Enzymes are proteins that catalyze, or greatly speed up, the many vital chemical reactions occurring in the body at all times.

This means that the amount of "starting" chemical in the reaction, or substrate, is disappearing more rapidly, while the amount of "finished" chemicals, or products, is accumulating more rapidly. While this might be desirable in the short term, what happens when the amount of product is sufficient, but there is still plenty of substrate for the enzyme to work on?

Fortunately for cells, they have a way to "talk" to enzymes from upstream, as it were, to let them know it's time to slow or shut down. That way is the feedback inhibition of enzymes, a form of feedback regulation.

Enzymes are flexible proteins that speed up biochemical reactions by making it easier for the substrate molecule to assume the physical arrangement of the product molecule, with the two usually being very closely chemically related.

When an enzyme binds with its specific substrate, it often induces a conformational change in the molecule, urging it in the direction of being more energetically inclined to take the shape of the product molecule. In chemical accounting terms, this facilitation of a reaction that would otherwise occur too slowly for life happens because the enzyme lowers the activation energy of the reaction.

Some enzymes act by bringing two substrate molecules physically closer together by bending, which makes the reaction occur more quickly because the substrates can then more readily exchange electrons, the stuff of chemical bonds.

When it is time to order an enzyme to stop, the cell has a number of ways to do this.

One is through competitive inhibition of the enzyme, which happens when a substance that very closely resembles the substrate is introduced to the environment. This "tricks" the enzyme into attaching to the new substance instead of its intended target. The new molecule is called a competitive inhibitor of the enzyme.

In noncompetitive inhibition, a newly introduced molecule also binds to the enzyme, but at a spot removed from where it exerts its activity on its substrate, called an allosteric site. This interferes with the enzyme by altering its shape.

In allosteric activation, the basic chemistry is the same as in noncompetitive inhibition, except in this case, the enzyme is told to speed up, not slow down, by the change in shape the molecule binding to the allosteric site induces.

In feedback inhibition, a product is used to regulate the reaction that generates that product. This occurs because the product itself as able to act as an enzyme inhibitor at certain concentrations, multiple reactions "upstream" of where it is formed.

When a molecule, which you can think of as C, feeds back two steps in a reaction to act as an allosteric inhibitor of the production of B from molecule A, it is because too much C has built up in the cell. With less A being converted to B thanks to the allosteric inhibition by C, less B is made into C, and this occurs until enough C is consumed to draw it away from the A-to-B enzyme to get the reactions going again.

The synthesis of ATP, the universal fuel currency of living cells, is controlled by feedback inhibition.

Adenosine triphosphate, or ATP, is a nucleotide made from ADP, or adenosine diphosphate, by attaching a phosphate group to ADP. ATP comes from cellular respiration, and ATP acts as an allosteric inhibitor of the enzymes at various steps in the cellular respiration process.

Although ATP is a fuel molecule and thus indispensable, it is short-lived and spontaneously reverts to ADP when found in high concentrations. This means that an excess of ATP would only go to waste if the cell went to the trouble of synthesizing greater amounts than it does thanks to feedback inhibition.