Which statement is the most accurate description of feedback inhibition?

Question

BlackTom AI · Accepted Answer

To approach this question, let's consider what feedback inhibition means in metabolic pathways and which component exerts the control.
Option 1: 'The end product feeds back and inhibits the enzyme immediately upstream that directly produced the end product.' This statement describes inhibition targeting the enzyme that produced the last product, i.e., the enzyme immediately upstream. In many classic descriptions, feedback inhibition more commonly involves the end product inhibiting the first enzyme of the pathway (the initial committed step), not the penultimate one. So while it captures a form of inhibition, it is not the standard description of feedback inhibition.
Option 2: 'An early intermediate inhibits the enzyme that directly produces the end product, that is, the last step in the pathway.' Here the inhibitor is an intermediate rather than the end product, and it targets the final step. This deviates from the canonical feedback mechanism, where the end product is typically the inhibitor, and the target is often the first committed step rather than the last step. Therefore this is inaccurate as a general description of feedback inhibition.
Option 3: 'The end product of a metabolic pathway inhibits an early enzyme in the pathway.' This aligns with the classic concept of feedback inhibition: the pathway’s end product acts as a negative regulator of an early, usually the first or an early committed, enzyme, thereby downregulating the entire pathway when product levels are high. This description matches the standard mechanism.
Option 4: 'An early intermediate in a metabolic pathway feeds back to inhibit an early enzyme in the pathway.' In this case, the inhibitor is an intermediate rather than the end product, and although inhibition of an early enzyme occurs, the typical and most emphasized form of feedback inhibition involves the end product itself rather than an intermediate. This makes the statement inaccurate as the general description of feedback inhibition.
Overall, the statements differ in which molecule acts as the inhibitor (end product vs. intermediate) and which enzyme is targeted (early vs. last step). The most accurate, canonical description is that the end product inhibits an early enzyme in the pathway, which matches Option 3. The remaining options misstate either the inhibitor, the target enzyme, or both, leading to incorrect portrayals of feedback inhibition.

Which statement is the most accurate description of feedback inhibition?

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