Classification of Enzymes

Enzymes are generally classified on the basis of the type of reactions that they catalyse. 6 groups of enzymes can be recognised on this basis. The following table lists the 6 groups of enzymes along with example.

fig. 14.15 - Classification of Enzymes

Inhibition of Enzyme Action

There are four different ways by which the action of an enzyme can be inhibited.

Denaturation of Enzymes

Enzymes get denatured at very high temperatures, above 65⁰C. This denaturing occurs due to breakdown of the protein molecule.

Competitive Inhibition

Any chemical substance which has a molecular structure that closely resembles a substrate, can reduce or inhibit the activity of an enzyme. Such an inhibitor is called competitive inhibitor. This situation is comparable to a lock jammed by a key almost similar to the original one.

fig. 14.16 - Competitive Inhibition

Noncompetitive Inhibition

Chemical substances such as cyanides or phosphide can inhibit the action of repiratory enzymes. They are not similar to the substrate molecule and as such do not compete with it. However, such inhibitor may become attached any site on the enzyme, other than the substrate binding site.

fig. 14.17 - Noncompetitive Inhibition

Allosteric or Feed Back Inhibition

The activity of some enzymes, particularly those involved in metabolic pathways, are controlled by a self-regulating mechanism. Some specific substance, most often the product itself, acts as an inhibitor. Such an inhibitor binds to an enzyme at a specific site and modifies the active site of the enzyme. This prevents the binding of substrate molecule. Such sites on the enzymes are called allosteric sites and such enzymes are called allosteric enzymes.

fig. 14.18 - Allosteric Inhibition

Enzyme Nomenclature

The different kinds of enzymes are named in different ways.

• Most often enzymes are named by adding a suffix 'ase' to the root word of the substrate. For example, Lipase (fat hydrolysing enzyme), Sucrase (breaking down sucrose).

• Sometimes the enzymes are named on the basis of the reaction that they catalyse. For example, Polymerase (aids in polymerisation), Dehydrogenase (removal of H atoms).

• Some enzymes have been named based on the source from which they were first identified. For example, Papayin from papaya.

• The names of some enzymes ends with an 'in' indicating that they are basically proteins.For example, Pepsin, Trypsin etc.