Enzymes are biological catalysts. Their activity is measured by the amount of substrate decomposed or the amount of product formed during a reaction.

What are enzymes?

Enzymes are biologically active proteins. They are composed of thousands of amino acids. The substrate is a molecule on which an enzyme works. An enzyme combines with substrate and forms an enzyme-substrate complex. When the reaction progresses, this complex breaks into enzymes and products.

E + S → ES → E + P

Enzyme Action

Structure of enzyme

Enzyme structures have a key role in their activity. Enzymes are three-dimensional globular proteins. The three-dimensional structure of an enzyme depends upon the kind and sequence of amino acids they are made up of.

Primary structure

The long linear amide linkages chain is known as the primary structure of enzymes.

Secondary structure

There are two types of secondary structures:

  • Helical

Helical structures are formed by the H-bonds within the polypeptide bond.

  • Beta-pleated sheets

Beta pleated sheets are formed by the H-bond between two adjacent polypeptide chains.

Tertiary structure

The tertiary structure of an enzyme is the arrangement of amino acids in three-dimensional space.

Active site of enzymes

There is a specific site in the enzymes where the substrate binds, known as the active site. This is a pocket-like structure often called the receptor site of an enzyme. Specifically, the active site is a small portion of a large enzyme. This portion drives the whole mechanism of enzyme action.

The active site is also known as the binding or the catalyzing site.

Characteristics of enzymes

Enzymes have the following characteristics:

  • Enzymes are proteins.
  • They only catalyze the reaction. They do not affect the products.
  • They are required in very small amounts compared to the substrate.
  • Enzyme activity is sensitive to temperature, pH, etc.
  • Some enzymes may require cofactors for proper functioning.

Mechanism of action of an enzyme

Lock and key model

Hermann Emile Fischer (1852-1919) proposed a lock and key model in 1894. This model states:

“Enzymes are very specific in their action. They react only with the specific substrate and change it into products. The shape and size of active sites of enzymes are different from each other. However, it is similar to the active site of the substrate. After completion of the reaction, enzymes are regenerated. Therefore, they can be used again and again”.

Lock and Key Model of enzyme action

According to this model, the shape of the active site is fixed. It cannot be modified. So, only a specific substrate can bind with the active site.

Not all types of reactions follow the lock and key model.

Induced fit model

This model is proposed by Daniel Edward Koshland Jr. (March 30, 1920 – July 23, 2007) in 1959.

According to this model:

“Active sites of enzymes are not fixed. They can be modified”

It enables enzymes to perform their activity more efficiently.

Induced fit model of enzyme action

Some of the factors which affect the enzyme activity are:

  • Temperature
  • pH
  • Concentration of enzyme
  • Concentration of substrate

Factors affecting the rate of enzyme activity

Enzymes are very specific. These are the factors that affect their activity:


With an increase in temperature the kinetic energy of molecules increases. So, the fast-moving enzyme and substrate molecules result in more collisions. This eventually leads to an increase in the rate of the reaction, according to the collision theory.

All enzymes work at a narrow specific temperature which is known as “optimum temperature”.

If the temperature is increased or decreased from the optimum temperature, the rate of reaction decreases. For example, if the temperature is increased the active sites of enzymes are damaged. These enzymes are said to be denatured and the structure of the enzyme becomes disrupted.

  • The optimum temperature of the human body enzymes is 37⁰ C (98.6⁰ Fahrenheit). If we increase the temperature to 40⁰ C (104⁰ Fahrenheit) the enzymes may get denatured. This means the tertiary and secondary structures of enzymes will be disrupted.
  • The optimum temperature reported for the growth of saccharomyces cerevisiae bacteria is 32.3⁰  C.

If enzymes are inactivated but not denatured, they can regain their catalytic activity when the temperature gets back to normal.

Temperature affects on enzyme activity


Enzymes work over a very narrow range of pH. The pH at which the maximum rate of enzymatic reaction occurs is known as optimum pH. It is between 6.00 to 8.00 for most of the enzymes.

For example

Human blood has an optimum pH range of 7.35 to 7.45.

If pH changes, an ionic distribution of acidic and basic groups over the molecule also changes. It leads to a change in the shape of the enzymes, which affects the enzyme’s activity.


There are some exceptions as some enzymes’ optimum pH does not lie in this range.

  • Pepsin has an optimum work in the pH range of 1 to 2.
  • Acid phosphatase works best in a pH range of 4 to 7.
  • Alkaline phosphatase has optimum activity in the pH range of 9 to 10.

rate of enzyme activity vs pH

Enzyme concentration

If we increase the concentration of enzymes while keeping the other conditions like temperature and pH constant, the rate of reaction is directly proportional to the concentration of enzyme. An increase in enzyme concentration means more available active sites, so, more substrate will combine with active sites increasing the amount of product.

Rate ∝ [Enzyme]

enzyme concentration affect on the enzyme activiy

It should be noted that the rate of reaction increases with an increase in enzyme concentration up to a certain limit. Beyond that, the enzymes are greater in number and the extra enzymes do find not any substrate to bind with. Therefore, the rate of reaction becomes constant.

Substrate concentration

When the concentration of substrate increases, the rate of reaction also increases initially. However, the further increase does not increase the rate of reaction as all the active sites of enzymes will get engaged i.e. when substrate molecules are greater in number than enzymes, there will be no availability of enzyme active sites for the substrate to combine with.

substrate concentration vs enzyme activity

Concentration of product

If there is an accumulation of products in the system, it may bring inhibition to the reaction. i.e. In such reactions, an increase in the concentration of products may decrease the rate of reaction.

Inhibitors and enzyme activity

Inhibition is referred to as the regulation of enzyme activity. Inhibitors are the substances that block the enzyme’s active sites. They do not let the enzymes transform substrates into products.

For example

Antibiotics, drugs, poisons, etc.

There are two types of inhibitions in enzymatic activity.

  • Competitive inhibition
  • Non-competitive inhibition

Competitive inhibition

It is a type of inhibition in which a structurally similar substance to the substrate acts as an inhibitor. It fits into the active sites of enzymes, occupying the active sites temporarily, and does not affect enzymes permanently. This inhibition can be controlled by increasing the concentration of the enzymes or the substrates.

Significance of competitive inhibitors

  • They follow the “lock and key” model.
  • They are used as drugs to treat bacterial infections.

For example

  • Malonate inhibits the formation of fumarate from succinate.
  • Methotrexate inhibits the regeneration of dihydrofolate from tetrahydrofolate.

competitive inhibition

Non-competitive inhibition

In this type of inhibition, inhibitors are loosely bonded to the enzymes. They change the shape of enzymes, making them unable to bind with the same substrate.


  • Heavy metals

The ions of heavy metals like mercury, silver, and copper break disulfide bonds by combining with the thiol group in the enzyme. Enzymes become denatured.

  • Cyanides

The cyanides combine with the iron of the prosthetic group and deactivate enzyme activity.

Enzyme Action

Feedback inhibition and enzyme activity

When the product formed in a reaction is greater, it can either increase or decrease the rate of reaction as a self synthesized factor in the reaction.

There are two types of feedback mechanisms.

  • Positive feedback

The products formed as a result of the reaction start catalyzing the reaction itself. This type of feedback inhibition is also known as “self catalyzed reaction”.

For example

Hemoglobin is a biological respiratory unit. It is an iron-containing protein complex. It consists of four subunits. When one of the subunits binds with oxygen during the respiration half cycle, it significantly increases the affinity of the other four subunits.

positive feedback

  • Negative feedback

The product binds with the enzyme’s active site and decreases the rate of reaction. This affects enzyme activity and is known as negative feedback inhibition. This type of reaction is also called self-destructive reaction.


  • Hexokinase is an enzyme that converts glucose-6-phosphate into fructose-6-phosphate. When glucose-6-phosphate is in abundance, it affects the activity of hexokinase.
  • Aspartate is converted into threonine by different enzymatic reactions. If threonine is in abundance it affects the activity of aspartate.

Negative feedback

Uses of enzyme

Enzymes are used in many industries. Some of the uses have been discussed here:

1. Enzymes are used in washing powder. For example, proteases remove protein-based stains.

2. They are used in the food industry. For example, carbohydrase is used in making chocolates, etc.

3. Enzymes are used in the leather industry. For example, proteolytic and lipolytic enzymes.

4. They are used in the digestion process. For example, lipase converts fats into fatty acids.

5. They are used in medicine. For example, lipases target adipose tissue.

Concepts Berg

What are the 4 factors that affect enzyme activity?

The four factors that affect enzyme activity are the following:

  • Temperature

As temperature increases, enzyme activity also increases.

  • pH

Enzymes work on a range of pH 6.00 to 8.00.

  • Substrate concentration

As substrate concentration increases, the rate of enzyme activity also increases.

  • Enzyme concentration

Enzymes concentration is directly proportional to the rate of enzyme activity.

Does size affect enzyme activity?

The rate of enzyme activity is inversely proportional to the size of the substrate. As substrate size increases, surface area decreases.

Why is it important to regulate enzyme activity?

It is important to regulate enzyme activity because:

  • They regulate metabolic pathways.
  • They provide information about the specificity of an enzyme.
  • They identify catalytic groups at the active sites.

What household products work by means of an enzyme?

  • Laundry detergents are used as household products. They contain enzymes that dissolve fats and proteins. The proteins and fats cause stains on clothes.
  • Some food like bananas, mangoes, pineapples, etc contains digestive enzymes, etc.

Why does boiling affect enzyme activity?

Boiling causes the breakage of secondary bonds, resulting in distortion of the three-dimensional structure of enzymes. This way, substrate molecules cannot bind to the enzymes. Such enzymes are said to be denatured.

What happens to enzyme activity during fever?

The optimum temperature of human body enzymes is 37⁰ C. When a person has a fever, the temperature of the body increases. This results in the denaturation of enzymes making them unable to perform their function.

Why do drugs inhibit the activity of enzymes?

Drugs are temporarily or permanently bonded to the enzymes and block their active site, in such a way that the substrate molecules will not be able to bind with the enzyme’s active site.

What are enzyme poisons?

Chemical substances that block the active sites of enzymes are known to be inhibitors. They react with the enzyme but do not transform into products. For example, cyanide, antibiotics, and some drugs.

What is the effect of coenzymes and cofactors on enzyme activity?

Coenzymes are molecules that are loosely bonded to the active sites of the enzymes.

A cofactor is a non-proteinous part of the enzymes.

These two molecules help an enzyme function properly.

What do you mean by optimal pH for an enzyme”?

The pH at which the enzyme activity is maximum is known as optimum pH. It ranges from 6.00 to 8.00 for most of the enzymes.

For example

Human blood has an optimum pH of 7.35 to 7.45.

How can allosteric regulation change the activity of an enzyme?

In allosteric regulation, the allosteric inhibitor binds to the enzymes, but not at the active site. These inhibitors inactivate the enzymes by changing their shape. This way, the substrate molecule cannot bind to the active site of the enzyme.

What is the optimum pH for most of the enzymes?

The optimum pH for most of the enzymes is 6.00 to 8.00, but some enzymes have different optimum pH ranges.


  • Enzyme activity (ncbi.nlm.nih.gov)
  • Principles of Chemical Kinetics second Edition by James E.House (Illinois State University and Illinois Wesleyan University)