Any substance that slows down or stops an enzyme catalysed reaction is an inhibitor. Broadly there are two types of inhibition that can affect enzyme catalysed reactions. These are competitive and non-competitive inhibition.
The necessity of the substrate to be able to fit into the active site on the enzyme is a feature of enzyme action, which allows competitive inhibition to occur. The following example shows the principle.
One of the enzymes needed for the respiration catalyses the oxidation (by the removal of two hydrogen atoms) of succinate acid ((a) in the diagram) to fumarate.
If malonate ((b) in the diagram) is added to a mixture of succinate and the enzyme, the action of the enzyme is strongly inhibited. This is because the structure of malonate allows it to fit into the active site of the enzyme. As there is no reaction when this occurs products are not quickly released and the site is temporarily blocked. The inhibition is called competitive because the two molecules compete on roughly equal terms for the active site on the enzyme.
Because the inhibitor molecules and the substrate molecules compete for the active site of the enzyme, changing the proportions of these molecules changes the degree of inhibition. If the concentration of substrate is increased enough then the effect of the inhibition can be removed.
When the inhibitor molecules are different in shape to the substrate and bind to the enzyme at a site away from the active site the inhibition is non-competitive as the inhibitor is not competing for the active site.
When a non-competitive inhibitor binds to the enzyme molecule it does so at a site other than the active site. This binding causes the enzyme's tertiary structure to change, which changes the shape of the active site so the substrate can no longer fit. This process is shown in the diagram below