In metabolic regulation, feedback inhibition is best described as?

• A. Enzyme degradation
• B. Product inhibits upstream enzyme
• C. Substrate activates downstream enzyme
• D. Product inhibits downstream enzyme

Answer: (B)

Feedback inhibition is a regulatory strategy where the end product of a metabolic pathway slows the pathway by acting on an enzyme earlier in the sequence. This ensures that when the product accumulates, the entire flux through the pathway is reduced, preventing wasteful overproduction and maintaining balance in the cell. The product typically inhibits an upstream (early) enzyme, often through allosteric binding, which quickly downregulates the pathway’s activity. This rapid control contrasts with enzyme degradation, which removes enzymes entirely and is a slower form of regulation. It also differs from scenarios where a substrate activates a downstream enzyme, which would boost later steps rather than curb the overall flow, or where a product inhibits a downstream enzyme, which would not efficiently limit the entire pathway. So, the description that the product inhibits an upstream enzyme best captures how feedback inhibition functions.