hormones Flashcards
hormones
Hormones are chemical messengers secreted into blood by endocrine or ductless glands.
The term hormone was introduced by Satrling in 1905 and comes from the Greek word “Hormacin” meaning “excite” or “arouse”.
Hormones are substances synthesized in the body in small quantities but have a profound biochemical effect in the control and regulation of metabolic events and contribute in some cases to intercellular and intracellular communication.
A characteristic of hormones is that they are released, usually by specialized secretory cells in endocrine glands, and are carried by the circulation, to target cells where they have an effect.
major hormone secreting glands are
The major hormone secreting glands are:
Pituitary
Thyroid & Parathyroid
Adrenal
Pancreas
Ovaries
Testes
functions of hormones
Reproduction,
Growth and development,
Maintenance of internal environment, and
Energy production, utilization and storage.
classification of hormones is based upon
Chemical Composition
Location of Receptors
Solubility Properties
based on chemical composition
Hormones can be classified chemically into three major groups.
Steroids Hormones: These hormones are fat soluble compounds like adrenal cortical hormone and sex hormones (androgens and estrogens).
Amines: these hormones are small, water-soluble compounds containing amino groups like epinephrine, thyroxine and several derivatives of tyrosine.
Peptides/Protein Hormones: these are either large protein or small or medium size peptides e.g., insulin, glucagon and pituitary hormones.
based on mechanism
Hormones that bind to Intracellular Receptors: These include androgens, calcitrol, estrogens, glucocorticoids, mineralocorticoids, progestins, reticnoic acid, and thyroid hormones (T3 and T4).
Hormones that bind to Cell Surface Receptors: These can be further classified on the basis of secondary messengers. It can be cAMP, cGMP, DAG, InsP3 and calcium ion.
on basis of solubility
On the basis of solubility, hormones can be classified as:
Lipid-Soluble Hormones: The lipid soluble hormones include steroid and thyroid hormones.
Water-Soluble Hormones: The water soluble hormones include amines, peptide hormones, and the eicosanoid hormones.
steroid
These include the sex hormones and the hormones from adrenal cortex.
These are synthesized in mammals by the ovary (or testis), adrenal cortex, corpus luteum and the placenta.
sex hormones
Sex Hormones
Three types of sex hormones are recognized :
Estrogens (female or ovarian or follicular hormones)
Androgens (male or testicular hormones)
Progestins (corpus luteal hormones).
Ovarian hormones
Mammalian ovary contains ovarian follicles and corpus lutea.
Hormones produced mainly in the follicles are known as estrogens.
Chemically, the estrogens are derivatives of a C18 hydrocarbon, estrane.
chemistry of ovarian hormones
The three compounds of this group with hormonal activity are :
β-estradiol
Estriol
Estrone
All these are characterized by the absence of a CH3 group at carbon 10 and by the aromatic nature of ring A, making the OH group phenolic in character.
Of all these, β-estradiol is most potent physiologically, estrone less potent and estriol is least active.
Their relative potencies are 50 : 5 : 1 respectively.
Although ovary is the chief source of estrogens, they are in smaller amounts also produced by the testis and the adrenal cortex.
testicular hormone
These hormones are secreted mainly by the testes, the male reproductive organs and are called as androgens.
Chemically, these are derivatives of a C19 hydrocarbon, androstane.
They have no side chain on the 17-carbon of steroid skeleton.
hormones secreted from testes
There are many hormones secreted from testes with androgenic activity.
The three important ones are :
Testosterone
Androsterone
Dehydroepiandrosterone
Testosterone is most potent of all these and dehydroepiandrosterone is least active.
The relative potency ratio of these three forms is 20 : 7 : 1.
A few testicular hormones are also produced by the adrenal gland.
corpus luteal hormones
The hormones secreted by the ovarian bed, corpus luteum are collectively called as gestagens or progestins.
The principal gestogen is progesterone.
Progesterone is a C21 steroid and is secreted by the corpus luteum during the second half of the menstrual cycle.
Chemically, progesterone is one of the pregnane derivatives.
adrenal corticol hormones
Adrenal cortex secretes some 40-50 closely related C21 steroids, collectively called as corticosteroids.
The corticosteroids may be grouped under two categories :
Mineralocorticoids. — concerned primarily with the transport of electrolytes and the distribution of water in tissues, e.g., aldosterone and deoxycorticosterone.
Glucocorticoids. — concerned primarily with the metabolism of carbohydrates, proteins and fats, e.g., cortisone, cortisol and corticosterone.
compoun of adrenal gland
The estimated 24-hour production of major compounds of human adrenal gland is :
Cortisol
8-24 mg
Corticosterone
1.5−4 mg
Aldosterone
0.04−0.2 mg
Aldosterone is 30 times more active than deoxycorticosterone.
Deoxycorticosterone, in its turn, is 4 times more potent than cortisone and cortisol in maintenance of life.
Corticosterone is least active in this regard.
biosynthesis of androgens and estrogens
The androgens are synthesized from cholesterol by the gonadal tissues, chiefly from testes.
In the testes, cholesterol is converted to pregnenolone, progesterone and 17α-hydroxyprogesterone.
17α-hydroxyprogesterone is then converted to androstenedione, which is then converted to testosterone and androsterone.
Testosterone can be converted to estradiol by the action of aromatase.
Testosterone can also be converted to dihydrotestosterone by 5α-reductase.
biosynthesis of progesterone
In the corpus luteum, placenta and adrenal cortex, progesterone is synthesized from pregnenolone by 3β-hydroxysteroid dehydrogenase.
Progesterone is an intermediate common to the biosynthesis of all steroid hormone from cholesterol.
biosynthesis of glucorticoids
Biosynthesis of Glucocorticoids
Progesterone is converted to either 17α-hydroxyprogesterone by 17α-hydroxylase or to 11-deoxycorticosterone by 21α-hydroxylase.
17α-Hydroxyprogesterone is then converted to 11-deoxycortisol by 21α-hydroxylase.
11-deoxycortisol is then converted by 11β-hydroxylase to cortisol.
11-deoxycorticosterone is converted to corticosterone by 11β-hydroxylase.
synthesis of mineralcorticoids
Synthesis of Mineralocorticoids
Corticosterone is converted to aldosterone by 18-hydroxylase.
This reaction is stimulated by angiotensin II, a hormone produced in angiotensin by angiotensin-converting enzyme.
peptide hormones
Peptide hormones are most prevalent and consist of chains of amino acids.
Larger peptides are usually referred to as proteins, while complex protein hormones with carbohydrate side chains are called glycoprotein hormones
peptide hormones are classified as
Peptide Hormones are classified as:
Hormones of the pancreas
Hormones of the Hypophysis
Hormones of the Parathyroid
Hormones of the Gastrointestinal Tract
Hormones of the Corpus Luteum
hormones of the pancreas
Each pancreas has about 1,00,000 islets of Langerhans, which are clusters of various types of cells.
These islets in mammals contain at least 4 major types of cells:
α cells
β cells
δ cells
F cells
The β cells contain granules which manufacture a hormone insulin, store it and eventually release it directly into the bloodstream at the appropriate times.
The α cells contain granules which produce another hormone, glucagon.
The δ cells secrete a hormone, somatostatin.
The F cells secrete pancreatic polypeptide.
The α and β cells make up 20% and 75% of the total, respectively.
insulin
Human insulin is a molecule of 5,734 Da which falls on the borderline between a large polypeptide and a small protein.
It is composed of two separate peptide chains, designated the A chain and the B chain.
These two chains are joined together by two disulfide bridges.
The human insulin A chain consists of 21 amino acid residues and the B chain of 30 amino acid residues.
A third disulfide exists in the A-chain between the cysteine 6 and 11.
biosynthesis of insulin
Insulin is synthesized as a pre-prohormone, that is a pre-proinsulin.
The initial portion of 23 amino acids is called the leader sequence and directs the synthesis of pro-insulin. The leader sequence is cleaved off.
The pro-insulin which is synthesized has the conformation necessary for forming the proper disulphide bridges between A- and B-chains.
It undergoes a series of site-specific cleavages forming the mature insulin and an equimolecular amount of connecting peptide (C-peptide) of 31 amino acids.
glucagon
Glucagon has a molecular weight of 3,485 Da and is comprised of a single amino acid chain of 29 amino acids that is devoid of disulfide linkages.
Histidine is the N-terminal amino acid and threonine, the C-terminal amino acid.
Glucagon’s N-terminal histidine at position #1 is essential for its biological activity.
The adjacent residues, #2 (serine) and #5 (threonine), are involved in implementing the adenylyl cyclase activity which is key to the initiation of glucagon-mediated biological responses.
Residues #19–#26 (blue color) have an equal potential to stabilize the structure either as β sheets, or as α-helices.
The C-terminal two residues, asparagine (#28) and threonine (#29), increase glucagon’s tight binding to its receptor.
Unlike insulin, it contains no cystine, proline or isoleucine, but possesses methionine and tryptophan in appreciable amounts.
somatostatin
Somatostatin
Two forms of somatostatin exists; S14 and S28, which are single chain polypeptides of 14 and 28 amino acids, respectively.
Somatostatin-14 has one disulfide bond.
It is produced by the δ-cells of the endocrine pancreas and dispersed cells in the gastrointestinal tract.
pancreatic polypeptide
Pancreatic Polypeptide (PP)
Pancreatic polypeptide (PP) is a 36-amino-acid peptide that is known to stimulate the gastric secretion of HCl and pepsin.
Most of the PP is expressed and then secreted by the F cells of the pancreas.
PP circulates in the plasma as a dimer with a half-life of 6–7 minutes.
amylin
The pancreatic β-cell islets produce the peptide hormone amylin.
It has also been formerly known as islet amyloid polypeptide (IAPP).
Amylin is a 37-amino-acid peptide.
Amylin and insulin are co-secreted from the pancreatic beta cells at the time of food intake.
hormones of hypophysis
- Hormones of the Hypophysis or Pituitary Gland
Hypophysis or pituitary gland consists of 3 lobes:
An anterior lobe or adenohypophysis
An intermediate lobe
A posterior lobe or neurohypophysis
All the three lobes secrete substances having hormonal activity.
The secretion of pituitary hormones is controlled by the hormones (or factors) released from hypothalamus, a region of the brain immediately proximal to the pituitary.
These hormones are called as hypothalamo-releasing hormones or hypothalamic factors.
hormones of anterior pitiutary
Hormones of Anterior Pituitary (Adenohypophysis)
Anterior pituitary gland (Adenohypophysis) secrete a number of hormones of vital importance and all of them are protein in nature, and these nourish the secretory activities of other glands.
These hormones are called as tropins or trophic hormones.
thyrotropin or thyroid stimulating hormone
A. Thyrotropin or Thyroid-Stimulating Hormone, TSH.
The molecule of TSH is a dimer made up of α and β subunits.
It is a glycoprotein with molecular weight about 30,000.
Each molecule has 8-9 cystine residues and the disulfide groups are present as intrachain linkages rather than interchain linkages.
adrenalcorticotropic hormone
B. Corticotropin or Adrenocorticotrophic Hormone, ACTH.
Corticotropin is a straight chain polypeptide with a molecular weight of about 4,500 and consists of 39 amino acid residues.
The most potent segment of activity is from residue 15 to 18 (Lys.Lys.Arg. Arg).
The release of ACTH is controlled by CRH.
gonadotropins
. Gonadotropins or Gonadotrophic Hormones, GTH.
Gonadotropins include:
Follicle Stimulating Hormone, FSH
Luteinizing hormone, LH
Prolactin, PRL
The gonadotropins, FSH and LH, are both regulated by the hypothalamic gonadotrophin releasing hormone, GnRH.
follcile stimulating hormone
Follicle-stimulating hormone, FSH
It is a glycoprotein that contains galactose, mannose, galactosamine, glucosamine, sialic acid, fucose and uronic acid.
It has a molecular weight of about 30,000 in man.
lutenizing hormone
Luteinizing hormone, LH
It is a peptide hormone with molecular weight of about 26,000 (in man).
It lacks tryptophan but has a high content of cystine and proline.
Each molecule contains 10 glucosamine and 3 galactosamine residues.
Prolactin PRL, Luteotrophic Hormone, LTH
Prolactin PRL, Luteotrophic Hormone, LTH
This is also a peptide hormone with 198 amino acid residues and a molecular weight of about 23,500.
It has 3 disulfide bonds between cysteine residues at 4-11, 58-173 and 190-198.
It differs from FSH and LH in that it contains no carbohydrate.
somatotropin
D. Somatotropin or Somatotrophic hormone, STH or Growth Hormone, GH.
Somatotropin obtained from human hypophysis is a protein with molecular weight 27,000.
It has 191 amino acids and consists of 2 disulfide bridges between adjacent cysteine residues.
The N-terminal and C-terminal residues are both phenylalanine.
Hormones of Posterior Pituitary or Neurohypophysis
Two important hormones are secreted from the posterior pituitary in both males and females.
These are:
Oxytocin (OT)
Vasopressin (VP), the antidiuretic hormone,
The two hormones are structurally closely related nonapeptides that are derived from a common ancestral gene.
oxytocin
Oxytocin
It is a nonapeptide amide.
A disulfide bond is present to link the two cysteine residues present in the molecule.
vasopressin
t is also a cyclic nonapeptide amide and resembles oxytocin except that isoleucine is replaced by phenylalanine and leucine by arginine.
hormones of the parathyroid
Parathyroids secrete a hormone called parathyroid hormone (parathormone, PTH) or Collip’s hormone.
It is a linear polypeptide consisting of 84 amino acids (molecular weight=9300) with no cysteine residues and hence no disulfide bridges.
A fragment of the intact hormone consisting of the first 34 amino acids at the N-terminal region of the molecule is sufficient for the peptide to exert its entire spectrum of characteristic biological effects.
hormones of the gastrointestinal tract
- Hormones of the Gastrointestinal Tract
The gastrointestinal hormones are a family of polypeptides produced by specialized endocrine cells present in the stomach, intestine, and colon which function both as traditional hormonal messengers and as neurotransmitters.
Of the many regulatory gastrointestinal peptides, following are well understood are considered to be the authentic hormones:
Gastrin
Secretin
Cholecystokinin (CCK)
gastrin
Gastrin
It is produced G cells in the pyloric mucosa, which is apparently stimulated by the proteins present in food or possibly by HCl.
The main form of gastrin obtained from G cells is a heptadecapeptide.
At the N-terminal end of the heptadecapeptide, there is a pyroglutamyl residue, and at the c-terminal end, there is a phenylalaninamide residue.
Two forms of heptadecapeptide exist:
One in which the tyrosyl residue at postion 12 is free (Gastrin 1)
The other in which it is sulphated.
secretin
Secretin has only 27 amino acids with molecular weight of 3,056.
The entire molecule of 27 amino acids is required for full biological activity.
Secretin is localized to the upper duodenal and jejunal S cells.
Cholycystokinin, CCK
It is also secreted by the upper part of the small intestine.
Cholecystokinin is a polypeptide with 33 amino acid residues and a molecular weight of 3,883.
It is, however, noteworthy that the last 5 amino acids towards the C-terminal in the gastrin and cholecystokinin are exactly the same.
It is in this terminal portion of these hormones that the principal activity resides.
thyroid hormones
Thyroidal Hormones
The mammalian thyroid gland biosynthesizes, stores, and secretes two molecular species of thyroid hormone:
Thyroxine (T4; 3,5,3′,5′-tetraiodothyronine)
Triiodothyronine (T3; 3,5,3′-triiodothyronine)
Thyroid hormones are poorly soluble in water, and more than 99% of the T3 and T4 circulating in the blood is bound to a carrier proteins.
The principle carrier of thyroid hormones is thyroxin-binding globulin.
calcitonin
[Calcitonin (CT) is a small polypeptide hormone secreted by the specialized C cells that are found primarily in the thyroid gland.
It consisting of a straight chain peptide of 32 amino acids, with a seven-membered disulfide ring at the N-terminus and a prolinamide residue at the C-terminus. ]
hormones of adrenal medulla
Hormones of Adrenal Medulla
The adrenal medulla produces two hormones:
Epinephrine or Adrenaline
Norepinephrine or Noradrenaline
Chemically, these two hormones are catecholamines (dihydroxyphenylamines) and are closely related to tyrosine and phenylalanine.
Norepinephrine, however, differs from epinephrine structurally in having a hydrogen atom in place of the methyl group.
