(NOV 2020-1)
13. The following statements were made of suggest the existence of an enzyme-substrate complex.
A. at constant concentration of enzyme, the reaction increase with rate increase with increasing substrate concentration
B. An enzyme-catalyzed reaction has a maximal velocity
C. At constant concentration of the enzyme and substrate, an increase in the reaction rate is
observed.
D. An enzyme catalyzed reaction is not influenced by high substrate concentration.
Which of the above statements suggest the existence of an enzyme-substrate complex?
(1) A and B                                                          (2) B and C
(3) A and C                                                          (4) D only

The correct answer is (1) A and B.

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Introduction

The concept of the enzyme-substrate complex is foundational for understanding enzyme catalysis and kinetics. Several observable phenomena indicate its existence, such as the dependence of reaction rates on substrate concentration and the achievement of a maximal reaction velocity (Vmax). This article clarifies which statements about enzyme-catalyzed reactions support the enzyme-substrate complex model and elaborates on their significance.

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Analysis of the Statements

Statement A

“At constant concentration of enzyme, the reaction rate increases with increasing substrate concentration.”

• This reflects the enzyme’s binding to substrate to form an ES complex.

• Initial increase in substrate concentration enhances complex formation, thereby increasing the reaction rate.

• Indicates classic substrate dependence essential for ES complex formation.

Statement B

“An enzyme-catalyzed reaction has a maximal velocity.”

• At high substrate concentrations, enzyme active sites become saturated, and all enzymes form ES complexes.

• The reaction rate reaches a plateau (Vmax), reflecting a maximum number of ES complexes converting substrate to product.

• Demonstrates the finite capacity of enzyme molecules for substrates, confirming ES complex formation and saturation kinetics.

Statement C

“At constant concentration of the enzyme and substrate, an increase in the reaction rate is observed.”

• At constant enzyme and substrate concentration, reaction rate should not increase.

• Rate depends on these concentrations; no reason for rate increase at stable concentrations.

• Does not conclusively suggest ES complex existence.

Statement D

“An enzyme-catalyzed reaction is not influenced by high substrate concentration.”

• False; enzyme reactions are influenced by substrate concentration until saturation is reached.

• Saturation kinetics explain why rate plateaus at very high substrate, but substrate concentration still influences rate up to that point.

• Contradicts the enzyme-substrate complex concept.

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Why A and B Suggest ES Complex Existence

• Both reflect classic Michaelis-Menten kinetics, which describe the formation and breakdown of enzyme-substrate complexes.

• Increasing substrate concentration increases the formation of enzyme-substrate complexes, boosting initial reaction rate (A).

• The saturation of enzyme active sites leads to Vmax, indicating all enzyme molecules are engaged in ES complex formation (B).

• These properties could not occur without the transient formation of the enzyme-substrate complex.

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Supporting Concepts: Michaelis-Menten Model

E+S⇌k−1k1ES→k2E+PE+Sk1k−1ESk2E+P

• $E$: enzyme

• $S$: substrate

• $ES$: enzyme-substrate complex

• $P$: product

Reaction velocity depends on ES complex concentration; saturation kinetics produce the Vmax plateau.

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Summary Table

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Conclusion

Statements A and B—the increase of reaction rate with substrate concentration and the existence of maximal velocity—are strong evidence for the formation of an enzyme-substrate complex. This concept is fundamental for understanding enzyme kinetics and underpins many biological processes and experimental designs, making it essential knowledge for life science students and researchers.