Enzymes are protein machines that need to take on 3D shapes in order to function properly. Enzymes become inactive when they lose their 3D structure. One way this happens is because the temperature gets too hot and the enzyme denatures, or unfolds. Another way that enzymes become inactive is when their activity is blocked by a chemical inhibitor. There are different types of inhibitors. Competitive inhibitors bind to and block the enzymes active site. Non-competitive inhibitors bind to a site other than the active site, but cause the active site to be non-functional.

The atoms in enzymes normally vibrate, but not so much that the molecule unfolds. Increasing the temperature of the enzyme increases the amount of vibration. Too much jiggling and the enzyme begins to lose its proper shape. Enzymes have an optimal temperature range in which they are most active. Enzyme activity increases as the temperature reaches this optimum range, but sharply decreases after this range is passed. Most animal enzyme lose activity above 40 degrees Celsius. There are bacteria called extremophiles that can survive in hot springs. Their enzymes can withstand temperatures that boil water.

Enzymes have a region called the active site, which is responsible for performing the chemical reaction that is the main purpose of enzyme. Just like the rest of the enzyme, the active site needs to have a proper 3-D shape in order to work. The active site is like the mouth of the enzyme. The side groups of certain amino acids stick into the space of the active site, much like teeth in the mouth. These side groups are responsible for making the chemical reaction happen. Just as teeth need to be aligned in order to chew food, the side groups cannot complete the reactions if the active site is not in its 3-D shape.

Another way enzymes become less effective is because their activity is blocked by a chemical inhibitor. Competitive inhibitors are molecules that bind to the active site of the enzyme. The active is where the substrate, the molecule that enzyme is supposed to modify, binds, so the competitive inhibitor competes with the substrate for the active site. Many competitive inhibitors are known as reversible inhibitors, because though they bind the active site they can fall off. This turns the enzyme back on.

Another type of enzyme inhibitor is called non-competitive inhibitors. These type chemicals do not bind to the active site, but to another site on the enzyme. However, the binding of the inhibitor at the other site cause a change in the shape of the protein that either closes or blocks the active site. Non-competitive inhibitors are also called allosteric inhibitors, since allosteric sites are regulatory sites that are not the active site. Some enzymes are multiple enzymes that come together into what is called an enzyme complex. An allosteric inhibitor can turn off all enzymes in a complex by binding to one allosteric site.