Principles of Biochemistry (2+1)

9.2.1 Functioning of Hormones

Hormone action begins with the binding of the hormone to a receptor on (or in) a target cell. Binding of any hormone induces a conformational change in its receptor, and this change is detected by other macromolecules. Hence the hormone-receptor interaction can be transduced from one molecule to another in the following ways.

a. Action on enzymes

A hormone receptor may be a rate-limiting enzyme or be coupled to a rate-limiting enzyme.

b. Action on receptors

A hormone receptor regulate adenylate cyclase inter post by a another protein which functions between there receptor and the adenylate cyclase.

c. Action of acceptors

The protein directly activated by hormone -receptor interaction is sometimes referred to as an acceptor protein. GTP-binding proteins (G-Proteins) are acceptors for all receptors that activate or inhibit adenylate cyclase.

Mechanism of action

First step

The first step in the action of a hormone is its binding to a specific molecule or set of molecules, called the hormone receptor, which is located on the cell surface or in the cytosol of the target cell. Such a receptor has a very high specificity and affinity for its hormones molecule.

The receptors for the water soluble peptide and amine hormones are located on the outer surface of the target cells. The receptors for the lipid soluble steroid hormones, which readily pass through the plasma membrane of their target cells, are specific proteins located in the cell cytosol.

Second step

Once the hormones receptor on or in the target cell is occupied by the hormone molecule, the receptor undergoes a characteristic change which forms or sets free an intracellular messenger molecule, often called the second messenger. This messenger conveys the signal from the hormones receptor to some enzyme or molecular system in the cell that carries out the instructions brought by the hormone.

Third step

The intracellular messenger either regulates a specific enzymatic reaction or causes an inactive gene or set of genes to be expressed.

There are several different secondary messenger systems but they all are quite similar in overall mechanism, though the substances involved in those mechanisms and effects are different.