Subtopic: Enzyme Classification
Keyword Definitions:
• Hexokinase: An enzyme that phosphorylates glucose to glucose-6-phosphate using ATP.
• Ligase: Enzyme catalyzing bond formation with ATP hydrolysis.
• Transferase: Enzyme transferring functional groups between molecules.
• Oxidoreductase: Enzyme catalyzing oxidation–reduction reactions.
• Reductase: Enzyme involved in reduction reactions.
Lead Question - 2013
Hexokinase is ?
a) Ligase
b) Transferase
c) Oxidoreductase
d) Reductase
Explanation: Hexokinase catalyzes transfer of phosphate from ATP to glucose, forming glucose-6-phosphate. This is a classic example of a transferase because it moves a functional group (phosphate) from one molecule to another. Correct answer is b) Transferase.
1) Which enzyme catalyzes the first step of glycolysis?
a) Hexokinase
b) Phosphofructokinase
c) Pyruvate kinase
d) Aldolase
Explanation: The first step of glycolysis is phosphorylation of glucose to glucose-6-phosphate, catalyzed by hexokinase. This reaction traps glucose inside cells and prepares it for further metabolism. Other enzymes act in later steps. Correct answer is a) Hexokinase.
2) A patient has a mutation in hexokinase causing impaired glucose phosphorylation. Which metabolic pathway is most affected?
a) Glycolysis
b) TCA cycle
c) Glycogenolysis
d) Urea cycle
Explanation: Hexokinase initiates glycolysis by phosphorylating glucose. A mutation reduces glycolytic flux, impairing ATP production and energy metabolism. TCA cycle and glycogenolysis are downstream or parallel processes, while urea cycle is unrelated. Correct answer is a) Glycolysis.
3) Hexokinase belongs to which enzyme class?
a) Transferase
b) Ligase
c) Hydrolase
d) Lyase
Explanation: Hexokinase is a transferase, transferring phosphate groups from ATP to glucose. Ligases form bonds using ATP, hydrolases break bonds using water, and lyases cleave without hydrolysis. Correct answer is a) Transferase.
4) Which isoenzyme of hexokinase has high Km and high Vmax for glucose?
a) Glucokinase
b) Hexokinase I
c) Hexokinase II
d) Hexokinase III
Explanation: Glucokinase, the hepatic isoform of hexokinase, has a high Km and Vmax, allowing liver to regulate blood glucose by storing excess as glycogen. Hexokinases I–III have low Km, efficiently trapping glucose at low concentrations. Correct answer is a) Glucokinase.
5) A neonate presents with hypoglycemia and impaired liver glycogen storage. Which enzyme defect is suspected?
a) Glucokinase
b) Pyruvate kinase
c) Hexokinase I
d) Aldolase
Explanation: Deficiency of glucokinase impairs phosphorylation of glucose in liver, reducing glycogen synthesis and leading to hypoglycemia. Pyruvate kinase deficiency causes hemolytic anemia, not hypoglycemia. Correct answer is a) Glucokinase.
6) Assertion-Reason: Assertion: Hexokinase is a transferase. Reason: Transferases catalyze transfer of groups like phosphate from one molecule to another.
a) Both true, Reason correct
b) Both true, Reason not explanation
c) Assertion true, Reason false
d) Both false
Explanation: Hexokinase catalyzes transfer of phosphate group from ATP to glucose, which is exactly what transferases do. Hence, both assertion and reason are true, and reason correctly explains the assertion. Correct answer is a) Both true, Reason correct.
7) A patient with MODY-2 has impaired glucose sensing in β-cells. Which enzyme mutation is most likely?
a) Glucokinase
b) Hexokinase II
c) Pyruvate carboxylase
d) PFK-1
Explanation: MODY-2 is caused by glucokinase mutation in pancreatic β-cells, impairing glucose sensing and insulin release. Hexokinase II, pyruvate carboxylase, and PFK-1 mutations cause other metabolic disorders. Correct answer is a) Glucokinase.
8) Fill in the blank: Hexokinase catalyzes the conversion of glucose to __________.
a) Glucose-6-phosphate
b) Fructose-6-phosphate
c) Pyruvate
d) Lactate
Explanation: Hexokinase phosphorylates glucose to glucose-6-phosphate, the first and committed step of glycolysis. This traps glucose in cells for metabolism. Correct answer is a) Glucose-6-phosphate.
9) Which tissue has glucokinase instead of hexokinase as primary glucose phosphorylating enzyme?
a) Liver
b) Muscle
c) Brain
d) RBC
Explanation: The liver contains glucokinase, allowing it to handle high glucose loads after meals. Muscle, brain, and RBCs rely on hexokinase with low Km, ensuring glucose utilization at low concentrations. Correct answer is a) Liver.
10) Choose correct statements about hexokinase:
1. It is inhibited by glucose-6-phosphate
2. It has low Km for glucose
3. It is present in liver
4. It traps glucose in cells
a) 1,2,4 correct
b) 1,3 correct
c) 2,3,4 correct
d) 1,2,3 correct
Explanation: Hexokinase has low Km, is inhibited by its product (glucose-6-phosphate), and traps glucose inside cells. In liver, glucokinase predominates instead. Hence statements 1,2,4 are correct. Correct answer is a) 1,2,4 correct.
Subtopic: Lactate Dehydrogenase Isoenzymes
Keyword Definitions:
• Lactate Dehydrogenase (LDH): An enzyme catalyzing interconversion of lactate and pyruvate.
• Isoenzymes: Different molecular forms of the same enzyme, catalyzing the same reaction but differing in tissue distribution.
• LDH1–LDH5: Five isoenzymes of LDH with distinct tissue localization (LDH1 heart, LDH5 liver/skeletal muscle).
• Serum LDH: Marker of tissue injury, elevated in myocardial infarction, liver disease, and hemolysis.
Lead Question - 2013
Which is predominant in normal healthy human?
a) LDH1
b) LDH2
c) LDH3
d) LDH4
Explanation: In normal healthy individuals, LDH2 is the predominant isoenzyme in serum, followed by LDH1. Elevated LDH1 greater than LDH2 suggests myocardial infarction. Thus, LDH isoenzyme patterns are clinically useful in diagnosis. Correct answer is b) LDH2.
1) Which LDH isoenzyme predominates in the heart?
a) LDH1
b) LDH3
c) LDH4
d) LDH5
Explanation: The LDH1 isoenzyme predominates in cardiac muscle. Its elevation is an important diagnostic marker of myocardial infarction. LDH5 predominates in liver and skeletal muscle, LDH2 in normal serum, while LDH3 and LDH4 are intermediate. Correct answer is a) LDH1.
2) A patient presents with chest pain and raised LDH1/LDH2 ratio. This indicates?
a) Myocardial infarction
b) Liver disease
c) Skeletal muscle injury
d) Renal failure
Explanation: In myocardial infarction, LDH1 becomes greater than LDH2 (known as LDH flip). This inversion of the LDH1/LDH2 ratio is a key biochemical marker supporting cardiac damage. Correct answer is a) Myocardial infarction.
3) LDH5 is predominantly found in?
a) Heart
b) Liver and skeletal muscle
c) Kidney
d) Brain
Explanation: The LDH5 isoenzyme predominates in liver and skeletal muscle. Its elevation indicates liver disease, hepatitis, or muscle injury. Thus, isoenzyme distribution helps identify organ-specific pathology. Correct answer is b) Liver and skeletal muscle.
4) A patient with hepatitis shows marked elevation of which LDH isoenzyme?
a) LDH1
b) LDH2
c) LDH3
d) LDH5
Explanation: Hepatic damage causes elevation of LDH5, as this isoenzyme is highly concentrated in liver cells. This helps distinguish liver involvement from cardiac or renal causes of raised LDH. Correct answer is d) LDH5.
5) Which isoenzyme is abundant in RBCs?
a) LDH1
b) LDH2
c) LDH4
d) LDH5
Explanation: LDH1 and LDH2 are present in high amounts in red blood cells. Hemolysis leads to elevated serum levels of these isoenzymes, helping to identify intravascular hemolysis. Correct answer is a) LDH1.
6) A patient with hemolytic anemia will show elevation of?
a) LDH2 and LDH1
b) LDH3 and LDH4
c) LDH5
d) None
Explanation: Hemolysis releases LDH1 and LDH2 from red blood cells into circulation, causing elevated serum levels. This supports diagnosis of hemolytic anemia. Correct answer is a) LDH2 and LDH1.
7) Match the following:
1. LDH1 - Heart
2. LDH2 - RBCs
3. LDH3 - Lung
4. LDH5 - Liver
a) 1,2,3,4 correct
b) 1,2 correct
c) 1,3,4 correct
d) 2,4 correct
Explanation: LDH isoenzymes show tissue specificity: LDH1-heart, LDH2-RBCs, LDH3-lungs, and LDH5-liver/skeletal muscle. All associations are correct. Correct answer is a) 1,2,3,4 correct.
8) LDH3 elevation is seen in?
a) Lung diseases
b) Liver injury
c) Myocardial infarction
d) Kidney disorders
Explanation: LDH3 is abundant in lungs, so its elevation is typically seen in pulmonary embolism or lung diseases. This pattern helps in organ-specific diagnosis. Correct answer is a) Lung diseases.
9) Which isoenzyme of LDH is thermolabile?
a) LDH1
b) LDH2
c) LDH4
d) LDH5
Explanation: LDH5 is heat-labile, meaning it is inactivated at higher temperatures, while LDH1 is relatively heat-stable. This property helps distinguish between different isoenzymes during electrophoresis. Correct answer is d) LDH5.
10) A 50-year-old man presents with elevated LDH5 and SGPT. This combination suggests?
a) Myocardial infarction
b) Acute hepatitis
c) Pulmonary embolism
d) Hemolysis
Explanation: Raised LDH5 along with elevated liver enzymes like SGPT strongly indicates hepatic damage, as both markers are liver-associated. Thus, this biochemical profile is consistent with acute hepatitis. Correct answer is b) Acute hepatitis.
Subtopic: Hydrolases
Keyword Definitions:
• Enzyme: Biological catalyst that speeds up biochemical reactions.
• IUB system: International Union of Biochemistry classification of enzymes.
• Hydrolases: Enzymes catalyzing hydrolysis reactions.
• EC number: Enzyme Commission number denoting enzyme class.
• Substrate: Molecule upon which enzyme acts.
Lead Question - 2013
According to IUB system, hydrolases belong to which class ?
a) EC-1
b) EC-2
c) EC-3
d) EC-4
Answer & Explanation:
Correct answer: c) EC-3
Hydrolases belong to EC-3 in the IUB enzyme classification. These enzymes catalyze hydrolytic cleavage of bonds using water. Examples include esterases, lipases, and proteases. EC-1 represents oxidoreductases, EC-2 transferases, and EC-4 lyases. Thus, the correct class for hydrolases is EC-3 in enzyme nomenclature.
1) Which class of enzymes catalyzes oxidation-reduction reactions?
a) EC-1
b) EC-2
c) EC-3
d) EC-4
Answer & Explanation:
Correct answer: a) EC-1
Oxidoreductases belong to EC-1. They catalyze oxidation-reduction reactions, involving transfer of electrons or hydrogen atoms between molecules. Examples include dehydrogenases and oxidases. This class is crucial in energy metabolism and respiration. Hydrolases are EC-3, not EC-1. Hence, oxidoreductases are represented by EC-1.
2) Which enzyme class includes transfer of functional groups?
a) EC-1
b) EC-2
c) EC-3
d) EC-5
Answer & Explanation:
Correct answer: b) EC-2
Transferases belong to EC-2. They catalyze transfer of functional groups such as phosphate, amino, or methyl groups from one molecule to another. Examples include kinases and transaminases. Hydrolases are EC-3, oxidoreductases EC-1, while isomerases belong to EC-5. Hence, EC-2 class is for transferases.
3) A patient has high amylase levels. Which enzyme class does amylase belong to?
a) Hydrolases
b) Oxidoreductases
c) Transferases
d) Lyases
Answer & Explanation:
Correct answer: a) Hydrolases
Amylase belongs to hydrolases (EC-3). It catalyzes hydrolysis of starch into sugars using water. Elevated amylase is often seen in acute pancreatitis. This clinical finding helps in diagnosis. Hence, amylase clearly belongs to the hydrolase class of enzymes.
4) Lyases belong to which EC class?
a) EC-1
b) EC-2
c) EC-4
d) EC-6
Answer & Explanation:
Correct answer: c) EC-4
Lyases are classified under EC-4. They catalyze addition or removal of groups from double bonds without hydrolysis or oxidation. Examples include decarboxylases and aldolases. They differ from hydrolases (EC-3) which use water to break bonds. Thus, lyases are EC-4 enzymes in IUB classification.
5) Which enzyme catalyzes peptide bond hydrolysis?
a) Protease
b) Kinase
c) Isomerase
d) Synthase
Answer & Explanation:
Correct answer: a) Protease
Proteases are hydrolases (EC-3) that break peptide bonds by hydrolysis. They are crucial for protein digestion and turnover. Examples include trypsin and pepsin. Kinases are transferases, isomerases rearrange molecules, and synthases catalyze synthesis reactions. Therefore, proteases catalyze peptide bond hydrolysis under hydrolases class.
6) Which class includes ATP synthase?
a) Lyases
b) Isomerases
c) Ligases
d) Hydrolases
Answer & Explanation:
Correct answer: c) Ligases
ATP synthase belongs to ligases (EC-6). It catalyzes the joining of molecules, in this case ADP and phosphate to form ATP, using energy. Ligases are responsible for bond formation with energy input, unlike hydrolases which break bonds. Hence, ATP synthase is a ligase enzyme.
7) A patient with bile salt deficiency shows impaired lipid digestion. Which enzyme class is affected?
a) Hydrolases
b) Transferases
c) Oxidoreductases
d) Isomerases
Answer & Explanation:
Correct answer: a) Hydrolases
Lipases, which digest triglycerides into fatty acids and glycerol, belong to hydrolases (EC-3). Their activity is reduced without bile salts, impairing lipid digestion. Clinical findings include steatorrhea. Thus, hydrolases (lipases) are the enzyme class most affected in bile salt deficiency states.
8) Which enzyme rearranges glucose-6-phosphate to fructose-6-phosphate?
a) Isomerase
b) Transferase
c) Hydrolase
d) Ligase
Answer & Explanation:
Correct answer: a) Isomerase
Phosphoglucose isomerase is an isomerase (EC-5). It catalyzes reversible isomerization of glucose-6-phosphate to fructose-6-phosphate in glycolysis. Isomerases catalyze intramolecular rearrangements. Hydrolases break bonds with water, ligases join molecules, and transferases move groups. Hence, the correct answer is isomerase.
9) Which enzyme deficiency causes Gaucher’s disease?
a) Glucocerebrosidase
b) Hexokinase
c) Catalase
d) Aldolase
Answer & Explanation:
Correct answer: a) Glucocerebrosidase
Gaucher’s disease results from deficiency of glucocerebrosidase, a lysosomal hydrolase (EC-3). This leads to accumulation of glucocerebrosides in macrophages. Clinical features include hepatosplenomegaly and bone crises. Thus, the hydrolase class enzyme deficiency is responsible for Gaucher’s disease pathology.
10) Enzymes that catalyze bond formation with ATP hydrolysis are?
a) Ligases
b) Hydrolases
c) Transferases
d) Lyases
Answer & Explanation:
Correct answer: a) Ligases
Ligases (EC-6) catalyze joining of two molecules with ATP hydrolysis. Examples include DNA ligase and aminoacyl-tRNA synthetase. They are different from hydrolases (EC-3) which break bonds. Hence, ligases represent enzymes forming new bonds using ATP energy.
Topic: Proteolytic Enzymes
Subtopic: Serine Proteases
Keyword Definitions:
• Protease: Enzyme that hydrolyzes peptide bonds in proteins.
• Serine protease: Protease that uses a serine residue in its active site for catalysis.
• Pepsin: Aspartic protease found in the stomach.
• Carboxypeptidase: Zinc-containing protease that cleaves terminal amino acids.
• Trypsin: Classic example of a serine protease.
Lead Question - 2013
Which of the following is serine protease?
a) Pepsin
b) Trypsin
c) Carboxypeptidase
d) None
Answer & Explanation:
Correct answer: b) Trypsin
Trypsin is a classical serine protease, using serine in its catalytic triad (Ser-His-Asp). It plays a critical role in digestion by cleaving peptide bonds at lysine and arginine residues. Pepsin is an aspartic protease, and carboxypeptidase is a zinc metalloprotease, not serine-based.
1) Which amino acid is essential in the catalytic triad of serine proteases?
a) Serine
b) Glycine
c) Histidine
d) Proline
Answer & Explanation:
Correct answer: a) Serine
The catalytic triad of serine proteases includes serine, histidine, and aspartate. Serine provides the nucleophilic hydroxyl group that attacks the peptide bond. This mechanism is a hallmark of enzymes like trypsin, chymotrypsin, and elastase, making serine indispensable in their function.
2) A patient with pancreatic insufficiency has difficulty digesting proteins. Which protease is primarily affected?
a) Trypsin
b) Pepsin
c) Rennin
d) Lysozyme
Answer & Explanation:
Correct answer: a) Trypsin
Pancreatic insufficiency impairs secretion of trypsin, a major serine protease secreted as trypsinogen and activated in the intestine. Its deficiency leads to impaired protein digestion, malabsorption, and weight loss. Pepsin acts in the stomach, but trypsin is the main protease for intestinal protein digestion.
3) Which of the following is not a serine protease?
a) Elastase
b) Chymotrypsin
c) Carboxypeptidase
d) Trypsin
Answer & Explanation:
Correct answer: c) Carboxypeptidase
Carboxypeptidase is a zinc metalloprotease, not a serine protease. In contrast, elastase, chymotrypsin, and trypsin are classic serine proteases containing the catalytic triad. They act in protein digestion and tissue remodeling. Hence, the exception among the options is carboxypeptidase.
4) In chymotrypsin, which residue stabilizes the transition state?
a) Histidine
b) Serine
c) Aspartate
d) Glycine
Answer & Explanation:
Correct answer: d) Glycine
In chymotrypsin, glycine contributes to the oxyanion hole that stabilizes the negative charge on the transition state intermediate during peptide bond cleavage. This stabilization is essential for efficient catalysis, complementing the role of serine, histidine, and aspartate in the catalytic triad.
5) A child with cystic fibrosis shows steatorrhea and protein malabsorption. Which enzyme activity is lacking?
a) Pepsin
b) Trypsin
c) Amylase
d) Lipase
Answer & Explanation:
Correct answer: b) Trypsin
In cystic fibrosis, pancreatic ducts are obstructed, reducing secretion of enzymes including trypsin. Lack of trypsin impairs protein digestion, causing malabsorption and nutritional deficiency. Lipase deficiency contributes to steatorrhea, but protein digestion specifically requires trypsin, a serine protease.
6) Which protease is active in acidic pH?
a) Pepsin
b) Trypsin
c) Chymotrypsin
d) Elastase
Answer & Explanation:
Correct answer: a) Pepsin
Pepsin is an aspartic protease active at acidic pH in the stomach. Serine proteases like trypsin, chymotrypsin, and elastase require alkaline pH for optimal activity in the intestine. Thus, pepsin is unique in functioning at acidic gastric conditions, unlike serine proteases.
7) Which enzyme activates trypsinogen into trypsin?
a) Enteropeptidase
b) Pepsin
c) Elastase
d) Kinase
Answer & Explanation:
Correct answer: a) Enteropeptidase
Enteropeptidase (also called enterokinase) activates trypsinogen into trypsin in the duodenum. Trypsin then autocatalytically activates more trypsinogen and other zymogens. This activation cascade is crucial for protein digestion. Without enteropeptidase, protein malabsorption occurs.
8) In acute pancreatitis, early activation of which protease leads to autodigestion?
a) Trypsin
b) Elastase
c) Chymotrypsin
d) Pepsin
Answer & Explanation:
Correct answer: a) Trypsin
In acute pancreatitis, premature activation of trypsinogen to trypsin inside the pancreas triggers autodigestion. Trypsin further activates other zymogens, amplifying tissue injury. This explains abdominal pain, elevated serum amylase, and lipase. Thus, trypsin activation is central in the pathogenesis of pancreatitis.
9) Which protease breaks down elastin fibers in lungs?
a) Elastase
b) Trypsin
c) Pepsin
d) Carboxypeptidase
Answer & Explanation:
Correct answer: a) Elastase
Elastase, a serine protease, digests elastin fibers. Neutrophil elastase contributes to lung tissue destruction in chronic smokers and emphysema, especially when unchecked by α1-antitrypsin deficiency. Trypsin and pepsin do not specifically degrade elastin. Thus, elastase plays a critical role in lung pathology.
10) Which serine protease is inhibited by α1-antitrypsin?
a) Elastase
b) Carboxypeptidase
c) Pepsin
d) Renin
Answer & Explanation:
Correct answer: a) Elastase
α1-antitrypsin inhibits neutrophil elastase to protect lung tissue from degradation. Deficiency of α1-antitrypsin leads to unchecked elastase activity, causing emphysema and liver disease. This clinical link highlights the importance of elastase regulation by serine protease inhibitors.
Topic: Enzyme Classification and Properties
Subtopic: Fastest Acting Enzyme
Keyword Definitions:
Enzyme: Biological catalyst that speeds up reactions without being consumed.
Catalase: Enzyme that decomposes hydrogen peroxide rapidly into water and oxygen.
Trypsin: Serine protease breaking peptide bonds in proteins during digestion.
LDH (Lactate Dehydrogenase): Enzyme converting lactate to pyruvate during metabolism.
Reaction Rate: Speed at which a chemical reaction proceeds, influenced by enzyme activity.
Lead Question - 2013
Fastest acting enzyme ?
a) LDH
b) Trypsin
c) Catalase
d) None
Explanation: The fastest acting enzyme in the human body is Catalase. It rapidly decomposes millions of hydrogen peroxide molecules into water and oxygen per second, protecting cells from oxidative damage. LDH and Trypsin act slower compared to Catalase. Hence, the correct answer is c) Catalase.
1) Which enzyme catalyzes the breakdown of starch into maltose?
a) Lipase
b) Amylase
c) Sucrase
d) Pepsin
Explanation: Amylase hydrolyzes starch into maltose, beginning carbohydrate digestion in the mouth and continuing in the duodenum. Pepsin digests proteins, Lipase acts on fats, and Sucrase breaks sucrose. Hence, the correct answer is b) Amylase.
2) In myocardial infarction, which enzyme rises first?
a) Creatine kinase MB
b) LDH
c) Amylase
d) Lipase
Explanation: In acute myocardial infarction, Creatine kinase MB (CK-MB) rises first, within 3–6 hours, peaking at 24 hours. LDH rises later, while Amylase and Lipase are not cardiac markers. Hence, the correct answer is a) Creatine kinase MB.
3) Which class of enzymes catalyzes oxidation-reduction reactions?
a) Transferases
b) Oxidoreductases
c) Lyases
d) Hydrolases
Explanation: Oxidoreductases catalyze oxidation-reduction reactions, involving electron transfer between molecules. Transferases shift functional groups, Hydrolases break bonds using water, and Lyases cleave bonds without hydrolysis. Hence, the correct answer is b) Oxidoreductases.
4) A patient with acute pancreatitis shows elevated serum levels of:
a) Lipase
b) Amylase
c) Both a and b
d) Lactate dehydrogenase
Explanation: In acute pancreatitis, both serum amylase and lipase levels are elevated due to leakage from inflamed pancreatic cells. LDH is not diagnostic here. Hence, the correct answer is c) Both a and b.
5) Which enzyme is used in PCR technique?
a) Taq polymerase
b) DNA ligase
c) DNA polymerase I
d) Reverse transcriptase
Explanation: Taq polymerase, a thermostable DNA polymerase from Thermus aquaticus, is used in PCR to amplify DNA, withstanding high denaturation temperatures. DNA ligase joins DNA fragments, DNA polymerase I is bacterial enzyme, and reverse transcriptase synthesizes DNA from RNA. Hence, the correct answer is a) Taq polymerase.
6) Which enzyme deficiency causes Phenylketonuria (PKU)?
a) Phenylalanine hydroxylase
b) Tyrosinase
c) Homogentisate oxidase
d) Dopa decarboxylase
Explanation: PKU is due to deficiency of Phenylalanine hydroxylase, leading to accumulation of phenylalanine and its metabolites, causing intellectual disability if untreated. Tyrosinase defect causes albinism, while Homogentisate oxidase deficiency causes alkaptonuria. Hence, the correct answer is a) Phenylalanine hydroxylase.
7) Enzyme used to dissolve blood clots clinically:
a) Trypsin
b) Streptokinase
c) Lipase
d) Urease
Explanation: Streptokinase is a fibrinolytic enzyme used clinically to dissolve blood clots in myocardial infarction and pulmonary embolism. Trypsin digests proteins, Lipase acts on fats, Urease hydrolyzes urea. Hence, the correct answer is b) Streptokinase.
8) A 40-year-old diabetic patient develops ketoacidosis. Which enzyme increases ketone body formation?
a) HMG CoA lyase
b) Glucose-6-phosphatase
c) Pyruvate kinase
d) Citrate synthase
Explanation: In diabetic ketoacidosis, HMG CoA lyase catalyzes the breakdown of HMG-CoA to acetoacetate, a ketone body. Other enzymes are not directly involved in ketogenesis. Hence, the correct answer is a) HMG CoA lyase.
9) Which coenzyme is required by transaminases?
a) Biotin
b) Pyridoxal phosphate
c) FAD
d) NAD+
Explanation: Transaminases require Pyridoxal phosphate (Vitamin B6 derivative) as a coenzyme for transferring amino groups. Biotin acts in carboxylation, FAD and NAD+ act in redox reactions. Hence, the correct answer is b) Pyridoxal phosphate.
10) A 25-year-old male with jaundice shows increased unconjugated bilirubin. Which enzyme deficiency is likely?
a) Glucose-6-phosphate dehydrogenase
b) UDP-glucuronyl transferase
c) Glucokinase
d) Lactate dehydrogenase
Explanation: In hereditary unconjugated hyperbilirubinemia (Crigler-Najjar syndrome), deficiency of UDP-glucuronyl transferase prevents bilirubin conjugation. G6PD deficiency causes hemolysis, not impaired conjugation. Hence, the correct answer is b) UDP-glucuronyl transferase.
Topic: Bioenergetics
Subtopic: High Energy Compounds
Keyword Definitions:
High energy compound: Molecule that stores significant free energy in its chemical bonds, released during hydrolysis.
ADP (Adenosine diphosphate): Intermediate energy carrier, forms ATP upon phosphorylation.
Creatine phosphate: High energy molecule that rapidly donates phosphate to ADP to regenerate ATP.
Glucose-6-phosphate: Phosphorylated glucose intermediate in glycolysis; lower energy than ATP.
Fructose-6-phosphate: Glycolytic intermediate, not considered high energy compound.
Lead Question - 2013
Which of the following is high energy compound?
a) ADP
b) Glucose-6-phosphate
c) Creatine phosphate
d) Fructose-6-phosphate
Explanation:
The correct answer is c) Creatine phosphate. Creatine phosphate contains a high-energy phosphate bond capable of rapidly transferring its phosphate to ADP to form ATP. ADP, glucose-6-phosphate, and fructose-6-phosphate are lower energy compounds and cannot provide immediate energy for muscular contraction.
1) Which molecule serves as a temporary energy reservoir in muscle?
a) ATP
b) Creatine phosphate
c) NADH
d) Pyruvate
Explanation:
Correct answer: b) Creatine phosphate. Creatine phosphate rapidly donates its phosphate to ADP to regenerate ATP during short bursts of muscular activity. It acts as a temporary energy reservoir, sustaining immediate energy demands before glycolysis or oxidative phosphorylation increases ATP production.
2) Which enzyme catalyzes ATP formation from creatine phosphate?
a) Creatine kinase
b) ATP synthase
c) Hexokinase
d) Pyruvate kinase
Explanation:
Correct answer: a) Creatine kinase. Creatine kinase transfers a phosphate group from creatine phosphate to ADP, regenerating ATP. This reaction is rapid, reversible, and critical during initial seconds of high-intensity muscle contraction, maintaining ATP supply until slower metabolic pathways take over.
3) Which of the following is considered a universal energy currency?
a) GTP
b) ATP
c) NADH
d) FADH2
Explanation:
Correct answer: b) ATP. ATP stores and releases energy in its terminal phosphate bond, powering diverse cellular reactions. GTP is used in specific reactions, while NADH and FADH2 are electron carriers contributing indirectly to ATP synthesis. ATP serves as the primary universal energy currency in all cells.
4) A patient with creatine kinase deficiency will show?
a) Reduced ATP regeneration in muscles
b) Elevated serum glucose
c) Impaired glycolysis
d) High creatinine excretion
Explanation:
Correct answer: a) Reduced ATP regeneration in muscles. Creatine kinase deficiency limits transfer of phosphate from creatine phosphate to ADP, impairing rapid ATP replenishment during high-intensity muscle activity. Glycolysis and creatinine excretion are not directly affected, and serum glucose remains normal.
5) Which compound has the highest phosphate transfer potential?
a) ATP
b) Glucose-6-phosphate
c) Creatine phosphate
d) AMP
Explanation:
Correct answer: c) Creatine phosphate. Creatine phosphate possesses a high-energy phosphate bond that can transfer its phosphate to ADP more readily than ATP. Glucose-6-phosphate and AMP have lower phosphate transfer potential, making creatine phosphate the most immediate energy donor for rapid ATP regeneration.
6) In which cellular compartment is creatine phosphate primarily found?
a) Cytosol of muscle fibers
b) Mitochondrial matrix
c) Nucleus
d) Lysosome
Explanation:
Correct answer: a) Cytosol of muscle fibers. Creatine phosphate is localized in the cytosol near myofibrils, providing a rapid phosphate source to regenerate ATP during muscle contraction. Mitochondria synthesize ATP, but creatine phosphate acts as a cytosolic energy buffer for immediate energy demands.
7) Which of the following is an energy-rich phosphate compound in liver?
a) Glucose-6-phosphate
b) Creatine phosphate
c) Phosphoenolpyruvate
d) Fructose-6-phosphate
Explanation:
Correct answer: c) Phosphoenolpyruvate. Phosphoenolpyruvate (PEP) has a high-energy phosphate bond used in glycolysis to generate ATP via pyruvate kinase. Creatine phosphate is mainly in muscle, while glucose-6-phosphate and fructose-6-phosphate are lower-energy intermediates.
8) A patient with muscle fatigue shows decreased creatine phosphate. What happens?
a) Reduced rapid ATP supply
b) Accumulation of ADP
c) Delayed muscle contraction
d) All of the above
Explanation:
Correct answer: d) All of the above. Low creatine phosphate limits rapid ATP regeneration, causing ADP accumulation and delayed muscle contraction. This highlights creatine phosphate’s role in immediate energy buffering, especially during short, high-intensity activities before glycolysis or oxidative phosphorylation compensates.
9) Which high-energy compound buffers ATP levels during sudden exercise?
a) ADP
b) Creatine phosphate
c) AMP
d) NADH
Explanation:
Correct answer: b) Creatine phosphate. Creatine phosphate rapidly donates phosphate to ADP to maintain ATP concentration during sudden muscular exertion. ADP and AMP are not immediate donors, and NADH is an electron carrier, contributing indirectly to ATP production via oxidative phosphorylation.
10) Which enzyme synthesizes ATP from ADP using creatine phosphate?
a) Creatine kinase
b) Hexokinase
c) ATP synthase
d) Pyruvate kinase
Explanation:
Correct answer: a) Creatine kinase. Creatine kinase catalyzes the reversible transfer of phosphate from creatine phosphate to ADP, rapidly regenerating ATP in muscles. This immediate ATP replenishment supports high-energy demands before glycolysis or mitochondrial oxidative phosphorylation can produce sufficient ATP.
Topic: Bioenergetics
Subtopic: High-Energy Molecules
Keyword Definitions:
High-energy molecule: Compound storing large amounts of free energy in chemical bonds, usable for cellular work.
ATP (Adenosine Triphosphate): Primary energy currency of the cell, hydrolysis releases ~7.3 kcal/mol under standard conditions.
GTP (Guanosine Triphosphate): High-energy molecule similar to ATP, used in protein synthesis and signaling.
Creatine phosphate: Muscle energy reservoir, transfers phosphate to ADP to regenerate ATP rapidly.
Glucose-6-phosphate: Glycolytic intermediate with moderate energy, not a primary energy donor.
Lead Question - 2013
Which energy molecule gives 10.5 kcal/molecule?
a) ATP
b) GTP
c) Creatine phosphate
d) Glucose-6-phosphate
Explanation:
The correct answer is c) Creatine phosphate. Creatine phosphate stores high-energy phosphate bonds, releasing approximately 10.5 kcal/mol when transferring phosphate to ADP to form ATP. ATP releases only ~7.3 kcal/mol. Glucose-6-phosphate and GTP release less or different energy amounts, making creatine phosphate the fastest immediate energy donor.
1) Which enzyme catalyzes phosphate transfer from creatine phosphate to ADP?
a) Creatine kinase
b) ATP synthase
c) Hexokinase
d) Pyruvate kinase
Explanation:
Correct answer: a) Creatine kinase. Creatine kinase catalyzes the rapid transfer of phosphate from creatine phosphate to ADP, regenerating ATP. This reaction buffers ATP levels in muscle during high-intensity exercise. Hexokinase phosphorylates glucose, pyruvate kinase acts in glycolysis, and ATP synthase generates ATP in mitochondria.
2) Which energy molecule is immediately used for muscle contraction?
a) ATP
b) Creatine phosphate
c) GTP
d) NADH
Explanation:
Correct answer: b) Creatine phosphate. During sudden muscle contraction, creatine phosphate rapidly donates its phosphate to ADP, forming ATP. ATP generated from mitochondria or glycolysis is slower. GTP and NADH are not immediate energy donors for rapid muscular work.
3) Which high-energy compound is used in protein synthesis?
a) ATP
b) GTP
c) Creatine phosphate
d) NADPH
Explanation:
Correct answer: b) GTP. GTP provides energy for translation during protein synthesis, including tRNA binding and translocation. ATP is more general, Creatine phosphate is muscle-specific, and NADPH functions mainly in biosynthetic reactions and antioxidant defense.
4) A patient with muscle fatigue and low creatine phosphate will experience?
a) Delayed ATP regeneration
b) Rapid muscle exhaustion
c) Accumulation of ADP
d) All of the above
Explanation:
Correct answer: d) All of the above. Decreased creatine phosphate reduces rapid ATP regeneration during high-intensity exercise, leading to quick muscle exhaustion, ADP accumulation, and delayed contraction. This demonstrates creatine phosphate’s critical role as a fast energy reservoir in skeletal muscle.
5) Which compound has higher energy than ATP?
a) Glucose-6-phosphate
b) ATP
c) Creatine phosphate
d) Fructose-6-phosphate
Explanation:
Correct answer: c) Creatine phosphate. The phosphate bond in creatine phosphate stores more energy (~10.5 kcal/mol) than ATP (~7.3 kcal/mol). Glucose-6-phosphate and fructose-6-phosphate are lower-energy intermediates. Creatine phosphate acts as an immediate energy donor in muscle, supporting rapid ATP replenishment.
6) In which organ is creatine phosphate most abundant?
a) Liver
b) Skeletal muscle
c) Brain
d) Heart
Explanation:
Correct answer: b) Skeletal muscle. Creatine phosphate is concentrated in skeletal muscle fibers to buffer ATP levels during sudden contractions. Heart muscle also contains it but in lower amounts. Liver and brain primarily use ATP generated from oxidative metabolism.
7) Which enzyme deficiency reduces muscle phosphocreatine availability?
a) Creatine kinase
b) Hexokinase
c) Lactate dehydrogenase
d) Pyruvate dehydrogenase
Explanation:
Correct answer: a) Creatine kinase. Creatine kinase deficiency limits phosphate transfer from creatine phosphate to ADP, reducing ATP regeneration in muscles. Hexokinase, LDH, and pyruvate dehydrogenase deficiencies affect glycolysis or oxidative metabolism but not immediate phosphocreatine-mediated ATP buffering.
8) Which molecule provides energy for rapid ATP regeneration in neurons?
a) Creatine phosphate
b) ATP
c) GTP
d) NADH
Explanation:
Correct answer: a) Creatine phosphate. Neurons use creatine phosphate to quickly regenerate ATP during transient high-energy demands, such as action potential firing. ATP alone is insufficient for sudden energy needs. GTP and NADH contribute indirectly via metabolic pathways but do not provide immediate phosphate for ATP regeneration.
9) Which energy molecule is hydrolyzed to produce ~7.3 kcal/mol under standard conditions?
a) ATP
b) GTP
c) Creatine phosphate
d) Fructose-6-phosphate
Explanation:
Correct answer: a) ATP. Hydrolysis of ATP releases approximately 7.3 kcal/mol under standard conditions, providing energy for most cellular processes. Creatine phosphate releases more energy (~10.5 kcal/mol) when transferring phosphate to ADP, making it a rapid, high-energy donor. GTP is similar to ATP but context-specific.
10) A 30-year-old athlete performs sprinting. Which energy system acts first?
a) Creatine phosphate system
b) Glycolysis
c) Oxidative phosphorylation
d) Pentose phosphate pathway
Explanation:
Correct answer: a) Creatine phosphate system. During the first seconds of sprinting, the creatine phosphate system rapidly donates phosphate to ADP, regenerating ATP for immediate energy. Glycolysis and oxidative phosphorylation act later for sustained energy, while the pentose phosphate pathway provides NADPH and ribose-5-phosphate, not immediate ATP.
Topic: Vitamins
Subtopic: Vitamin B12 (Cobalamin)
Keyword Definitions:
Vitamin B12: Water-soluble vitamin essential for DNA synthesis, fatty acid and amino acid metabolism.
Methylcobalamin: Active coenzyme form of Vitamin B12 involved in homocysteine to methionine conversion.
Homocysteine: Sulfur-containing amino acid converted to methionine by methylcobalamin.
Methylmalonyl CoA: Intermediate in odd-chain fatty acid metabolism converted to succinyl CoA by B12-dependent enzyme.
Pyruvate to lactate conversion: Catalyzed by lactate dehydrogenase, independent of Vitamin B12.
Lead Question - 2013
All are true about Vitamin B12, except?
a) Active form is methylcobalamin
b) Requires for conversion of homocysteine to methionine
c) Requires in metabolism of methylmalonyl CoA
d) Requires for conversion of pyruvate to lactate
Explanation:
The correct answer is d) Requires for conversion of pyruvate to lactate. Vitamin B12 functions as a coenzyme for methylcobalamin and adenosylcobalamin in homocysteine-methionine conversion and methylmalonyl CoA metabolism. Pyruvate to lactate conversion is catalyzed by lactate dehydrogenase and is independent of B12.
1) Which enzyme requires Vitamin B12 as a cofactor?
a) Methionine synthase
b) Lactate dehydrogenase
c) Pyruvate kinase
d) Hexokinase
Explanation:
Correct answer: a) Methionine synthase. Methionine synthase uses methylcobalamin (Vitamin B12) to convert homocysteine to methionine, essential for methylation reactions. Lactate dehydrogenase, pyruvate kinase, and hexokinase function independently of B12, participating in glycolysis and lactate metabolism.
2) Which metabolic pathway is affected by Vitamin B12 deficiency?
a) Odd-chain fatty acid metabolism
b) Glycolysis
c) Citric acid cycle
d) Electron transport chain
Explanation:
Correct answer: a) Odd-chain fatty acid metabolism. B12 deficiency impairs conversion of methylmalonyl CoA to succinyl CoA, disrupting odd-chain fatty acid and amino acid metabolism. Glycolysis, citric acid cycle, and ETC are not directly dependent on Vitamin B12.
3) Clinical manifestation of B12 deficiency includes?
a) Megaloblastic anemia
b) Neuropathy
c) Glossitis
d) Hyperglycemia
Explanation:
Correct answer: d) Hyperglycemia. Vitamin B12 deficiency causes megaloblastic anemia, neuropathy, and glossitis due to impaired DNA synthesis and myelin formation. Hyperglycemia is unrelated. Recognition of neurological and hematological signs helps diagnose B12 deficiency early.
4) Which form of Vitamin B12 participates in methylmalonyl CoA mutase reaction?
a) Adenosylcobalamin
b) Methylcobalamin
c) Cyanocobalamin
d) Hydroxocobalamin
Explanation:
Correct answer: a) Adenosylcobalamin. Adenosylcobalamin is the cofactor for methylmalonyl CoA mutase, converting methylmalonyl CoA to succinyl CoA. Methylcobalamin acts in methionine synthase reaction. Cyanocobalamin and hydroxocobalamin are precursors converted into active forms.
5) Which lab test indicates functional B12 deficiency?
a) Elevated methylmalonic acid
b) High serum glucose
c) Low ALT
d) High creatinine
Explanation:
Correct answer: a) Elevated methylmalonic acid. B12 deficiency leads to accumulation of methylmalonic acid due to impaired conversion to succinyl CoA. Serum glucose, ALT, and creatinine are unrelated markers. Measuring methylmalonic acid is a sensitive test for subclinical B12 deficiency.
6) Vitamin B12 absorption requires which factor?
a) Intrinsic factor
b) Folic acid
c) Vitamin D
d) Iron
Explanation:
Correct answer: a) Intrinsic factor. Intrinsic factor secreted by gastric parietal cells binds B12 for ileal absorption. Folic acid, vitamin D, and iron are unrelated to B12 absorption. Deficiency of intrinsic factor causes pernicious anemia due to impaired B12 uptake.
7) Which neurological sign may appear in B12 deficiency?
a) Paresthesia
b) Hyperreflexia
c) Muscle atrophy
d) Hyperpigmentation
Explanation:
Correct answer: a) Paresthesia. Vitamin B12 deficiency causes demyelination, leading to paresthesia in extremities. Hyperreflexia or muscle atrophy may appear in severe cases, but paresthesia is an early and consistent neurological sign. Hyperpigmentation is unrelated.
8) Which form of B12 is used in homocysteine to methionine conversion?
a) Methylcobalamin
b) Adenosylcobalamin
c) Cyanocobalamin
d) Hydroxocobalamin
Explanation:
Correct answer: a) Methylcobalamin. Methylcobalamin acts as a cofactor for methionine synthase, converting homocysteine to methionine. Adenosylcobalamin participates in methylmalonyl CoA metabolism. Cyanocobalamin and hydroxocobalamin are inactive forms converted into active coenzymes in vivo.
9) Which vitamin deficiency may cause both anemia and neuropathy?
a) Vitamin B12
b) Vitamin C
c) Vitamin D
d) Vitamin K
Explanation:
Correct answer: a) Vitamin B12. Deficiency of B12 causes megaloblastic anemia due to impaired DNA synthesis and neurological symptoms due to demyelination. Vitamins C, D, and K deficiencies do not typically cause both anemia and neuropathy simultaneously.
10) Which is a common cause of B12 deficiency?
a) Pernicious anemia
b) Iron deficiency
c) Vitamin C deficiency
d) Zinc deficiency
Explanation:
Correct answer: a) Pernicious anemia. Pernicious anemia results from autoimmune destruction of parietal cells, reducing intrinsic factor and B12 absorption. Iron, vitamin C, and zinc deficiencies are unrelated to B12 metabolism. Recognizing pernicious anemia is essential for early supplementation and neurological protection.
Topic: Bioenergetics
Subtopic: Electron Transport Chain (ETC) and Oxidative Phosphorylation
Keyword Definitions:
ATP (Adenosine Triphosphate): Primary energy currency of the cell, generated by phosphorylation of ADP.
Electron Transport Chain (ETC): Series of protein complexes in inner mitochondrial membrane transferring electrons to oxygen.
FoF1-ATPase: Enzyme complex (ATP synthase) using proton gradient to synthesize ATP from ADP and inorganic phosphate.
Oxidative phosphorylation: Process of ATP generation driven by ETC and proton-motive force.
ADP kinase: Enzyme catalyzing interconversion of ADP and ATP, not part of ETC.
Lead Question - 2013
ATP is generated in ETC by?
a) Na⁺ ATPase
b) Cl⁻ ATPase
c) FoF1-ATPase
d) ADP Kinase
Explanation:
The correct answer is c) FoF1-ATPase. In mitochondria, the FoF1-ATPase (ATP synthase) uses the proton gradient created by ETC complexes to phosphorylate ADP to ATP. Na⁺ and Cl⁻ ATPases are unrelated ion pumps, and ADP kinase catalyzes ATP-ADP interconversion outside the ETC system.
1) Which complex of ETC transfers electrons to oxygen?
a) Complex I
b) Complex IV
c) Complex II
d) Complex III
Explanation:
Correct answer: b) Complex IV. Complex IV (cytochrome c oxidase) transfers electrons from cytochrome c to oxygen, forming water. Complex I and II donate electrons from NADH and FADH2 respectively, while Complex III transfers electrons to cytochrome c. This final step is essential for establishing the proton gradient driving ATP synthesis.
2) Which ion gradient drives FoF1-ATPase activity?
a) Sodium gradient
b) Potassium gradient
c) Proton gradient
d) Chloride gradient
Explanation:
Correct answer: c) Proton gradient. Protons pumped across the inner mitochondrial membrane by ETC complexes generate an electrochemical gradient. FoF1-ATPase uses this proton-motive force to synthesize ATP from ADP and Pi. Sodium, potassium, and chloride gradients are not directly used in mitochondrial ATP production.
3) Clinical deficiency of FoF1-ATPase may lead to?
a) Lactic acidosis
b) Hypoglycemia
c) Hyperuricemia
d) Hemolysis
Explanation:
Correct answer: a) Lactic acidosis. FoF1-ATPase deficiency reduces ATP production by oxidative phosphorylation, forcing reliance on anaerobic glycolysis, increasing lactate accumulation and leading to lactic acidosis. Hypoglycemia, hyperuricemia, and hemolysis are not primary features of ATP synthase defects.
4) Which coenzyme delivers electrons from NADH to Complex I?
a) NAD⁺
b) FAD
c) Coenzyme Q
d) FMN
Explanation:
Correct answer: d) FMN. In Complex I, flavin mononucleotide (FMN) accepts electrons from NADH and passes them to iron-sulfur clusters. Coenzyme Q receives electrons later, FAD functions in Complex II, and NAD⁺ is the oxidized form of NADH. FMN is critical in initiating electron flow in ETC.
5) Inhibition of which ETC complex blocks ATP synthesis completely?
a) Complex I
b) Complex IV
c) Complex III
d) FoF1-ATPase
Explanation:
Correct answer: d) FoF1-ATPase. Even if electron flow occurs, inhibiting ATP synthase prevents ATP production because the proton gradient cannot be converted into chemical energy. Complex inhibition reduces gradient generation, but complete blockage of ATP synthesis occurs only if FoF1-ATPase is nonfunctional.
6) Which molecule acts as a mobile electron carrier between Complex III and IV?
a) Coenzyme Q
b) Cytochrome c
c) NADH
d) FADH2
Explanation:
Correct answer: b) Cytochrome c. Cytochrome c is a small heme protein shuttling electrons from Complex III to Complex IV. Coenzyme Q transfers electrons between Complex I/II and III. NADH and FADH2 are electron donors entering the ETC but not mobile carriers between complexes.
7) Which clinical sign indicates mitochondrial ATP deficiency?
a) Muscle weakness
b) Hyperpigmentation
c) Jaundice
d) Polyuria
Explanation:
Correct answer: a) Muscle weakness. ATP deficiency due to impaired ETC or ATP synthase manifests as fatigue and muscle weakness, especially in high-energy demanding tissues. Hyperpigmentation, jaundice, and polyuria are unrelated to mitochondrial ATP defects.
8) Which cofactor is required by Complex IV?
a) Copper
b) Zinc
c) Iron
d) Magnesium
Explanation:
Correct answer: a) Copper. Complex IV contains copper centers that facilitate electron transfer to oxygen. Iron is present in heme groups, but copper is essential for catalytic activity. Zinc and magnesium are not components of ETC complexes.
9) Which molecule is directly synthesized by FoF1-ATPase?
a) ATP
b) GTP
c) NADH
d) FADH2
Explanation:
Correct answer: a) ATP. FoF1-ATPase couples proton flow across the inner mitochondrial membrane to phosphorylation of ADP, forming ATP. GTP, NADH, and FADH2 are produced in other metabolic pathways, not directly by ATP synthase.
10) A 25-year-old patient with ETC defect may have increased levels of?
a) Lactate
b) ATP
c) Oxygen
d) Pyruvate kinase
Explanation:
Correct answer: a) Lactate. ETC defects impair oxidative phosphorylation, forcing cells to rely on anaerobic glycolysis, leading to lactate accumulation. ATP production decreases, oxygen consumption remains normal or elevated, and pyruvate kinase activity is not directly affected. Lactic acidosis is a key clinical clue.
Topic: Bioenergetics
Subtopic: Electron Transport Chain and Oxidative Phosphorylation
Keyword Definitions:
Atractiloside: A compound that inhibits mitochondrial oxidative phosphorylation by blocking specific ETC components.
Oxidative Phosphorylation: Process of ATP generation using energy released by electrons passing through the ETC and a proton gradient.
Electron Transport Chain (ETC): Series of protein complexes transferring electrons from NADH/FADH2 to oxygen in mitochondria.
Uncoupler: Molecule that dissipates the proton gradient, producing heat instead of ATP.
Complex I: NADH dehydrogenase complex in ETC transferring electrons to coenzyme Q.
Complex III: Cytochrome bc1 complex transferring electrons from coenzyme Q to cytochrome c.
Lead Question - 2013
Atractiloside act as?
a) Uncoupler
b) Inhibitor of oxidative phosphorylation
c) Inhibitor of complex I of ETC
d) Inhibitor of complex III of ETC
Explanation:
The correct answer is b) Inhibitor of oxidative phosphorylation. Atractiloside inhibits ATP synthesis by blocking mitochondrial oxidative phosphorylation. It prevents ADP phosphorylation to ATP by impairing electron flow indirectly, without acting as an uncoupler or targeting specific ETC complexes like Complex I or III, leading to reduced energy production.
1) Which ETC complex transfers electrons from NADH to coenzyme Q?
a) Complex I
b) Complex II
c) Complex III
d) Complex IV
Explanation:
Correct answer: a) Complex I. NADH dehydrogenase (Complex I) accepts electrons from NADH, transferring them to coenzyme Q, while pumping protons into the intermembrane space. Complex II uses FADH2, Complex III transfers electrons to cytochrome c, and Complex IV reduces oxygen to water, completing electron flow.
2) A 30-year-old patient with atractiloside poisoning may present with?
a) Muscle weakness
b) Lactic acidosis
c) Hypoglycemia
d) Hypercalcemia
Explanation:
Correct answer: b) Lactic acidosis. By inhibiting oxidative phosphorylation, atractiloside prevents efficient ATP generation, forcing reliance on anaerobic glycolysis. This leads to accumulation of lactate and metabolic acidosis. Muscle weakness may occur secondarily, but lactic acidosis is a primary clinical manifestation.
3) Which molecule is directly synthesized by oxidative phosphorylation?
a) ATP
b) GTP
c) NADH
d) Pyruvate
Explanation:
Correct answer: a) ATP. Oxidative phosphorylation uses the proton gradient generated by the ETC to convert ADP to ATP. GTP is produced mainly in the TCA cycle, NADH is an electron donor for ETC, and pyruvate is formed in glycolysis, not directly by oxidative phosphorylation.
4) Which cofactor is required by Complex IV to reduce oxygen?
a) Copper
b) Iron
c) Magnesium
d) Zinc
Explanation:
Correct answer: a) Copper. Complex IV (cytochrome c oxidase) contains copper centers that transfer electrons to oxygen, forming water. Iron is present in heme groups but copper is essential for the catalytic function. Magnesium and zinc are not components of Complex IV.
5) Atractiloside belongs to which type of ETC inhibitor?
a) Complex-specific
b) Uncoupler
c) ATP synthase inhibitor
d) General oxidative phosphorylation inhibitor
Explanation:
Correct answer: d) General oxidative phosphorylation inhibitor. Atractiloside inhibits ATP production without targeting a specific complex, unlike rotenone (Complex I) or antimycin A (Complex III). It prevents phosphorylation of ADP to ATP, reducing cellular energy and potentially causing lactic acidosis and other energy-deficiency symptoms.
6) Clinical feature of ETC inhibitor toxicity includes?
a) Fatigue
b) Hypotension
c) Polyuria
d) Hyperpigmentation
Explanation:
Correct answer: a) Fatigue. ETC inhibition reduces ATP synthesis, leading to energy deficiency in tissues, particularly muscles and the brain. Patients commonly present with fatigue, exercise intolerance, and sometimes lactic acidosis. Hypotension, polyuria, and hyperpigmentation are not primary features of ETC inhibitor toxicity.
7) Which is an example of a natural uncoupler?
a) DNP (2,4-dinitrophenol)
b) Atractiloside
c) Rotenone
d) Cyanide
Explanation:
Correct answer: a) DNP (2,4-dinitrophenol). DNP disrupts the proton gradient, generating heat instead of ATP. Atractiloside inhibits oxidative phosphorylation without uncoupling, rotenone inhibits Complex I, and cyanide inhibits Complex IV. Uncouplers increase oxygen consumption but reduce ATP synthesis efficiency.
8) Which coenzyme shuttles electrons between Complex I/II and III?
a) Coenzyme Q
b) Cytochrome c
c) NADH
d) FADH2
Explanation:
Correct answer: a) Coenzyme Q. Coenzyme Q (ubiquinone) is a lipid-soluble electron carrier transferring electrons from Complex I and II to Complex III. Cytochrome c carries electrons from Complex III to IV, while NADH and FADH2 act as electron donors, not carriers between complexes.
9) Which tissue is most sensitive to oxidative phosphorylation inhibitors?
a) Brain
b) Skin
c) Bone
d) Cartilage
Explanation:
Correct answer: a) Brain. Brain tissue has high energy demand and relies heavily on oxidative phosphorylation. Inhibition of ATP synthesis affects neurons first, leading to neurological symptoms. Skin, bone, and cartilage have lower immediate ATP demand and are less sensitive to ETC inhibitors.
10) Which clinical test indicates impaired oxidative phosphorylation?
a) Elevated blood lactate
b) Serum bilirubin
c) Blood urea
d) Serum creatinine
Explanation:
Correct answer: a) Elevated blood lactate. ETC inhibition decreases ATP production, forcing anaerobic glycolysis and lactate accumulation. Blood lactate measurement is a sensitive indicator of mitochondrial dysfunction. Bilirubin, urea, and creatinine are not direct markers of oxidative phosphorylation defects.