Biochemistry
7 questionsIn the malate shuttle, how many ATPs are produced from one NADH?
Which of the following statements about NADP is correct?
The mechanism of action of uncouplers of oxidative phosphorylation involves:
What is the mechanism of conversion of trypsinogen to trypsin?
Enzyme activity is expressed as?
Rate limiting enzyme in bile acid synthesis?
How many isoenzymes does lactate dehydrogenase (LDH) have?
NEET-PG 2015 - Biochemistry NEET-PG Practice Questions and MCQs
Question 521: In the malate shuttle, how many ATPs are produced from one NADH?
- A. 1 ATP
- B. 3 ATP
- C. 2 ATP
- D. 2.5 ATP (Correct Answer)
Explanation: ***2.5 ATP*** - In the **malate-aspartate shuttle**, mitochondrial **NADH** is regenerated from cytosolic NADH, and then enters the electron transport chain at **Complex I**. - **Complex I** entry means that NADH contributes to the pumping of enough protons to generate approximately **2.5 ATP** through oxidative phosphorylation. *1 ATP* - **1 ATP** is not the direct equivalent produced from the reoxidation of one NADH via the malate shuttle into the electron transport chain. - This value is typically associated with the direct hydrolysis of **ATP** or the energy equivalent of **GTP** produced in the citric acid cycle. *3 ATP* - Historically, **3 ATP** was the accepted stoichiometry for one NADH, but more accurate measurements of proton pumping and ATP synthase activity have revised this. - The value of 3 ATP per NADH does not reflect the most current understanding of mitochondrial bioenergetics. *2 ATP* - **2 ATP** is the approximate yield for **FADH2** entering the electron transport chain at **Complex II**, bypassing Complex I, and thus pumping fewer protons. - This value is not applicable to NADH transferred via the malate-aspartate shuttle, as NADH enters at Complex I.
Question 522: Which of the following statements about NADP is correct?
- A. Involved in fatty acid oxidation
- B. Involved in HMP shunt (Correct Answer)
- C. Involved in glycolysis
- D. Acts as a coenzyme form of Riboflavin
Explanation: ***Involved in HMP shunt*** - **NADPH**, the reduced form of NADP+, is primarily generated in the **hexose monophosphate shunt (HMP shunt)**, specifically during the oxidative phase. - The NADPH produced in the HMP shunt is crucial for **reductive biosynthesis** reactions and maintaining the **redox balance** of the cell. *Acts as a coenzyme form of Riboflavin* - **NADP is derived from Niacin (Vitamin B3)**, not Riboflavin (Vitamin B2). - **Flavin adenine dinucleotide (FAD)** and **flavin mononucleotide (FMN)** are the coenzyme forms of Riboflavin. *Involved in glycolysis* - **NADP is not directly involved in glycolysis**; instead, **NAD+** is the primary coenzyme that accepts electrons in glycolysis, specifically during the oxidation of glyceraldehyde-3-phosphate. - While some enzymes in glycolysis can interact with NADP+ under specific conditions, its main role is not within the glycolytic pathway. *Involved in fatty acid oxidation* - **Fatty acid oxidation (beta-oxidation)** primarily utilizes **NAD+** and **FAD** as electron acceptors. - **NADP+** is not a direct participant in the electron transport chain during fatty acid breakdown.
Question 523: The mechanism of action of uncouplers of oxidative phosphorylation involves:
- A. Inhibition of ATP synthase
- B. Stimulation of ATP synthase
- C. Disruption of proton gradient across the inner membrane (Correct Answer)
- D. Blocking electron transport chain complexes
Explanation: ***Disruption of proton gradient across the inner membrane*** - Uncouplers such as **2,4-dinitrophenol** increase the permeability of the **inner mitochondrial membrane** to protons. - This dissipates the **proton motive force** that is normally used by ATP synthase to produce ATP, leading to the uncoupling of electron transport from ATP synthesis. *Inhibition of ATP synthase* - Inhibitors of ATP synthase directly block the enzyme's activity, preventing the synthesis of ATP while the **proton gradient** remains intact. - This mechanism is distinct from uncouplers, which allow electron transport to continue while dissipating the proton gradient. *Stimulation of ATP synthase* - Uncouplers do not stimulate ATP synthase; rather, their action prevents ATP synthase from effectively utilizing the **proton gradient** for ATP production. - Stimulation of ATP synthase would lead to increased ATP synthesis, which is contrary to the effect of uncouplers. *Blocking electron transport chain complexes* - Inhibitors of the **electron transport chain** (e.g., cyanide, rotenone) directly prevent the flow of electrons, thereby preventing the pumping of protons and the formation of a **proton gradient**. - Uncouplers, in contrast, allow electron transport to proceed but dissipate the proton gradient after it has been established.
Question 524: What is the mechanism of conversion of trypsinogen to trypsin?
- A. Hydrolysis
- B. Phosphorylation
- C. Removal of part of protein (Correct Answer)
- D. Removal of Carboxyl group
Explanation: ***Removal of part of protein*** - The conversion of **trypsinogen to trypsin** is an example of **proteolytic activation**, where a specific part of the inactive precursor (zymogen) is cleaved off. - This cleavage occurs at the N-terminus of trypsinogen by **enteropeptidase (or enterokinase)** in the duodenum, exposing the active site and forming active trypsin. *Hydrolysis* - While the removal of a part of the protein involves **hydrolysis of peptide bonds**, this option is too general. - It does not specify the selective nature of the cleavage that leads to activation, nor the fact that it's a specific segment being removed. *Phosphorylation* - **Phosphorylation** is a common mechanism for regulating enzyme activity, but it involves the addition of a **phosphate group**, not the removal of a protein segment. - This process is typically mediated by kinases and does not activate trypsinogen. *Removal of Carboxyl group* - The activation of trypsinogen involves the removal of a small N-terminal peptide, not specifically the removal of a **carboxyl group** from the protein. - While enzymatic cleavage does involve breaking peptide bonds, stating "removal of carboxyl group" is imprecise and does not accurately describe the mechanism.
Question 525: Enzyme activity is expressed as?
- A. Millimoles/lit
- B. Milli gm/lit
- C. Mg/dl
- D. Micromoles/min (Correct Answer)
Explanation: ***Micromoles/min*** - Enzyme activity is typically measured by the rate at which an enzyme converts its **substrate into product**. - This rate is often expressed as the amount of product formed (e.g., **micromoles**) or substrate consumed per unit of time (e.g., **per minute**). *Millimoles/lit* - This unit expresses **concentration** (moles per liter) rather than a rate of reaction. - While enzyme reactions involve changes in substrate/product concentration, this unit alone does not describe the **activity or catalytic speed** of the enzyme. *Milli gm/lit* - This unit also expresses **concentration by mass** (milligrams per liter), not enzyme activity. - It does not account for the **time-dependent nature** of enzyme catalysis or the molar quantity of reactants/products. *Mg/dl* - This unit represents **concentration by mass** (milligrams per deciliter), commonly used for measuring substances like glucose or cholesterol in blood. - It is not appropriate for expressing the **catalytic rate or activity** of an enzyme.
Question 526: Rate limiting enzyme in bile acid synthesis?
- A. Desmolase
- B. 21α-hydroxylase
- C. 7α-hydroxylase (Correct Answer)
- D. 12α-hydroxylase
Explanation: ***7α-hydroxylase*** - This enzyme, specifically **cholesterol 7α-hydroxylase**, catalyzes the first and rate-limiting step in the classic pathway of **bile acid synthesis**, converting cholesterol to 7α-hydroxycholesterol. - Its activity is tightly regulated, primarily by the availability of cholesterol and feedback inhibition by bile acids, making it a key control point. *Desmolase* - **Cholesterol desmolase** (CYP11A1) is the rate-limiting enzyme in **steroid hormone synthesis** in the adrenal glands, converting cholesterol to pregnenolone. - It is not involved in the committed steps of bile acid synthesis from cholesterol. *21α-hydroxylase* - **21α-hydroxylase** (CYP21A2) is crucial in the synthesis of **cortisol and aldosterone** from progesterone and 17-hydroxyprogesterone, respectively. - Deficiency in this enzyme is the most common cause of **congenital adrenal hyperplasia**, but it has no direct role in bile acid synthesis. *12α-hydroxylase* - **12α-hydroxylase** (CYP8B1) is an enzyme involved in the later steps of bile acid synthesis, specifically in the formation of **cholic acid** from 7α-hydroxy-4-cholesten-3-one. - While essential for synthesizing primary bile acids, it is not the *rate-limiting enzyme* for the overall pathway; 7α-hydroxylase holds that distinction.
Question 527: How many isoenzymes does lactate dehydrogenase (LDH) have?
- A. 5, based on H and M polypeptide subunits (Correct Answer)
- B. 7, based on H and M polypeptide subunits
- C. 9, based on H and M polypeptide subunits
- D. 3, based on H and M polypeptide subunits
Explanation: **5, based on H and M polypeptide subunits** - **Lactate dehydrogenase (LDH)** is a tetrameric enzyme, meaning it is composed of four polypeptide subunits. - These subunits can be either **H (heart)** type or **M (muscle)** type, leading to five distinct isoenzymes (**LDH-1, LDH-2, LDH-3, LDH-4, LDH-5**) based on their combinations (HHHH, HHHM, HHMM, HMMM, MMMM). *7, based on H and M polypeptide subunits* - While LDH is composed of two types of subunits, H and M, the possible combinations of these four subunits result in **five distinct isoenzymes**, not seven. - Seven isoenzymes are not a recognized number for LDH. *9, based on H and M polypeptide subunits* - The combination of two types of subunits in a tetrameric structure cannot yield nine unique isoenzymes. - This number is incorrect and not supported by the biochemistry of LDH. *3, based on H and M polypeptide subunits* - Three isoenzymes would imply either fewer than four subunits or a more restricted combination, which is not the case for LDH's tetrameric structure with H and M subunits. - This number is insufficient to account for all possible combinations.
Pathology
1 questionsWhat is the first cellular response observed after a sharp nerve cut?
NEET-PG 2015 - Pathology NEET-PG Practice Questions and MCQs
Question 521: What is the first cellular response observed after a sharp nerve cut?
- A. Chromatolysis (Correct Answer)
- B. Polymorphic arrangement
- C. Increased protein synthesis
- D. Macrophage activation
Explanation: ***Chromatolysis*** - **Chromatolysis** is the dissolution of the Nissl bodies (rough endoplasmic reticulum) in the neuron cell body following axonal injury, which is the **first observable cellular response** [1]. - This process is a preparatory step for neuronal regeneration, indicating the cell's attempt to repair the damaged axon [1]. *Polymorphic arrangement* - This term is not typically used to describe an immediate cellular response to a nerve cut; it might refer to diverse cell shapes or arrangements in different contexts but is not a recognized initial post-injury event. - The neuron's immediate response involves changes within the cell body, not a re-arrangement of its cellular structure with other cells. *Increased protein synthesis* - While increased protein synthesis does occur during neuronal repair and regeneration, it is a consequence of chromatolysis and part of a later, more sustained response, not the very first visible cellular change [1]. - **Chromatolysis precedes** and facilitates the subsequent increase in protein synthesis necessary for axonal regrowth [1]. *Macrophage activation* - **Macrophage activation** is a crucial part of the inflammatory response and debris clearance following nerve injury, but it is not the *first cellular response* of the neuron itself [2]. - Macrophages migrate to the site of injury hours to days after the initial insult, whereas chromatolysis begins within the neuron's cell body much earlier [2]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Peripheral Nerves and Skeletal Muscles, pp. 1254-1256. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Manifestations Of Central And Peripheral Nervous System Disease, pp. 697-698.
Pharmacology
2 questionsWhich of the following drugs is not used for the treatment of H. Pylori?
Among the following statins, which has the longest half-life?
NEET-PG 2015 - Pharmacology NEET-PG Practice Questions and MCQs
Question 521: Which of the following drugs is not used for the treatment of H. Pylori?
- A. Bismuth
- B. Domperidone (Correct Answer)
- C. Clarithromycin
- D. Amoxicillin
Explanation: ***Correct: Domperidone*** - Domperidone is a **prokinetic drug** used to treat nausea, vomiting, and gastric motility disorders, but it has **no direct antibacterial activity** against *H. pylori*. - It works by blocking **dopamine receptors** in the chemoreceptor trigger zone and peripherally. - Therefore, it is **NOT used for *H. pylori* eradication**. *Incorrect: Bismuth* - **Bismuth subsalicylate** is a key component of **quadruple therapy** for *H. pylori* eradication, particularly in cases of antibiotic resistance or treatment failure. - It has **bactericidal effects** against *H. pylori*, disrupts its cell wall, and inhibits its adherence to the gastric mucosa. *Incorrect: Amoxicillin* - **Amoxicillin** is a penicillin-class antibiotic commonly used in various *H. pylori* eradication regimens, including **triple therapy**. - It acts by **inhibiting bacterial cell wall synthesis**, leading to bacterial lysis. *Incorrect: Clarithromycin* - **Clarithromycin** is a macrolide antibiotic frequently included in standard **triple therapy** for *H. pylori* eradication. - It inhibits **bacterial protein synthesis** by binding to the 50S ribosomal subunit.
Question 522: Among the following statins, which has the longest half-life?
- A. Pravastatin
- B. Simvastatin
- C. Lovastatin
- D. Rosuvastatin (Correct Answer)
Explanation: **Rosuvastatin** - **Rosuvastatin** has the longest half-life among the commonly used statins, approximately **19 hours**, allowing for consistent lipid-lowering effects. - Its prolonged presence in the body contributes to its effectiveness in reducing **LDL-C** at lower doses. *Pravastatin* - **Pravastatin** has a relatively short half-life of about **1.8 hours**, requiring daily dosing to maintain therapeutic concentrations. - Its hydrophilic nature means it is less likely to penetrate non-hepatic tissues, potentially reducing extrahepatic side effects. *Simvastatin* - **Simvastatin** has a short half-life of about **3 hours**, necessitating daily administration. - It is a **prodrug** that requires hepatic activation to its active beta-hydroxy acid form. *Lovastatin* - **Lovastatin** also has a short half-life, around **3 hours**, and is a **prodrug** like simvastatin. - It is often recommended to be taken in the evening due to the diurnal rhythm of cholesterol synthesis.