (Correct Answer)
- D. Enolase
Explanation: ***Hexokinase*** - This enzyme catalyzes the **first step of glycolysis**, the phosphorylation of glucose to **glucose-6-phosphate**, which requires the consumption of one molecule of **ATP**. - ATP is hydrolyzed to **ADP**, providing the necessary phosphate group and energy for this irreversible reaction. - Note: Hexokinase is one of **two ATP-consuming steps** in glycolysis (the other being phosphofructokinase in step 3). *Pyruvate kinase* - This enzyme catalyzes the **final step of glycolysis**, converting **phosphoenolpyruvate (PEP)** to pyruvate. - This reaction involves the **production of ATP** from ADP, not its consumption, as it's one of the substrate-level phosphorylation steps. *Isomerase* - Isomerase enzymes, like phosphoglucose isomerase, convert one isomer to another (e.g., glucose-6-phosphate to fructose-6-phosphate). - These reactions generally involve an **internal rearrangement of atoms** and do not directly consume or produce ATP. *Enolase* - Enolase catalyzes the reversible conversion of **2-phosphoglycerate to phosphoenolpyruvate (PEP)**, releasing a molecule of water. - This step occurs before the ATP-generating step catalyzed by pyruvate kinase and **does not consume or produce ATP**.
Question 88: All are activated by insulin except?
- A. Lipoprotein lipase
- B. Pyruvate kinase
- C. Acetyl-CoA carboxylase
- D. Hormone sensitive lipase (Correct Answer)
Explanation: ***Hormone sensitive lipase*** - **Insulin** is an **anabolic hormone** that promotes energy storage; it **inhibits** hormone-sensitive lipase (HSL) activity which is responsible for **fat breakdown (lipolysis)**. - When insulin levels are high, the body stores fat rather than breaks it down, thus **decreasing** HSL activity. *Lipoprotein lipase* - **Insulin activates lipoprotein lipase (LPL)**, an enzyme that breaks down triglycerides in **chylomicrons** and **VLDL** into fatty acids for storage in adipose tissue. - This activation promotes the uptake of fatty acids into fat cells, aligning with insulin's role in **energy storage**. *Pyruvate kinase* - **Insulin activates pyruvate kinase** in glycolysis, promoting the conversion of **phosphoenolpyruvate to pyruvate**. - This enzyme's activation enhances glucose utilization and energy production following a meal when insulin levels are high. *Acetyl-CoA carboxylase* - **Insulin activates acetyl-CoA carboxylase (ACC)**, the **rate-limiting enzyme in fatty acid synthesis**. - Activation of ACC leads to the production of **malonyl-CoA**, which commits acetyl-CoA to fatty acid synthesis, storing excess energy as fat.
Question 89: 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 90: 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.