Classification of Hormones, Mechanism of Hormone Action

Classification of Hormones

Below you will find a summary of the five major classifications of hormones. Among them are:

• By chemical nature
• According to the Mechanism of Action
• Based on the nature of the action
• By effect
• Based on the stimulation of endocrine glands

By chemical nature

Steroid hormones

These hormones are primarily derived from cholesterol, i.e., Testosterone, Oestrogen, Progesterone, and so on.

Amine hormones

Amines are the building blocks of hormones. Tyrosine is the amino acid that is derived from amin­e hormones. Example – norepinephrine, epinephrine, T3, T4, etc.

Peptide hormone

A few amino acid residues make up these hormones, and they are simple linear chains. Example - Neither oxytocin nor vasopressin contains more than 9 amino acids.

Protein hormone

When these hormones are produced, a larger number of amino acids are also formed. These are characterized by their primary, secondary, and tertiary structures. Examples – STH, Glucagon, insulin, etc.

Glycoprotein hormones

Glycoproteins make up these hormones. It is a conjugated protein with mannose, galactose, or fucose as one of the carbohydrate groups. Examples – FSH, TSH, LH, etc.

Eicosanoid hormone

A variety of arachidonic acids make up eicosanoids, which are small fatty acid derivatives. Examples – prostaglandins.

According to the mechanism of action

Group I hormones

Hormone receptor complexes (HRCs) form intracellular receptors by which hormones exert their biochemical effects. Hormones like T3, T4, and T5 are lipophilic and made up of cholesterol (except T3). Circulation occurs in association with transport proteins, and their half-lives are fairly long (hours or days). Examples – testosterone, progesterone, T3, T4, estrogen, etc.

Group II hormones

A hormone's role in the body is to bind to specific cell surface (plasma membrane) receptors and stimulate the release of specific molecules, such as second messengers, which then perform biochemical functions. Lipophobic in nature, hormones are usually transported free and are short-lived (in minutes).

Based on the chemical nature of second messengers, Group II hormones can be divided into three groups:

• cAMP is a second messenger. Examples – LH, ACTH, FSH, etc.
• Second, phospholipid/inositol/Ca++ are messengers. Examples – GnRH, Gastrin, TRH, etc.
• No information is available regarding the second messenger. Examples – insulin, LTH, Oxytocin, STH, etc.

Based on the nature of the action

Local hormones

The paracrine secretion of these hormones gives them specific local effects. Examples – testosterone

General hormones

Hormones are transported distally by circulation to target organs and tissues. Examples – thyroid hormone, insulin, etc

By effect

Kinetic hormone

Color migration, muscle contraction, glandular secretion, etc. may be caused by these hormones. Examples – MSH, epinephrine, pineal in, etc.

Metabolic hormones

This hormone functions to regulate the rate of metabolism and balance the reaction in the body. Examples – glucagon, PTH, insulin, etc.

Morphogenetic hormones

The hormones responsible for growth and differentiation play a role in these processes. Examples – FSH, STH, thyroid hormones, LTH, etc.

By stimulating endocrine glands

Tropic hormones

A large number of endocrine glands secrete hormones in response to these hormones. Example - The thyroid gland produces TSH in response to pituitary stimulation.

Non - tropic hormones

Other tissues outside the endocrine system can also be affected by hormones. Example - Thyroid hormone increases the metabolic rate of nearly all cells, which increases O2 consumption.

Mechanism of hormone action

The release of hormones into the bloodstream is the pathway by which they are carried to their target sites. Depending on whether the hormone is lipid-soluble (permeable to the plasma membrane) or water-soluble (binds to a cell-surface receptor), the target cell will be triggered. In target cells, liposoluble hormones bind to receptor proteins on plasma membranes. The plasma membrane of a cell contains proteins that bind to water-soluble hormones. When a receptor is stimulated, a change in activity occurs, which may trigger a feedback loop with the original hormone-producing cells.

Water-soluble hormone - A lipophobic hormone binds to a receptor on, or within, its plasma membrane to initiate an intracellular signaling cascade.

Hormone - An enzyme that is released by a cell and sends out a chemical message to its counterparts in other parts of the body.

Lipid soluble hormone - Lipophilic hormone that passes through cell membranes, binds to intracellular receptors and controls the expression of genes.

Communication between cells is enabled by hormones, which are chemical messengers. A hormone is a protein that is secreted by glands of the endocrine system that regulates many other processes and systems, including reproduction and development.

Hormone signaling

Hormones are secreted directly into the extracellular space by glands of the endocrine system. The circulatory system then transports hormones to target cells after diffusing them into the bloodstream through the capillaries. Hormones can therefore have effects on tissues and organs far from their sites of production, or they can have systemic effects on the entire body. In the thyroid gland, for example, hormone-producing cells called thyrocytes reside within a specific endocrine gland. Hormones leave their cells of origin using exocytosis or other membrane transport mechanisms. Cells of different types are found within several different tissues and may receive various hormonal signals. Diabetes, for example, involves a variety of physiological effects across the entire body due to insulin. The response of different tissues to hormones is different.

Therefore, hormonal signaling is complicated to dissect. Hormones exert their effects either by interacting with specific receptor proteins within a target cell's cytoplasm or by binding specific receptor proteins within the cell's membrane. As a result of both hormone complexes, the nucleus of the cell will be activated in a chain of events that actively regulate gene expression. The original hormone-producing cells may recognize the reaction of the target cells and downregulate hormone production. A negative feedback loop is a homeostatic process.

Activation of nuclear hormone receptors by lipid-soluble hormones: nuclear hormone receptors within the cell are activated by lipid-soluble hormones. The plasma membrane can be crossed by lipid-soluble hormones.

Steps of hormone signaling

• An organ or tissue synthesizes a particular hormone.
• Storage of the hormone and its secretion.
• The movement of the hormone from the tissue or cell to the target cell, tissue, or organ.
• An intracellular receptor protein or a cell membrane recognizes the hormone.
• The process through which hormonal signals are relayed and amplified.
• Possible feedback to the hormonal production.

Hormone classes

Hormones fall into three categories:

• Peptide - A peptide is a modified amino acid or long (protein) or short (peptide) chain of amino acids. In addition, they may contain carbohydrates.
• Lipid - -Lipids: Steroid hormones containing lipids synthesized from cholesterol and eicosanoids containing lipids formed from the fatty acid chains of plasma membrane phospholipids.
• Monoamine: A hormone formed by aromatic amino acids like the amino acids tyrosine, phenylalanine, and tryptophan.

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