In feedback inhibition, which enzyme is inhibited by the end product?

• A. The first enzyme.
• B. The last enzyme in the pathway.
• C. An intermediate enzyme.
• D. All enzymes in the pathway.

Answer: (B)

Feedback inhibition works by the end product signaling the pathway to slow down or stop, and the most common target is the first enzyme in the sequence, often the committed step. Blocking the entry point of the pathway efficiently reduces the production of all downstream intermediates, so the cell avoids wasting resources if enough product is already present. If regulation hit the last enzyme instead, the final step would stop, but intermediates could still accumulate and energy would be wasted while the rest of the pathway kept running up to the last step. While some pathways can have regulation at other points, the hallmark of feedback inhibition is control at or near the initial step, which is why inhibiting the first enzyme is the typical and most effective form of this regulation.