Hormones classification

Hormones classification, Hormones are chemical messengers produced by glands in the endocrine system and are classified based on their chemical structure and function.

Hormones classification, (Peptide Hormones)

Peptide hormones vary in size, from 3 to more than 200 amino acid residues. They include the pancreatic hormones insulin, glucagon, and somatostatin; the parathyroid hormone calcitonin and all the hormones of the hypothalamus and pituitary glands. These hormones are synthesized on ribosomes in the form of longer, precursor proteins, then packed into secretory vesicles and proteolytically cleaved to form the active peptides. In many peptide hormones, the terminal residues are modified as in TRH.

INSULIN: 

Insulin is a small protein (Mr 5,800) with two polypeptide chains, A and B, joined by two disulfide bonds. It is synthesized in the pancreas as an inactive signal chain precursor, preproinsulin with an amino-terminal “signal sequence” that directs its passage into a secretory vesicle. Proteolytic removal of the signal sequence and forming three disulfide bonds produces proinsulin, Which is stored in secretory granules in pancreatic beta cells.

When blood glucose is elevated sufficiently to trigger insulin by specific proteases, which cleave two peptide bonds to form the mature insulin molecule and C peptide, released into the blood by exocytosis, all three fragments have physiological effects insulin stimulates glucose uptake and fat synthesis, and C peptide acts through G protein-coupled receptors in various tissues to migrate effects of reduced insulin synthesis, such as diabetic nerve pain. There are other cases in which prohormone protein undergoes specific cleavage to produce several active hormones.

Concentration of Peptide Hormones:

The concentration of peptide hormone in secretory granules is so high that vesicle contents are virtually crystalline, when the contents are released by exocytosis, a large amount of hormones is released suddenly. The capillaries that serve peptide-producing endocrine glands are fenestrated so the hormone molecules readily enter the bloodstream for transport to target cells elsewhere. All peptide hormones act by binding to receptors in the plasma membrane. They cause the generation of a second messenger in the cytosol, which changes the activity of intracellular enzymes, thereby altering the cell’s metabolisms.

2- Catecholamine Hormones:

The water-soluble compounds epinephrine ( adrenaline ) and norepinephrine noradrenaline ) are catecholamines, named for the structurally related compound catechol. They are synthesized from tyrosine.

Production of  Catecholamine Hormones:

Catecholamines are produced in the brain and other neural tissues and function as neurotransmitters, but epinephrine and norepinephrine are also hormones, Catecholamines are highly concentrated in secretory granules and released by exocytosis and they act through surface receptors to generate intracellular second messengers. They mediate a wide variety of physiological responses to acute stress.

3- Eicosanoid Hormones:

The Eicosanoid Hormones ( prostaglandins, thromboxane’s, leukotrienes, and lipoxins ) are derived from the 20-carbon polyunsaturated fatty acids arachidonate and eicosapentaenoic acid.

Pathway of Eicosanoid Hormones:

Unlike the described, they are non-synthesized in advance and stored they are produced when needed. The enzymes of the pathways leading to prostaglandins and thromboxane are very widely distributed in mammalian tissues, most cells can produce these hormone signals, and the cells of many tissues can respond to them through specific plasma membrane receptors. The eicosanoid hormones are paracrine hormones, secreted into the intestinal fluid and acting on nearby cells.

Role of Prostaglandin in Eicosanoid Hormones:    

• Some prostaglandins promote the concentration of smooth muscle, including that of the intestine and uterus. They also mediate pain and inflammation in some tissues. Many anti-inflammatory drugs act by inhibiting steps in prostaglandin synthetic pathways.

Role of Thromboxanes in Eicosanoid Hormones:

• Thromboxane regulates platelet function and therefore blood clotting. Leukotrienes LTC4 act through plasma membrane receptors to stimulate the contraction of smooth muscle in the intestine, pulmonary airways, and trachea. They are mediators of anaphylaxis and an immune overresponse that can include airway constriction, altered heartbeat, shock, and sometimes death. Lipoxins are short-lived eicosanoid derivatives with potent effects or immune function, they appear in the bloodstream as inflammation ends.

4- Steroid Hormones:

The steroid hormones are corticosteroid hormones and sex hormones are synthesized from cholesterol in several endocrine tissues. They travel to their target cells through the bloodstream, bound to carrier proteins. More than 50 corticosteroid hormones are produced in the adrenal cortex by reactions that remove the side chain from the D ring of cholesterol and introduce oxygen to keto and hydroxyl groups.

Types of Sex Hormones:

• Androgen ( including testosterone )
• Estrogen ( including estradiol )

They are synthesized in the testes and ovaries. They affect sexual development, sexual behavior, and a variety of other reproductive and nonreproductive functions. Their synthesis also requires cytochrome P-450 enzymes that cleave the side chain of cholesterol and introduce oxygen atoms.

All steroid hormones act through nuclear receptors to change the level of expression of specific genes. They can also have more rapid effects, mediated by receptors in the plasma membrane. Humans and other animals are exposed to many exogenous chemicals broadly referred to as ” endocrine receptors” ranging from environmental pollutants such as PCBs ( polychlorinated biphenyls ), pesticides, and pharmaceuticals to naturally occurring estrogens in plants, such as on soy products.

5- Vitamin D Hormones:

( 1 alpha,25- dihydroxy calcitriol )Iss produced from vitamin Denzyme-catalyzed hydroxylation in the liver and kidneys. Vitamins D is obtained in the diet or by photolysis of 7-dehydro-cholesteroll in skin exposed to sunlight.

Pathway of Vitamin D Hormones:

Calcitriol works in concerns with parathyroid hormone in Ca homeostasis, regulating the blood and the balance between Ca deposition and Ca mobilization from bone. Acting through nuclear receptors, calcitriol activates the synthesis of an intestinal Ca-binding protein essential for the uptake of dietary Ca. Inadequate dietary vitamin D or defects in the biosynthesis of calcitriol result in serious diseases such as rickets, in which bones are weak and malformed.

6- Retinoid Hormones:

The retinoid hormones are potent hormones that regulate the growth, survival, and differentiation of cells via nuclear retinoid receptors. The prohormone retinol is synthesized from beta-carotene, primarily in the liver, and many tissues convert retinol to the hormone retinoic acid. RA binds its specific receptors in the nucleus, forms a dimer with another nuclear protein, retinoid X receptor, and alters the rate of expression of genes responsive to RA. Pathway of Retinoid Hormones:

All tissues are retinoid targets, as all cell types have at least one form of nuclear retinoid receptors. In adults, the most significant targets include the cornea, skin, epithelia of the lungs and trachea, and the immune system, all of which undergo constant replacement of cells. RA regulates the synthesis of proteins essential for growth or differentiation. Excessive vitamin A can cause birth defects, and pregnant women are advised not to use retinoid creams that have been developed for the treatment of severe acne.

7- Thyroid Hormones: 

The thyroid hormones T4 ( Thyroxine ) and T3 ( triiodothyronine ) are synthesized from the precursor protein thyroglobulin are enzymatically iodinated in the thyroid gland, then two iodothyronine residues condense from the precursor to thyroxin. When needed, thyroxin is released by proteolysis. Condensation of monoiodotyrosine with diiodothyronine produced T3, which is also an active hormone released by proteolysis.

Pathway of Thyroid Hormones:

The thyroid hormones act through nuclear receptors to stimulate energy-yielding metabolism, especially in the liver, and muscle, by increasing the expression of genes encoding key catabolic enzymes. Underproduction of thyroxin slows metabolism and can be the cause of depression. When underproduction is the result of too little iodine in the diet, the thyroid gland enlarges in a futile attempt to produce more thyroxine.

What is Goiter, and its function?

This condition, called goiter, was once common in regions far from oceans ( which provide iodine in the form of fresh seafood ) and areas with low iodine soil ( yielding plants with low iodine ). Goiter has been almost eliminated in areas where iodine is routinely added to table salt.

8- Nitric Oxide Hormone:

Nitric oxide is a relatively stable free radical synthesized from molecular oxygen and the guanidinium nitrogen of arginine in a reaction catalyzed by NO synthase. This enzyme is found in many tissues and cell types: neurons, macrophages, hepatocytes, myocytes of smooth muscle, endothelial cells of the blood vessels, and epithelial cells of the kidney.

NO acts near its point of release, entering the target cell and activating the cytosolic enzyme guanylyl cyclase, which catalyzes the formation of the second messenger cGMP. A cGMP-dependent protein kinase mediates the effects of NO by phosphorylation key proteins, and altering their activities.

Example of Nitric Oxide: 

Phosphorylation of contractile proteins in the smooth muscle surrounding blood vessels relaxes the muscle, thereby lowering blood pressure.