(JUNE 2019)
47. Which one of the following statements is NOT correct?
(1) Allosteric enzymes do not obey Michaelis-Menten kinetics.
(2) The free-energy change provides information about the spontaneity but not rate of a reaction.
(3) Competitive and non-competitive inhibitions are kinetically indistinguishable.
(4) A Kcat/Km (M-1 S-1) of 2x 108 for an enzyme indicates that the value is close to diffusion-controlled rate of encounter.
The correct answer is (3) Competitive and non-competitive inhibitions are kinetically indistinguishable.
Introduction
Enzyme kinetics is fundamental to biochemistry, but several misconceptions persist regarding allosteric enzymes, thermodynamics of reactions, types of enzyme inhibition, and catalytic efficiency values. This article identifies the incorrect statement among common assertions and explains the correct scientific understanding. Clarifications help learners and professionals build accurate conceptual frameworks essential for research and exam preparation.
Statement Analysis
(1) Allosteric enzymes do not obey Michaelis-Menten kinetics.
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Correct: Allosteric enzymes tend to exhibit sigmoidal kinetics due to cooperative binding of substrates to multiple active sites rather than the hyperbolic kinetics described by Michaelis-Menten.
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They often do not fit the basic Michaelis-Menten model due to conformational changes affecting activity.
(2) The free-energy change provides information about the spontaneity but not rate of a reaction.
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Correct: Gibbs free energy change (ΔG) indicates whether a reaction is thermodynamically favorable (spontaneous) but does not specify reaction speed (kinetics).
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Reaction rate depends on activation energy and enzyme catalysis, not solely ΔG.
(3) Competitive and non-competitive inhibitions are kinetically indistinguishable.
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Incorrect:
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Competitive inhibition increases the apparent Km without changing Vmax; the inhibitor competes with substrate for the active site.
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Non-competitive inhibition decreases the apparent Vmax without affecting Km; the inhibitor binds an allosteric site affecting catalytic function regardless of substrate binding.
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These differences are kinetically distinguishable by enzyme assays and Lineweaver-Burk plots.
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(4) A kcat/Km (M−1s−1) of 2×108 for an enzyme indicates that the value is close to diffusion-controlled rate of encounter.
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Correct:
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The diffusion-limited rate constant is about 108−109 M−1s−1.
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A kcat/Km value near this range suggests the enzyme works at near the physical limit imposed by substrate diffusion.
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Summary Table
| Statement | Accuracy | Explanation |
|---|---|---|
| (1) | Correct | Allosteric enzymes show sigmoidal kinetics, not MM kinetics |
| (2) | Correct | ΔG indicates spontaneity, not rate |
| (3) | Incorrect | Competitive and non-competitive inhibitions have distinct kinetic signatures |
| (4) | Correct | kcat/Km close to diffusion-controlled limit signals maximal enzyme efficiency |
Conclusion
The statement that competitive and non-competitive inhibitions are kinetically indistinguishable is NOT correct, unlike the others which accurately describe enzyme behavior or thermodynamic principles. Recognizing these distinctions is crucial for enzyme kinetics analysis, drug design, and biochemical research.
This detailed analysis supports mastery of enzyme kinetics concepts critical for effective study and scientific investigation in life sciences.
35 Comments
Khushi Vaishnav
September 12, 2025Competitive and non-competitive inhibitions are kinetically indistinguishable this statement is NOT correct
Varsha Tatla
September 13, 2025Competative nd uncompitative inhibitors have distinct kinetics properties
Aakansha sharma Sharma
September 13, 2025(3) Competitive and non-competitive inhibitions are kinetically indistinguishable. Is incorrect
Kajal
September 14, 2025Competitive and non competitive inhibitor are kinetically distinguished by LB plot so this statement is wrong .
Pratibha Jain
September 14, 2025correct answer is (3) Competitive and non-competitive inhibitions are kinetically indistinguishable.
Rishita
September 14, 2025correct answer is (3) Competitive and non-competitive inhibitions are kinetically indistinguishable.
Mohd juber Ali
September 14, 2025The inncorrect statement is option 3
(1). Competative inhibition = affinity decrease and km high
(2). Non competative inhibition the affinity of E = ES and kdi=kdi’
So distinguisable
Aafreen Khan
September 14, 2025Competitive and non-competitive inhibitions are kinetically indistinguishable
Kanica Sunwalka
September 14, 2025competitive and non competitive inhibitions are kinetically indistinguishable
its an incorrect statement
Dharmpal Swami
September 14, 2025Competitive and non competitive inhibition are kinetically distinguishable
So option 3 incorrect
Santosh Saini
September 14, 2025In competitive inhibition affinity decreases and km increases , and in non-competitive inhibition affinity equal so km is stable (remain same) and Vmax decreases, so option 3 is incorrect
Shivani
September 14, 2025Competitive and no-competitive inhibitions are kinetically indestinguishable
Pooja
September 14, 2025Competitive and non-competitive inhibitions are kinetically indistinguishable
Soniya Shekhawat
September 14, 2025In competitive inhibition km increase affinity decrease and not change in V maX BUT IN non competitive inhibition decrease of V maX and affinity is equal to km
yashika
September 14, 2025Compet and non compet are kineticalky distinguish
Konika Naval
September 14, 2025Competitive and non-competitive inhibitions are kinetically indistinguishable.
Vanshika Sharma
September 14, 2025Competitive and non competitive inhibitors are kinetically indistinguishable
Sakshi Kanwar
September 14, 2025Competitive and non-competitive inhibitions are kinetically indistinguishable.
They both can be easily distinguishable by graph
Kirti Agarwal
September 14, 2025Competitive and uncompetitive inhibitors are kinetically indistinguishable
Sakshi yadav
September 14, 2025Competitive and non-competitive inhibitions are kinetically indistinguishable.
Muskan singodiya
September 14, 2025Compitetive and non competitive inhibitions are kineticly indistinguishable
Palak Sharma
September 14, 2025Competitive and uncompetitive inhibitors are kinetically distinguishable by LB plot.
Ajay Sharma
September 14, 2025Allosteric enzymes do not obey michaelis menten
Gibbs free energy only indicate spontaneity not reaction speed
Diffusion limited rate constant is around 10^9-10^10
So the right incorrect answer is that Competitive and non-competitive inhibitions are kinetically indistinguishable cause they can be by lb plot and enzyme assays
Tanvi Panwar
September 14, 2025Competitive and uncompetitive inhibitors are kinetically indistinguishable.
Deepika sheoran
September 15, 2025Competative and Non competative inhibitions are kinetically indistinguishable.
So option 3 incorrect.
Anurag Giri
September 15, 2025The correct answer is (3) Competitive and non-competitive inhibitions are kinetically indistinguishable In competitive inhibition km increase affinity decrease and not change in V maX BUT IN non competitive inhibition decrease of V maX and affinity is equal to km
Nilofar Khan
September 15, 2025correct answer is (3)
Competitive and non-competitive inhibitions are kinetically indistinguishable. In competitive inhibition km increase and affinity decrease (km is inversely proportional to affinity)
In noncompetitive affinity is equal to km
Payal Gaur
September 15, 20253. Competitive and non competitive inhibitors are kineticlly indistinguishable. Because in competitive km is increase and affinity decrease Vmax increase and non competitive affinity is equal to km(Vmax decrease).
Anjana sharma
September 16, 2025Competitive and non-competitive inhibitions are kinetically indistinguishable.
Khushi Agarwal
September 16, 2025The correct answer is (3)
Competitive and non-competitive inhibitions are kinetically indistinguishable
Simran Saini
September 16, 2025Competitive and non-competitive inhibitions are kinetically indistinguishable.
Avni
September 16, 2025Competitive and non-competitive inhibitions are kinetically indistinguishable.
Priti khandal
September 17, 20253is correct sir
Muskan Yadav
September 18, 2025Competitive and non-competitive inhibitions are kinetically indistinguishable.
Minal Sethi
September 19, 2025competitive and non-competitive are kinetically distinguishable