Enzyme: Induction and Repression and Allosteric Regulation

Enzyme induction and enzyme repression are important regulatory mechanisms by which the cells control the synthesis of enzymes according to their metabolic needs or requirement.

These mechanisms operate mainly at the genetic level by regulating the expression of genes responsible for enzyme production.

These genes (which are responsible for enzyme synthesis) were activated or supressed by some factors, due to which the cells synthesize the enzyme at the time of need or requirement (to perform the metabolize of any substrate) or supresses the enzyme synthesis (when there is no need to perform metabolic activity).

Enzyme suppression is the process which inhibit the genes which is responsible for enzyme synthesis. when the enzymes are not required or when there is no need to perform any metabolic reaction some factors, supress the genes and inhibit enzyme synthesis.

Enzyme Induction:

In enzyme induction, the presence of a specific substrate or inducer activates the gene that codes for a particular enzyme.

Normally, a regulatory protein called a repressor remains attached to a specific region of DNA and blocks transcription of the gene. When the inducer molecule enters the cell, it binds to the repressor and changes its shape, causing it to detach from DNA. This allows RNA polymerase to transcribe the gene into mRNA, which is then translated into the required enzyme. As a result, enzyme synthesis increases in the presence of the substrate.

Example: This can be explained by lac operon model. When lactose enters the cell, it acts as an inducer by binding to the repressor protein and changing its shape. Due to this change, the repressor detaches from the DNA, allowing RNA polymerase to transcribe the genes into mRNA.

The mRNA is then translated into enzymes such as β-galactosidase, which help in lactose metabolism. Thus, the presence of lactose induces the synthesis of enzymes required for its utilization.

Enzyme Repression:

In enzyme repression, the process is reversed. When the end product of a metabolic pathway accumulates in the cell in more quantity, it acts as a corepressor. This corepressor binds to an inactive repressor protein and activate it.

The activated repressor then binds to the operator region of DNA and blocks transcription of the enzyme-coding genes. Due to this, mRNA is no longer produced, enzyme synthesis decreases or stops. In this way, the cell prevents unnecessary enzyme production when no metabolic process is required.

Example: this can be explained by tryptophane operon model. When tryptophan accumulates in excess, it acts as a corepressor by binding to the inactive repressor protein and activating it.

The activated repressor then binds to the operator region of DNA and blocks the action of RNA polymerase. As a result, transcription stop and mRNA is not formed, and enzyme synthesis supressed.

In this way, the cell avoids unnecessary production of enzymes when sufficient tryptophan is already available and there is no need to synthesize tryptophane.

Allosteric Regulation of Enzymes

Allosteric regulation is an mechanism by which the activity of enzymes is controlled or regulated inside the cell. The word allosteric is derived from two Greek words: “allo” meaning other and “stereos” meaning space or site. Therefore, an allosteric enzyme contains a regulatory site that is different from the active site.

In this type of regulation, a small molecule known as an allosteric effector or modulator binds to a specific region of the enzyme called the allosteric site. This binding causes a change in the three-dimensional shape (conformation) of the enzyme. As a result, the activity of the enzyme either increases or decreases.

Types of Allosteric Regulation

Allosteric regulation is mainly of two types:

1. Positive Allosteric Regulation (Activation)

In positive regulation, an activator molecule binds to the allosteric site and changes the enzyme into a more active form. This increases the affinity of the enzyme for its substrate, so the reaction proceeds faster.

Example: Binding of ATP to certain metabolic enzymes can enhance their activity when energy is needed rapidly.

2. Negative Allosteric Regulation (Inhibition)

In negative regulation, an inhibitor molecule binds to the allosteric site and changes the shape of the enzyme in such a way that substrate binding becomes difficult. Due to this, enzyme activity decreases.

Example: In many biosynthetic pathways, the final product when not required or formed in much more quantity, then it binds with enzyme regulatory site and inhibits the enzyme activity. This process is called feedback inhibition.