Enzymes - Classification of Enzymes

Enzymes are biological catalysts used to speed up biochemical reactions in living organisms. Enzymes, unlike inorganic catalysts, are big molecules or macromolecules. Most enzymes in living organisms are protein molecules, some are RNA molecules. There are myriads of chemical reactions happening in living organisms continuously, and each reaction is catalyzed by a different enzyme. The reactants of the reaction catalyzed by an enzyme are called its substrates. Since they are big molecules, enzymes have a particular 3-dimensional structure. An enzyme has a pocket called an active site, into which its substrate fits. In the active site, the substrate reaches a suitable conformation, so its bonds are broken easily and new bonds are formed, leading to the formation of the products.

Chemical reactions

Chemical reactions involve chemical changes. A chemical change involves the breaking and making of bonds, while a physical change only involves a change of state or a change in shape or form. A chemical reaction can be represented by:

Reactants → Products

The rate of a chemical reaction is given by

Rate of a chemical reaction

Where ∆[P] is the increase/decrease in molar (moles per liter solution) concentration of the products/reactants,

            ∆t is the time taken.


If the reactions in a living organism were to take place by themselves without catalysts, the reactions would take forever to form the products. Life is possible because the biochemical reaction occurs at a reasonable rate because the rates are accelerated by the enzymes.

Classification, Nomenclature, and Examples of Enzymes

  • Oxidoreductases

These catalyze oxidation and reduction reactions,e.g. pyruvate dehydrogenase, which catalyzes the oxidation of pyruvate to acetyl coenzyme A.

  • Transferases

These catalyze the transfer of a chemical group from one compound to another. An example is a transaminase, which transfers an amine group from one molecule to another.

  • Hydrolases

They catalyze the hydrolysis of a bond. For example, the enzyme pepsin hydrolyzes peptide bonds in proteins.

  • Lyases

These catalyze breakage of bonds without catalysis, e.g. aldolase (an enzyme in glycolysis) catalyzes the splitting of fructose-1, 6-bisphosphate to glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.

  • Isomerases

They catalyze the formation of an isomer of a compound, example, phosphoglucomutase catalyzes the conversion of glucose-1-phosphate to glucose-6-phosphate (transfer of a phosphate group from one position to another in the same compound) in glycogenolysis (conversion of glycogen to glucose for quick release of energy.

  • Ligases

Ligases catalyze the joining of two molecules. For example, DNA ligase catalyzes the joining of two fragments of DNA by forming a phosphodiester bond.


Co-factors are non-proteinous substances that associate with enzymes. A cofactor is essential for the functioning of an enzyme. An enzyme without a cofactor is called an apoenzyme. An apoenzyme and its cofactor together constitute the holoenzyme.

There are three kinds of cofactors present in enzymes:

  • Prosthetic groups: These are cofactors tightly bound to an enzyme at all times. A fad is a prosthetic group present in many enzymes.


  • Coenzyme: A coenzyme is bound to an enzyme only during catalysis. At all other times, it is detached from the enzyme. NAD+ is a common coenzyme.


  • Metal ions: For the catalysis of certain enzymes, a metal ion is required at the active site to form coordinate bonds. Zn2+ is a metal ion cofactor used by a number of enzymes.

To learn more about enzymes and their action mechanism, visit Byju’s.

Practise This Question

If we assume that the 'lock and key model' of the enzyme-substrate interaction is correct, which of these conditions is absolutely true?