Enzymes (Kinetics, Mechanism, Clinical Significance) MCQs

Biochemistry · 43 free questions with answers & explanations.

1. An enzyme has Km = 2 mM and Vmax = 100 nmol/min. At a substrate concentration of 2 mM, the reaction velocity is:
2. A competitive inhibitor increases the apparent Km of an enzyme without changing its Vmax. On a Lineweaver-Burk (double-reciprocal) plot, competitive inhibition is characterized by:
3. Serum troponin I (cTnI) is measured in a patient presenting with chest pain. Which property makes cTnI a superior biomarker of myocardial injury compared to total CK?
4. Covalent catalysis by serine proteases (e.g., trypsin, chymotrypsin) involves a catalytic triad. Which amino acid acts as a nucleophile, directly attacking the peptide bond of the substrate?
5. Enzyme A has Km = 0.1 mM and Vmax = 100 nmol/min/mg. Enzyme B has Km = 2 mM and Vmax = 100 nmol/min/mg. At a substrate concentration of 0.1 mM, Enzyme A operates at approximately 50% Vmax while Enzyme B operates at approximately 5% Vmax. This difference in Km BEST reflects which property of these enzymes?
6. A patient taking methotrexate for rheumatoid arthritis develops mucositis and pancytopenia. Methotrexate is a competitive inhibitor of dihydrofolate reductase (DHFR). Leucovorin rescue involves administering folinic acid (5-formyltetrahydrofolate). The Lineweaver-Burk (double reciprocal) plot for competitive inhibition shows which characteristic change compared to the uninhibited reaction?
7. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are used clinically to assess hepatocellular damage. Both are transaminases requiring PLP. In alcoholic liver disease, the AST/ALT ratio typically exceeds 2:1. This is primarily because:
8. Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most common enzyme deficiency worldwide. Oxidative stress triggers hemolytic crises in G6PD-deficient erythrocytes because red cells cannot regenerate NADPH. Which downstream antioxidant molecule's activity is most directly dependent on NADPH availability?
9. An enzyme's Km for substrate A is 0.5 mM. A competitive inhibitor raises the apparent Km to 2 mM without altering Vmax. A non-competitive inhibitor reduces Vmax by 50% without altering Km. If you add both inhibitors simultaneously, what happens to Vmax and apparent Km?
10. Serum LDH isoenzyme analysis in a patient with chest pain shows a flipped LDH-1/LDH-2 ratio (LDH-1 > LDH-2) at 48 hours. LDH-1 is rich in H (heart) subunits. In contrast, elevated LDH-5 is most specific for which condition?
11. Allosteric enzymes like ATCase (aspartate transcarbamoylase) show sigmoidal (S-shaped) velocity-substrate curves rather than hyperbolic Michaelis-Menten kinetics. Which parameter, used in place of Km for allosteric enzymes, indicates the substrate concentration at which velocity is half-maximal?
12. An enzyme follows Michaelis-Menten kinetics. When substrate concentration equals the Km, the reaction velocity (v) = Vmax/2. A competitive inhibitor doubles the apparent Km without changing Vmax. At substrate concentration = Km (without inhibitor), what fraction of Vmax is achieved in the presence of the competitive inhibitor (assuming inhibitor concentration equals Ki)?
13. Aspartate aminotransferase (AST) is released from hepatocytes and cardiac myocytes after cellular injury. AST requires pyridoxal-5'-phosphate (PLP) and catalyzes: Aspartate + alpha-ketoglutarate ↔ Oxaloacetate + Glutamate. In severe liver disease, AST activity in serum may be falsely low despite extensive hepatocyte necrosis. Which mechanism explains this paradox?
14. Serine proteases such as trypsin, chymotrypsin, and elastase share a catalytic triad (Ser-His-Asp). The serine acts as a nucleophile, attacking the peptide carbonyl carbon. Which catalytic mechanism is used by covalent acyl-enzyme intermediate formation?
15. Angiotensin-converting enzyme (ACE) inhibitors (e.g., lisinopril) are zinc metallopeptidases. The zinc ion is coordinated by two histidines and a glutamate in the active site and activates a water molecule for nucleophilic attack. Lisinopril competitively inhibits ACE with a Ki of approximately 0.27 nM. ACE normally cleaves the C-terminal dipeptide from angiotensin I to form angiotensin II. ACE ALSO inactivates bradykinin. Which clinical consequence results SPECIFICALLY from impaired bradykinin degradation?
16. In enzyme kinetics, a competitive inhibitor increases the apparent Km without changing Vmax, while an uncompetitive inhibitor DECREASES both Km and Vmax. Which type of inhibition is characteristic of aspirin's irreversible inhibition of cyclooxygenase (COX)?
17. Pyruvate kinase (PK) deficiency is the most common red cell glycolytic enzyme deficiency causing hereditary non-spherocytic hemolytic anemia. The mechanism of hemolysis is best explained by:
18. In a Lineweaver-Burk (double-reciprocal) plot, a pure non-competitive inhibitor produces which pattern?
19. Acid phosphatase (prostatic isoenzyme, PAP) was historically used as a tumor marker for prostate cancer. Which enzyme has now replaced it in routine clinical practice, and what is its major diagnostic limitation?
20. An enzyme follows Michaelis-Menten kinetics with Km = 2 mM and Vmax = 100 nmol/min. A competitive inhibitor is added at a concentration equaling its Ki. Substrate concentration is maintained at 2 mM. What is the observed reaction velocity?
21. Succinylcholine is hydrolyzed by pseudocholinesterase (butyrylcholinesterase). A patient with dibucaine-resistant pseudocholinesterase variant (dibucaine number ~20) receives succinylcholine for intubation. What is the expected clinical consequence and its mechanism?
22. Troponin T and Troponin I are released from cardiomyocytes after myocardial injury. Unlike creatine kinase-MB, cardiac troponins have almost no false-positive elevation from skeletal muscle disease. What biochemical property of cardiac isoforms accounts for their cardiac specificity?
23. Covalent (irreversible) inhibitors permanently inactivate enzymes by forming stable covalent bonds with active site residues. Which of the following correctly illustrates an irreversible enzyme inhibitor and its target residue?
24. Enzyme kinetics analysis of a reaction shows the following in the presence of compound X: Vmax is unchanged but Km increases significantly. What type of inhibition does compound X exhibit, and on a Lineweaver-Burk (double reciprocal) plot, where do the lines with and without inhibitor intersect?
25. Lactate dehydrogenase (LDH) has five isoforms composed of combinations of H (heart) and M (muscle) subunits. In acute myocardial infarction, the pattern LDH1 > LDH2 ('flipped LDH') is diagnostic. Which subunit composition represents LDH1 and why is it predominant in cardiac tissue?
26. An enzyme exhibits sigmoidal kinetics in its velocity-substrate concentration (v vs [S]) curve with a Hill coefficient (n) of 3.2. A drug that stabilises the T (tense, low-affinity) state of this enzyme would act as:
27. Prostate-specific antigen (PSA) is a serine protease of the kallikrein family. In the serum, PSA exists in two forms: free PSA and PSA complexed with alpha-1-antichymotrypsin (ACT). In benign prostatic hyperplasia (BPH), the ratio of free:total PSA is:
28. In a double-reciprocal (Lineweaver-Burk) plot, an inhibitor causes the lines at different inhibitor concentrations to intersect ON the x-axis (same x-intercept = same -1/Km, different y-intercepts). This inhibitor type is:
29. Alkaline phosphatase (ALP) isoenzymes can distinguish the source of elevation. The heat-stable ALP isoenzyme (resistant to heating at 56°C for 15 minutes) originates from:
30. A 55-year-old man with acute myocardial infarction has elevated serum troponin. His lactate dehydrogenase (LDH) isoenzyme pattern shows LDH-1 > LDH-2 ('flipped pattern'). The subunit composition of the LDH-1 isoenzyme that predominates in cardiac muscle is:
31. A patient with progressive muscle weakness and markedly elevated serum creatine kinase (CK) is found to have pure CK-MM elevation. The MB fraction is absent. Muscle biopsy reveals glycogen accumulation. The specific enzyme deficiency most likely causing CK-MM release in the setting of glycogen storage is:
32. Serum alkaline phosphatase (ALP) is elevated in a 14-year-old girl with no bone or liver disease. The family history reveals a similar finding in her father. This physiological variant of elevated ALP is most likely due to which isoenzyme source?
33. A researcher studying enzyme kinetics plots 1/v against 1/[S] (Lineweaver-Burk plot) for an enzyme with and without an inhibitor. The inhibitor increases both the x-intercept and y-intercept equally, and the lines cross on the y-axis. This pattern is characteristic of which type of inhibition?
34. Gamma-glutamyl transferase (GGT) is significantly elevated in a patient on long-term phenytoin therapy, but serum bilirubin, ALT, and ALP from bone are normal. The mechanism for this selective GGT elevation is:
35. In Michaelis-Menten kinetics, an inhibitor increases Km without changing Vmax. A second inhibitor decreases Vmax without changing Km. Which inhibition types are described respectively?
36. Isozymes of lactate dehydrogenase (LDH) are tetramers of H and M subunits. LDH-1 (HHHH) predominates in cardiac muscle and erythrocytes. How does the biochemical property of LDH-1 differ from LDH-5 (MMMM, liver/skeletal muscle), making LDH-1 better suited to aerobic tissue?
37. An enzyme follows Michaelis-Menten kinetics. A competitive inhibitor doubles the apparent Km without changing Vmax. If the original Km = 2 mM and the substrate concentration used in the assay is 8 mM, what is the velocity as a fraction of Vmax in the presence of the inhibitor?
38. Covalent modification of enzymes by phosphorylation is a key regulatory mechanism. Phosphorylation of glycogen synthase INACTIVATES it while phosphorylation of glycogen phosphorylase ACTIVATES it. What is the net effect when glucagon elevates cAMP?
39. An enzyme shows a Km of 5 mM and Vmax of 100 nmol/min. A new compound doubles the apparent Km to 10 mM without changing Vmax. A Lineweaver-Burk plot of the inhibited reaction would show:
40. Covalent catalysis is the mechanism used by serine protease enzymes. In the catalytic mechanism of chymotrypsin, which three amino acid residues form the catalytic triad and what are their functions?
41. An enzyme with Km = 2 mM and Vmax = 100 nmol/min is inhibited by a competitive inhibitor at concentration 5 mM with Ki = 1 mM. The apparent Km in the presence of the inhibitor is:
42. Allosteric enzymes differ from Michaelis-Menten enzymes in having sigmoidal kinetics. The Hill coefficient (n) for a perfectly cooperative allosteric enzyme equals the:
43. Irreversible inhibition of enzymes differs fundamentally from reversible inhibition. Aspirin (acetylsalicylic acid) irreversibly inhibits COX by: