Biochemistry
10 questionsRate limiting enzyme in bile acid synthesis?
Enzyme activity is expressed as?
What is the mechanism of conversion of trypsinogen to trypsin?
The mechanism of action of uncouplers of oxidative phosphorylation involves:
Which of the following statements about NADP is correct?
In the malate shuttle, how many ATPs are produced from one NADH?
Which of the following is not a substrate for gluconeogenesis?
Pyruvate dehydrogenase requires all cofactors except:
Which of the following processes primarily utilizes lactate produced anaerobically?
Citrate synthase is inhibited by -
NEET-PG 2015 - Biochemistry NEET-PG Practice Questions and MCQs
Question 541: 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 542: 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 543: 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 544: 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 545: 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 546: 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 547: Which of the following is not a substrate for gluconeogenesis?
- A. Leucine (Correct Answer)
- B. Lactate
- C. Propionate
- D. Glycerol
Explanation: ***Leucine*** - **Leucine** is an exclusively **ketogenic amino acid**, meaning its breakdown products can only be converted into **ketone bodies** or fatty acids, not glucose. - It does not have a carbon skeleton that can be directly converted into **pyruvate** or **oxaloacetate**, which are key intermediates in gluconeogenesis. *Lactate* - **Lactate** is a major substrate for gluconeogenesis, particularly during exercise or fasting. - It is converted to **pyruvate** by **lactate dehydrogenase**, and pyruvate can then enter the gluconeogenic pathway. *Propionate* - **Propionate** is a fatty acid with an odd number of carbon atoms, primarily derived from the catabolism of odd-chain fatty acids or from bacterial fermentation in the colon. - It can be converted into **succinyl CoA**, an intermediate of the citric acid cycle, which can then be used for gluconeogenesis. *Glycerol* - **Glycerol**, released during the breakdown of triglycerides, is an important substrate for gluconeogenesis. - It is phosphorylated to **glycerol-3-phosphate**, which is then oxidized to **dihydroxyacetone phosphate (DHAP)**, an intermediate in glycolysis and gluconeogenesis.
Question 548: Pyruvate dehydrogenase requires all cofactors except:
- A. Thiamin
- B. Pyridoxin (Vitamin B6) (Correct Answer)
- C. Riboflavin
- D. Niacin
Explanation: ***Pyridoxin (Vitamin B6)*** - **Pyridoxin** (vitamin B6) is a coenzyme for many enzymes involved in **amino acid metabolism**, but it is **not directly required** by the pyruvate dehydrogenase complex. - The pyruvate dehydrogenase complex uses **thiamine pyrophosphate**, **lipoic acid**, **FAD**, **NAD+**, and **Coenzyme A** as cofactors. *Thiamin* - **Thiamin pyrophosphate** (TPP), derived from thiamin (vitamin B1), is a crucial coenzyme for the **E1 subunit** of the pyruvate dehydrogenase complex. - It participates in the **decarboxylation** of pyruvate, releasing CO2. *Riboflavin* - **FAD** (flavin adenine dinucleotide), derived from riboflavin (vitamin B2), is a coenzyme for the **E3 subunit** (dihydrolipoyl dehydrogenase) of the pyruvate dehydrogenase complex. - It is involved in the **regeneration of oxidized lipoamide**. *Niacin* - **NAD+** (nicotinamide adenine dinucleotide), derived from niacin (vitamin B3), is a coenzyme for the **E3 subunit** of the pyruvate dehydrogenase complex. - It acts as an **electron acceptor** during the reoxidation of FADH2.
Question 549: Which of the following processes primarily utilizes lactate produced anaerobically?
- A. Cori cycle (Correct Answer)
- B. Gluconeogenesis
- C. TCA cycle
- D. Glycolysis
Explanation: ***Cori cycle*** - The **Cori cycle** (lactic acid cycle) involves the transport of **lactate** produced during anaerobic metabolism in muscles to the liver. - In the **liver**, this lactate is then converted back to **glucose** via gluconeogenesis, which can be returned to the muscles. *Gluconeogenesis* - **Gluconeogenesis** is the synthesis of glucose from non-carbohydrate precursors, one of which is lactate. - While it uses lactate, it is only one component of the broader **Cori cycle**, which describes the inter-organ cooperation. *Glycolysis* - **Glycolysis** is the metabolic pathway that breaks down glucose into pyruvate, which can then be converted to lactate under anaerobic conditions. - This process *produces* lactate but does not *utilize* it, acting upstream of lactate production. *TCA cycle* - The **TCA cycle** (Krebs cycle) is a central part of aerobic respiration that oxidizes acetyl-CoA to produce ATP, NADH, and FADH2. - It does not directly utilize lactate; instead, lactate is typically converted to pyruvate before potentially entering the TCA cycle under aerobic conditions.
Question 550: Citrate synthase is inhibited by -
- A. Insulin
- B. Glucagon
- C. ADP
- D. ATP (Correct Answer)
Explanation: ***ATP*** - **Citrate synthase**, a key enzyme in the Krebs cycle, is inhibited by **high levels of ATP**, indicating a high energy state in the cell. - This allosteric inhibition helps regulate the metabolic flux through the cycle, slowing it down when energy is abundant. *ADP* - **ADP** typically signifies a low energy state and would generally act as an **activator** rather than an inhibitor for metabolic pathways that produce ATP. - In this context, ADP would promote the activity of enzymes involved in energy generation, including those in the Krebs cycle. *Insulin* - **Insulin** is a hormone that promotes fuel storage and utilization, generally **activating** metabolic pathways rather than directly inhibiting enzymes like citrate synthase. - Its primary role is to regulate blood glucose levels and promote glucose uptake and utilization. *Glucagon* - **Glucagon** is a hormone that mobilizes fuel from storage and is typically associated with **catabolic processes**, often increasing metabolic activity in response to low blood glucose. - It does not directly inhibit citrate synthase; its main actions are on glucoregulation.