Suicidal inhibition These HIV protease inhibitors act by binding irreversibly to the active site of the viral protease enzyme, rendering it unable to function. This mechanism is known as suicidal inhibition. The key aspects are: - They bind irreversibly to the active site - Rendering the enzyme unable to function - This is considered suicidal inhibition The other proposed inhibition types (competitive, non-competitive, uncompetitive) do not accurately describe this mechanism.

1. Enzymology-Quiz
NAMRATA CHHABRA
MHPE, FAIMER FELLOW,
MD (BIOCHEMISTRY), M.B.B.S.

2. Case study 1
A 56- year- old female presents with difficulty opening her eye lids, as
well as inability to raise herself from sitting position.
She is diagnosed with "myasthenia gravis", a disease of extreme
fatigue, due to decreased concentration of Acetyl choline in her
muscles.
She has been prescribed physostigmine, a drug that increases the
amount of available Acetyl choline, by competitively inhibiting
acetylcholinesterase.
04/01/2021 Our Biochemistry 2

3. Case study 1
Which of the following statements is not true of competitive
inhibitors?
a) Vmax remains the same
b) Apparent Km in increased
c) Inhibitor is a structural analogue of the substrate
d) Inhibitor binds covalently to the enzyme
e) Increasing concentration of substrate can reverse the changes
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4. Answer
d) Inhibitor binds covalently to the enzyme
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5. Case study 2
A 55-year-old male presents with difficult breathing and swollen
ankles.
He is found to have a failing heart, resulting in blood backing up in
to his lungs (pulmonary congestion) and making it difficult for him to
breathe.
He is administered a drug that inhibits angiotensin converting enzyme
(ACE).
04/01/2021 Our Biochemistry 5

6. Case study 2
By inhibiting this enzyme, which of the following will change about the
reaction it catalyzes ?
a) Energy of activation
b) Net free energy change
c) Equilibrium concentration of substrate
d) Equilibrium concentration of product
e) Thermodynamics
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7. Answer
a) Energy of activation
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8. Case study 3
A 55-year-old male presents with familial hypercholesterolemia. His
serum cholesterol level has been reported to be 600 mg/dl. The familial
hypercholesterolemia is a disorder of defective LDL receptors. LDL fails
to get internalized due to faulty receptors. As a result, endogenous
cholesterol synthesis occurs uninhibited.
He is administered a drug that inhibits HMG Co A reductase. By
inhibiting this enzyme, which of the following will change about the
reaction it catalyzes ?
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9. Case study 3
a) Vmax
b) Km
c) Both Vmax and Km will decrease
d) Both Vmax and Km will increase
e) Thermodynamics
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10. Answer
b) Km
The drug acts by competitive
inhibition, therefore, Km will
increase, Vmax will remain
constant.
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11. Question
During an extended period of exercise, the enzymes involved in the
glycolytic pathway in muscle tissue are actively breaking down glucose
to provide the muscle energy. The liver, to maintain blood glucose
levels, is synthesizing glucose via the gluconeogenic pathway.
Which of the following enzymes involved in these pathways would be
most likely to exhibit Michaelis–Menten kinetics, that is, have a
hyperbolic curve when plotting substrate concentration versus
velocity of the reaction?
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12. Question
A) Fructose-1, 6-bisphosphatase
B) Hexokinase
C) Lactate dehydrogenase
D) Phosphofructokinase 1
E) Pyruvate kinase
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13. Answer
C) Lactate dehydrogenase
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14. Case study
A 54-year –old male was rushed to emergency when he collapsed in
the middle of a business meeting. Examination revealed excessive
sweating and high blood pressure. chest was conclusive of Acute
Myocardial infarction. Which biochemical investigation out of the
following would be of no help in the confirmation of diagnosis?
a) Cardiac Troponins
b) Serum myoglobin
c) Lactate dehydrogenase
d) Creatine Phospho kinase-MB(CPK-MB)
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15. Answer
c) Lactate dehydrogenase
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16. Case study
A 67- year-old army officer in good health previously presented with
sudden pain in the great toe. Serum uric acid level was higher, and a
diagnosis of gouty arthritis was made. He was advised bed rest, pain
killers and Allopurinol. What is the mechanism of action of Allopurinol
in lowering serum uric acid levels?
a) Suicidal inhibition
b) Noncompetitive inhibition
c) Allosteric inhibition
d) Feed back inhibition
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17. Answer
a) Suicidal inhibition
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18. Question
The activities of many enzymes, membrane transporters and other
proteins can be quickly activated or inactivated by phosphorylation of
specific amino acid residues. This regulation is called:
a) Allosteric modification
b) Covalent modification
c) Induction
d) Repression
e) Substrate level phosphorylation
04/01/2021 Our Biochemistry 18

19. Answer
b) Covalent modification
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20. Question
The drug Fluorouracil is recommended in the treatment of cancers. It
undergoes a series of changes and then binds to thymidylate synthase
enzyme resulting in its inhibition and blockage of cell division. This
mode of inhibition is most probably due to-
a) Allosteric inhibition
b) Competitive inhibition
c) Noncompetitive Inhibition
d) Suicidal inhibition
e) Uncompetitive inhibition
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21. Answer
d) Suicidal inhibition
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22. Question
The conversion of Pyruvate to oxaloacetate involves the participation
of---- as a coenzyme -
a) NAD+
b) NADPH
c) Biotin
d) FAD
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23. Answer
c) Biotin
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24. Question
Which out of the following coenzymes takes part in hydrogen transfer
reactions in the electron transport chain-
a) Tetrahydrofolate
b) Methyl Cobalamine
c) Co enzyme Q
d) Biotin
e) Folic acid
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25. Answer
c) Co enzyme Q
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26. Question
In the study of enzymes, a sigmoidal plot of substrate concentration [S]
versus reaction velocity (V) may indicate:
a) Michaelis-Menten kinetics
b) Cooperative binding
c) Competitive inhibition
d) Non-competitive inhibition
e) Suicidal inhibition
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27. Answer
b) Cooperative binding
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28. Question
A noncompetitive inhibitor of an enzyme :
a) Increases Km with little or no change in Vmax
b) Decreases Km and decreases Vmax
c) Decreases Vmax
d) Increases Vmax
e) Increases Km and increases Vmax
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29. Answer
c) Decreases Vmax
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30. Question
Which one of the following statements correctly describes allosteric
enzymes?
a) Effectors may enhance or inhibit substrate binding
b) They are usually not controlled by feedback inhibition
c) The regulatory site may be catalytic site
d) Michaelis Menten kinetics describe their activity
e) Positive co-operativity occurs in all allosteric molecules except
hemoglobin
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31. Answer
a) Effectors may enhance or inhibit
substrate binding
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32. Question
Which one of the following statements correctly describes allosteric
enzymes?
a) Regulatory molecules bind the active site
b) Regulatory molecules alter equilibrium but not activity
c) Regulatory molecules do not affect activity or equilibrium
d) Hyperbolic curve is obtained when reaction velocity is plotted
against substrate concentration
e) Binding of one site can affect other sites
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33. Answer
e) Binding of one site can affect other sites
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34. Question
Tissue plasminogen activator cleaves Arg-Val bond in plasminogen
(using water) to form active plasmin. This recombinant enzyme is used
to treat ischemic stroke and myocardial infarcts by its thrombolytic
activity to dissolve blood clots. This enzyme belongs to which of the
following classes of enzymes?
a) Oxidoreductases
b) Transferases
c) Hydrolases
d) Lyases
e) Ligases
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35. Answer
c) Hydrolases
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36. Question
A teenager finishes eating a hot fudge sundae and a milk shake. As her blood
sugar rises, the liver recruits both Glucokinase (km = 10.0m M) and
Hexokinase (km = 0.10mM) to metabolize the glucose.
Which of the following is a correct statement about the kinetics of these two
enzymes?
A. Hexokinase will metabolize 100 times more glucose molecules
B. Hexokinase will lower the energy of activation more than glucokinase
C. Hexokinase has a much higher affinity to glucose than glucokinase
D. Hexokinase will have a higher Vmax than glucokinase
E. Hexokinase will be induced more than glucokinase by the rising
concentration of glucose
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37. Answer
c) Hexokinase has a much
higher affinity to glucose than
glucokinase
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38. Question
Which of the following enzymes cleaves a carbon-carbon bond in
glycolysis?
A. Phospho triose isomerase
B. Aldolase
C. Phospho glycerate kinase
D. Enolase
E. Pyruvate Kinase
04/01/2021 Our Biochemistry 38

39. Answer
B. Aldolase
04/01/2021 Our Biochemistry 39

40. Case study
• An 8-year-old girl presents with precocious puberty, short stature,
and coffee-colored macules.
• The patient has been diagnosed with McCune-Albright syndrome that
results from estrogen production due to excessive Aromatase activity
from ovarian follicular cysts.
• To treat the precocious puberty and other symptoms, the patient has
been prescribed a drug that inhibits the Aromatase activity.
• The drug is known to act by binding to substrate binding site,
producing an increase in km.
04/01/2021 Our Biochemistry 40

41. Case study
Which of the following choices best describes the mechanism of
inhibition of the drug?
A. Competitive
B. Noncompetitive
C. Allosteric
D. Suicidal
E. Feedback inhibition
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42. Answer
A. Competitive
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43. Choose the correct Answer
The enzyme that catalyzes the change in location of the functional
group from one position to another within a compound is called:
a) Isomerase
b) Mutase
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44. Answer
b) Mutase
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45. Choose the correct Answer
The enzyme having low affinity for the substrate will have a
a) Low Km
b) High Km
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46. Answer
b) High Km
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47. Choose the correct Answer
The competitive inhibitors increase the …….. but…….. remains constant
a) Vmax/Km
b) Km/Vmax
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48. Answer
b) Km/Vmax
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49. Choose the correct Answer
The non- competitive inhibitors decrease the ……but the….remains
constant :
a) Vmax/Km
b) Km/Vmax
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50. Answer
a) Vmax/Km
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51. Choose the correct Answer
The process of induction of an enzyme is an example of….. control for
regulation of enzyme activity.
a) Coarse
b) Fine
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52. Answer
a) Coarse control
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53. Choose the correct Answer
Arsenate binds to SH group of an enzyme to inhibit its activity and this
mode of inhibition is :
a) Competitive
b) Non-competitive
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54. Answer
b) Non-competitive
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55. Choose the correct Answer
In normal health……..is in greater concentration in plasma
a) LDH-1
b) LDH-2
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56. Answer
b) LDH-2
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57. Choose the correct Answer
Which of the two enzymes is inhibited by feedback inhibition?
a) Hexokinase
b) Glucokinase
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58. Answer
a) Hexokinase
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59. Choose the correct Answer
Which of the two enzymes causes intravascular bleeding as a side
effect?
a) Streptokinase
b) tPA
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60. Answer
a) Streptokinase
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61. Choose the correct Answer
Which of the two enzymes has an optimum pH of 2.0 ?
a) Pepsin
b) Trypsin
04/01/2021 Our Biochemistry 61

62. Answer
a) Pepsin
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63. Choose the correct Answer
When the substrate concentration is equal to km the reaction velocity
is equal to :
a) Vmax
b) Vmax/2
04/01/2021 Our Biochemistry 63

64. Answer
b) Vmax/2
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65. Choose the correct Answer
The reaction rate is ………. when the enzyme activity is independent of
substrate concentration
a) Zero order
b) First order
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66. Answer
a) Zero order
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67. Choose the correct Answer
A deficiency in thiamine (vitamin B1) would most likely lead to which
of the following clinical manifestations?
A. A decrease in carboxylase enzyme activity
B. A decrease in serum lactate concentrations
C. A decrease in red blood cell transketolase activity
D. An increase in urinary methylmalonate
E. An increase in prothrombin time
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68. Answer
c) A decrease in
red blood cell
transketolase
activity
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69. Choose the correct Answer
Propionyl co A carboxylase catalyzes the c-c condensation, between
Propionyl co A and CO2. This enzymes belongs to which of the
following classes of enzymes?
a) Ligase
b) Lyase
c) Transferase
d) Hydrolase
e) Oxidoreductase
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70. Answer
a) Ligase
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71. Choose the correct Answer
HIV protease inhibitors (saquinavir & ritonavir) target the final step of
viral replication and inhibit the active site of the enzyme from
functioning. What is the type of enzyme inhibition?
a) Competitive
b) Non-Competitive
c) Un-Competitive
d) Suicidal
e) Allosteric
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72. Answer
a) Competitive
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73. Choose the correct Answer
Hemostasis and thrombosis are mediated by enzymes along cascading
steps of the blood coagulation pathways. Inactive precursor proteins
are activated by a process called:
a) Subunit aggregation
b) Zymogen activation
c) Induction
d) Positive cooperation
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74. Answer
b) Zymogen activation
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75. Choose the correct Answer
Which of the following is an early and a significant marker of Acute MI?
a) AST
b) Lactate dehydrogenase
c) GGT
d) CPK-MB
e) ALT
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76. Answer
d) CPK-MB
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77. Thank you
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