Biochemical reactions are occurring constantly in cells, converting starting substances to different products, usually with the help of enzymes that speed the reactions. Without enzymes, it would take far too long for most reactions to occur to be useful to the cell!

Since enzymes help control the rate of reactions, their activity is regulated so that appropriate amounts of starting materials, intermediate metabolites, and products are maintained in the cell. Excessive build-up or depletion of substances can have disastrous consequences on the health of the cell and organism.

In a regulatory process called feedback inhibition, the product of a reaction inhibits an enzyme at an earlier step in the metabolic pathway that produced it. The product binds to a regulatory site on the enzyme, which is distinct from the active site that binds to the substrate, thereby slowing or shutting down its own production. When the product binds to the regulatory site, it causes a conformational change in the enzyme that prevents it from binding to the substrate and continuing to catalyze the reaction.

This prevents excessively high levels of the product from accumulating in the cell. When its level drops low enough, the product no longer inhibits the enzyme, and the reaction can proceed as usual.