Enzymes and Regulation | Enzymology

What are enzymes?

Enzymes are protein molecule that facilitates a specific reaction towards equilibrium and acts as biocatalyst (catalysts accelerates chemical reaction) that participate in almost all the metabolic processes occurring in the body.

In the body, several biochemical changes occur in many steps, each step is catalyzed by a specific enzyme. In living beings, all biochemical activities require specific enzymes. The activity of enzymes may vary from one cell to another.

Classification of enzymes

Classification and different parts of the enzyme

Types of enzyme

Simple enzyme

They are made up of protein only like Pepsin, Trypsin, …

Holoenzyme or conjugated enzyme

The entire activated complex of the apoenzyme plus the co-factor is called the holoenzyme.

Apoenzyme + Co-factor = Holoenzyme

Apoenzyme (Protein part)

Many enzymes require a co-factor in order to function. The protein portion of the enzyme is called apoenzyme.


Enzymes are composed of one or several polypeptide chains. However, there are a number of cases in which non-protein constituents called cofactors are bound to the enzyme to make the enzyme catalytically active. In these instances, the protein portion of the enzymes is called the apoenzyme and the non-protein portion is called the cofactor.

Further Reading:  Principles of Inheritance and Variation

Three kinds of cofactors may be identified: Co-enzymes, Prosthetic groups, and Metal ions.


It is an organic compound that binds to the active sites of certain enzymes to assist in the catalysis of a reaction. When a cofactor is loosely attached to the protein and can be easily separated from it, it is called a coenzyme.

They are loosely bound to the apoenzyme. Coenzymes can function as an intermediate carrier of electrons during these reactions or be transferred between enzymes as functional groups. Their association with apoenzyme is only transient (usually occurring during the course of catalysis). Most of them are vitamin derivatives.

Prosthetic group

Co-factors are usually either nonprotein organic compounds or metal ions. A cofactor that is firmly attached to the apoenzymes (protein portion) is called a prosthetic group.

Metal activators

They form coordinate bonds with side chains at the active site and at the same time form one or more coordinate bond bonds with the substrate.


The molecules that are bound to and acted on by enzymes are termed substrates.

Classification of enzymes based on the occurrence

Isoenzymes or Isozymes

An enzyme may exist in two or more different molecular forms. Different species of the same enzyme are called isoenzymes.

Endoenzymes or Intracellular enzyme

These are the enzymes that act within the cell in which they are produced and are called endoenzymes.

Exoenzymes or Extracellular enzyme

These are the enzymes that act outside the cell from which they are produced and are called exoenzymes.

Chemical nature of enzyme

All enzymes isolated or purified so far are proteins in characters. Proteins are synthesized in the cell from 20 different kinds of amino acids. Each amino acid molecule has a carboxyl group (-COOH) and an amino group (-NH2). During protein synthesis, the carboxyl group of one amino acid links with the amino group of other amino acids thus forming peptide linkage.

Further Reading:  Principles of Inheritance and Variation

In protein 200 to 300 such linked peptide linkages are present. The arrangement of amino acids in protein molecules can be co-related with the arrangement of nucleotides (units of nucleic acid) in DNA molecules.

In certain cases, enzymes require other substances like calcium and magnesium ions whose presence in small amounts is responsible for the action of a given enzyme. In such a case, the term coenzymes are applied.

A complete enzyme, commonly known as holoenzyme, consists of coenzymes, plus protein apoenzymes. When cells secrete enzymes, these are often first produced in an inactive form of enzymes that are said to be proenzymes.

Mode of action of the enzyme

During biochemical reactions in the body, the enzymes first combine with the substrate to form an intermediate complex before yielding the products of the reactions.

In the process, the substrate molecules are thought to fit in substrate molecules ate thought to fit into the active sites located on the surface of the enzyme molecule just as one particular kind of key fits into one particular kind of lock. This results in the rapid formation named by adding the suffix “ase” to the name of the substrate on which they act. Some of them are:

Proteinases: Helps in the breakdown of protein into peptides and amino acids.

Amylases: Helps in the breakdown of starch into constituent sugars.

Lipases: Helps in the breakdown of fats into fatty acids.

Nucleases: This type of enzyme help in the breakdown of nucleic acids into nucleosides and nucleotides.

Factors affecting enzymatic activities

  1. Temperature: Temperature affects enzyme-catalyzed reactions in the same way that it affects ordinary chemical reactions. As the temperature rises, the rate of chemical reaction increases owing to an increase in the number of activated molecules. But when the temperature rises above a certain limit, the enzyme loses its activity as proteins; enzymes may be completely denatured at high temperatures. Therefore, for every enzyme under a given set of conditions, there is a temperature at which the activity of the enzyme is at a maximum. This is known as optimum temperature.
  2. Effect of pH: Each enzyme has its optimum pH at which the most rapid activity occurs. Extreme acidity or alkalinity usually causes irreversible destruction of the enzyme. Pepsin acts in the acidic medium of gastric juice and has an optimum pH of 2-0.
  3. Enzyme concentration: The concentration of enzyme is directly proportional to the chemical reaction up to a certain extent. An increase in the enzyme concentration would increase the rate of reaction.
  4. Substrate concentration: The concentration of substrate also shows the progressive activities up to a certain extent. An increase in the substrate concentration will increase the chance of a substrate molecule coming in contact with an active site.
  5. Product concentration: The increased concentration of product falls the rate of chemical reactions as there is a gradual decrease in the concentration of substrate.
  6. Other factors:
    1. Strong light inactivates the enzymes.
    2. Ultraviolet (UV) light is effective in destroying enzyme activity.
    3. The presence of salts may influence enzymatic activity by participating with the enzyme due to its proteinaceous nature.

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