feedback (end-product) inhibition
/ FEED-bak in-hih-BIH-shun /
Picture a factory assembly line that makes a product, with a smart rule: when the warehouse is full, the line shuts off the very first machine. There is no point making more raw output that just piles up. The cell uses exactly this trick. When a metabolic pathway has made enough of its final product, that product reaches back and switches off an early enzyme in its own pathway. This is feedback inhibition, also called end-product inhibition.
A metabolic pathway is a chain of enzyme-catalyzed steps, each turning one molecule into the next, until a useful end product appears. In feedback inhibition, the end product acts as an allosteric inhibitor of an enzyme near the start of the pathway — usually the first committed step. As the product accumulates, more of it binds that early enzyme and slows the whole pathway; as the product gets used up, it lets go, and the pathway speeds back up. The result is a self-correcting loop that keeps the product at a steady level without waste.
This is one of biology's most elegant control systems, an example of negative feedback that keeps the cell in balance (homeostasis). It is fast, automatic, and reversible, and it conserves resources by not overproducing. The same logic appears everywhere from amino-acid synthesis to body-wide hormone control. A key subtlety: the end product usually inhibits the first step, not a middle one, so the cell does not waste effort building intermediates it will only have to stall halfway through.
Bacteria build the amino acid isoleucine in five enzyme steps. When enough isoleucine has been made, it binds and shuts off the very first enzyme in that chain — so the cell stops pouring resources into making more, and resumes only when isoleucine runs low again.
The end product switches off the first enzyme — a self-correcting 'enough is enough' loop.
Feedback inhibition is usually allosteric and aimed at the first committed step — not the active site of the last enzyme, and not by destroying the enzyme.