Difficulty: Easy
Correct Answer: Catalyzed by a specific enzyme
Explanation:
Introduction / Context:
Cells organize chemistry into metabolic pathways, where each step converts a substrate into a product. This item probes the principle of enzyme specificity and control at the level of individual steps within a pathway.
Given Data / Assumptions:
Concept / Approach:
Enzyme specificity means a particular protein catalyst binds specific substrates and carries out a defined reaction with high selectivity. Although some steps are near-equilibrium and others are far-from-equilibrium (effectively irreversible in vivo), the unifying feature is that each step is catalyzed by its own enzyme, enabling regulation and flux control across the pathway.
Step-by-Step Solution:
1) Identify the common characteristic across all steps: enzymatic catalysis.2) Recognize that reversibility varies by step; it is not universal.3) End-product control (feedback inhibition) is one mechanism that targets specific regulatory enzymes, not every step.4) Therefore, the correct general statement is that each reaction is catalyzed by a specific enzyme.
Verification / Alternative check:
Biochemical maps (e.g., glycolysis, TCA cycle, urea cycle) list a distinct named enzyme for each reaction, confirming universality of step-specific catalysis.
Why Other Options Are Wrong:
Option A/B: Some steps are reversible, others effectively irreversible; neither property applies to all reactions. Option D: End-product inhibition regulates key nodes, not every reaction. Option E: Physiological rates and specificity require enzymes; spontaneous rates are insufficient.
Common Pitfalls:
Equating “pathways are regulated” with “every step is controlled by the end product,” and assuming uniform reversibility across all steps.
Final Answer:
Catalyzed by a specific enzyme
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