Active Site of an Enzyme

introduction image

The active site of an enzyme is the site which shows the highest metabolic activity by catalysing the enzyme-substrate complex into the products. The active site is found deep inside the enzyme which resembles a hole or small depression. An active site is a region which attaches the substrate molecule with the enzyme and thus catalysing the reaction.

As we know the enzyme is “Highly specific” molecule, but its specificity is due to the active site which allows the binding of a particular substrate. The amino acids residues are present around the active site which holds the substrate molecule at the right position while the reaction takes place. The substrate molecule shows high binding affinity towards the active site.

Content: Active Site of an Enzyme

  1. Definition of Active Site
  2. Reaction Mechanism of an Enzyme
  3. Characteristics
  4. Properties
  5. Role of Active Site

Definition of Active Site

The active site can define as the small region which appears like a cleft or cavity which is composed of about 10-15 amino acid residues. As the term defines, it is a site which activates the complex enzyme to bind with the particular substrate and induces the transition state of the substrate and stabilize the product formation. The active site also refers as Catalytic site.

The active site performs two functional activities:

  1. Binding activity
  2. Catalytic activity

functional activities of active siteBinding Activity: The binding activity is a property of active site which increases the binding affinity of the substrate with an enzyme.

Catalytic activity: It is a property of an active site which carries out the catabolic reaction where the enzyme and substrate react to form a product by reducing the activation energy.

Reaction Mechanism of an Enzyme

In an enzyme-catalyzed reaction, the substrate will attach to the enzyme’s active site. A specific substrate will bind to the active site of an enzyme. After the attachment of substrate and enzyme, an “Enzyme-substrate complex” forms. In the E-S complex, the substrate on enzyme activity will convert into a product. At last, the product gets released and the enzyme becomes free to reuse again.
enzyme catalyzed reaction

Example

Let us take an example, where sucrose is a substrate which combines with the active site of an enzyme “Sucrase”. After the binding of sucrose with an enzyme Sucrase, an E-S complex forms. Then a reaction between Sucrase and Sucrose takes place.  The reaction will change the structural conformation of the Sucrose refers to as “Transition state of the Sucrose”. The change in the structural configuration of a Sucrose leads to the conversion of the E-S complex into the E-P complex. At last, glucose and fructose released as products form the Sucrase enzyme.

Characteristics

  • An active site is a specific location found in the enzyme where a substrate binds to catalyze the reaction. It refers as “Enzyme catalytic surface”.
  • About 10-15 amino acid residues combine to form an active site.

active site of an enzyme

  • The active site possesses a specific geometrical shape and chemical signals which allow the enzyme for the recognition and binding of the specific substrate.
  • An active site will allow the specific substrate to bind whose shape is complementary to the active site. Therefore, a substrate is like is a key which can only fit into the particular lock i.e. active site.
  • The active site of an enzyme catalyzes many chemical or biological pathways.
  • After the formation of the enzyme-substrate complex, both substrate and active site change its structural configuration by bending the target bonds and breaking the substrate molecule into a product.
  • Enzymes show “Catalytic activity” which is due to its active site. It catalyzes a substrate into a product after complementary binding of the substrate with the active site of an enzyme based on geometric shape, size, charge and stereospecificity etc.
  • The active site of an enzyme induces the “Transition of the substrate”.

Properties

There are following characteristics of an active site which includes:

characteristics of active site of an enzyme

Hydrophobicity

The initial binding of substrate and enzyme is through the non-covalent bond. But the catalytic site involves hydrophobic interaction for the attachment of the substrate with the enzyme. Hydrophobic binding of the substrate to the active site of an enzyme increases the binding affinity. Other than hydrophobic interaction, there are three other mechanisms also like Vander Waal, hydrogen bond and electrostatic force of interaction which promotes the formation of E-S complex.

Flexibility

An active site shows flexibility as it can change its conformation which catalyzes the conversion of the substrate into a product.

Reactivity

The active site of an enzyme reacts with the specific substrate. Its reactivity depends upon the environmental conditions like temperature, pH, the concentration of enzyme and substrate etc. The active site of an enzyme combines with the substrate and lowers the activation energy to catalyze the reaction.

Net charge

The active site mainly consists of non-polar amino acid residues which carry no charge or having a 0 net charge. Some active site also consists of polar amino acids which can carry both positive and negative charge. The net charge of the catalytic site decides which amino acid will bind with the enzyme. There must be a complementary pairing between the active site and the substrate. The same charge on both the catalytic site and substrate will not form an E-S complex as there will occur repulsion between the two.

Role of Active Site

As we have discussed the active site performs two major activities like:

  • The binding of a substrate with an enzyme.
  • The catalytic activity by conversion of substrate into product.

Let us assume two conditions, One is the conversion of substrate into a product without enzyme and second in the presence of an enzyme.

First condition

transtion reaction without enzyme

In the first condition, we will discuss the transition reaction of the substrate into a product in the absence of an enzyme catalyst. For this, plot a graph between Reaction direction and Energy. In the absence of a “Catalyst”, a substrate (S) will require higher “Activation energy” to go into the transition state which we will represent as “St”. In the transition state, the substrate will change its conformation and thereby release a product (P).

Second condition

transtion reaction with enzyme

Here, we will discuss the transition reaction of the substrate into a product in the presence of an enzyme catalyst. For this also, plot a graph between Reaction direction and Energy. In the presence of a “Catalyst”, a substrate (S) will bind to the catalytic site of an enzyme. The enzyme being a catalytic agent undergo catalysis of a substrate.  The enzyme will modify the substrate and takes it to the transition state which we will represent as “ESt”. In the transition state the enzyme and substrate will react and there a change occurs in the configuration of the substrate. This change will lead to the formation of the enzyme product complex and finally release a product (P).

Therefore, we can conclude the active site of an enzyme lowers the activation energy by increasing the rate of reaction. The energy level of the substrate is higher than that of a product but lower than the transition state of the substrate. The activation energy is inversely proportional to the rate of reaction thus it decreases with the increase in the rate of reaction.

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