Enzyme–substrate interaction: What best describes the relationship between an enzyme and its reactant (substrate) molecule during catalysis?
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AA temporary, reversible association forming an enzyme–substrate complex.
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BA covalently stabilized association that permanently changes the enzyme.
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CA process in which the enzyme is permanently altered and cannot catalyze further reactions.
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DA non-complementary binding event with no specificity.
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EA random collision with no formation of an intermediate complex.
Answer
Correct Answer: A temporary, reversible association forming an enzyme–substrate complex.
Explanation
Introduction:Catalysis involves the transient formation of an enzyme–substrate (ES) complex. Understanding the temporary and reversible nature of ES is central to interpreting kinetic parameters and turnover.
Given Data / Assumptions:
- Standard Michaelis–Menten mechanism applies.
- Noncovalent interactions dominate ES formation (hydrogen bonds, ionic interactions, hydrophobic effects).
- Enzyme is regenerated after product release.
Concept / Approach:According to E + S ⇌ ES → E + P, ES is a transient complex. Binding is specific and reversible, enabling multiple catalytic cycles (turnover) without permanent enzyme consumption.
Step-by-Step Solution:1) E binds S via complementary shapes/chemistry to create ES.2) ES rearranges to a transition-state-like geometry, lowering activation energy.3) Chemistry occurs, producing product P and regenerating free E for another cycle.
Verification / Alternative check:Kinetic evidence: saturation behavior (Vmax) implies a finite ES complex population at high [S]; structural data show substrate analogs bound noncovalently and reversibly.
Why Other Options Are Wrong:
- (b) and (c) imply permanent chemical modification of the enzyme; ordinary catalysis is catalytic and recyclable.
- (d) Binding is complementary and specific, not random or non-complementary.
- (e) Productive catalysis involves a defined ES intermediate, not mere random collisions.
Common Pitfalls:Confusing transient covalent intermediates (in some serine/cysteine proteases) with permanent modification; assuming enzymes are consumed.
Final Answer:A temporary, reversible association forming an enzyme–substrate complex.