(JUNE 2019)
57. Choose the INCORRECT statement from the following statements made for an enzyme- catalyzed reaction
(1) The kinetic properties of allosteric enzyme do not diverge from Michaelis-Menten behaviour.
(2) In feedback inhibition, the product of a pathway inhibits an enzyme of the pathway
(3) An antibody that binds tightly to the analog of the transition state intermediate of the
reaction S → P, would promote formation of P when the analog is added to the reaction.
(4) An enzyme with K_cat = 1.4 x 10⁴ s^-1 and K_m = 9 x 10^-5 M has activity close to the diffusion controlled limit.

The correct answer is (1) The kinetic properties of allosteric enzyme do not diverge from Michaelis-Menten behavior.

Introduction

Understanding enzyme kinetics is essential in biochemistry, but common misconceptions often lead to incorrect conclusions about enzyme behavior, feedback mechanisms, and catalytic limits. This article clarifies why allosteric enzymes exhibit distinct kinetics from Michaelis-Menten models, the principle of feedback inhibition, the effect of antibodies mimicking transition states, and how to identify enzymes close to the diffusion limit based on catalytic constants.

Statement Analysis

(1) “The kinetic properties of allosteric enzyme do not diverge from Michaelis-Menten behaviour.”

• Incorrect statement.

• Allosteric enzymes show sigmoidal (S-shaped) kinetics due to cooperative binding of substrates or effectors, deviating from the classical Michaelis-Menten hyperbolic curve.

• Michaelis-Menten kinetics apply to enzymes with single or independent binding sites, not to allosteric enzymes.

(2) “In feedback inhibition, the product of a pathway inhibits an enzyme of the pathway.”

• Correct statement.

• Feedback inhibition is a common regulatory mechanism where pathway end products suppress activity of upstream enzymes to maintain metabolic balance.

(3) “An antibody that binds tightly to the analog of the transition state intermediate of the reaction S → P would promote formation of P when the analog is added to the reaction.”

• Correct statement.

• Antibodies binding transition state analogs stabilize transition states, often leading to transition state analog inhibition. However, in some cases, their presence can promote product formation by stabilizing intermediate states, depending on reaction conditions.

(4) “An enzyme with kcat=1.4×104 s−1kcat=1.4×104s−1 and Km=9×10−5 MKm=9×10−5M has activity close to the diffusion controlled limit.”

• Correct statement.

• kcat/Kmkcat/Km for this enzyme is approximately 1.5×108 M−1s−11.5×108M−1s−1, which is near the diffusion-controlled upper limit (~108−109108−109 M−1s−1M−1s−1) indicating extremely efficient catalysis.

Summary Table

Conclusion

The incorrect statement is (1), as allosteric enzymes do deviate from standard Michaelis-Menten behavior exhibiting cooperative substrate binding. The other statements accurately represent fundamental principles of enzymatic regulation, kinetics, and catalytic efficiency.