BIOM 3200 Chapter Notes - Chapter 4: Thyroid Hormone Receptor, Steroid Hormone Receptor, Hormone Response Element

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Unit 4: Endocrine
Endocrine glands and hormones
Endocrine glands
- Lack the ducts that are present in exocrine glands
- Secrete their products (biologically active molecules-hormones), into the blood.
- The blood carries the hormones to target cells that contain specific receptor proteins for the
hormones, can respond accordingly
- Many endocrine glands are organs that function to produce and secrete hormones regulate
- The pancreas functions as both an exocrine and an endocrine gland
The endocrine portion of the pancreas is composed of clusters of cells called the pancreatic
islets (islets of Langerhans)
-Endocrine system= heart, liver, adipose tissue, and kidneys
- The endocrine system is extended beyond these organs- many other organs secrete hormones
These organs may be categorized as endocrine glands even though they serve other
functions as well.
- Some specialized neurons (especially in the hypothalamus) secrete chemical messengers into
the blood rather than into a narrow synaptic cleft Neurohormones
- Many chemicals (eg. Norepinephrine) are secreted both as a neurotransmitter and a hormone
- Nervous vs. endocrine system are not differentiated based only on the chemicals they release
Hormones affect the metabolism of their target organs and help regulate total body
metabolism, growth, and reproduction
Chemical Classification of Hormones
Hormones secreted by different endocrine glands vary widely in chemical structure. All hormones,
however, can be divided into a few chemical classes.
1. Amines
- Derived from the amino acids tyrosine and tryptophan
- Includes the hormones secreted by the adrenal medulla, thyroid, and pineal glands.
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2. Polypeptides and proteins
- Proteins are large polypeptides the distinction isn’t “white and black”
- Antidiuretic hormone is a polypeptide with nine amino acids, too small to accurately be called a
protein.
If a polypeptide chain is larger than about 100 amino acids (eg. growth hormone with 191
amino acids), it can be called a protein.
Insulin blurs the two categories it is composed of two polypeptide chains derived from a
single, larger molecule.
3. Glycoproteins
- Consists of a protein bound to one or more carbohydrate groups
- Examples: follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
4. Steroids
- Derived from cholesterol after an enzyme cleaves off side chain attached to five-carbon “D” ring
- Includes testosterone, estradiol, progesterone, and cortisol
Hormone molecules can polar (water-soluble) or nonpolar (insoluble in water)
- Nonpolar hormones are soluble in lipids lipophilic hormones.
Lipophilic hormones can gain entry into their target cells
Polar hormones cannot pass through plasma membranes
- Lipophilic hormones steroid hormones and thyroid hormones
Steroid hormones
- Secreted by only two endocrine glands: the adrenal cortex and the gonads
The gonads secrete sex steroids
The adrenal cortex secretes corticosteroids (including cortisol and aldosterone) and small
amounts of sex steroids
Thyroid hormones
- Composed of two derivatives of the amino acid tyrosine bonded together
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When the hormone contains 4 iodine atoms, it is called tetraiodothyronine (T4), or
thyroxine
When it contains 3 atoms of iodine, it is called triiodothyronine (T3)
- Although these hormones are not steroids, they are like steroids in that they are relatively small,
nonpolar molecules.
Steroid and thyroid hormones are active when taken orally (as a pill)
- Sex steroids are the active agents in contraceptive pills, and thyroid hormone pills are taken by
people whose thyroid is deficient (hypothyroid)
- Polypeptide and glycoprotein hormones cannot be taken orally they would be digested into
inactive fragments before being absorbed into the blood
- Insulin-dependent diabetics must inject themselves with this hormone.
Polar, water-soluble hormones include: Polypeptides, Glycoproteins, and the catecholamine hormones
secreted by the adrenal medulla, epinephrine and norepinephrine
- Derived from the amino acid tyrosine
- Like the polypeptide and glycoprotein hormones, the catecholamines are too polar to pass
through the phospholipid portion of the plasma membrane.
- Melatonin (secreted by the pineal gland) is different
Derived from the nonpolar amino acid tryptophan
Melatonin pills are effective because they can pass through plasma membranes (like
steroids and thyroxine)
Also has some similarities to the polar hormones in terms of its effects on cells
Prohormones and Prehormones
**Hormone molecules that affect the metabolism of target cells are often derived from less active
precursor molecules
Prohormone a committed precursor of a hormone
- Polypeptide hormones: precursor may be a longer chained prohormone that is cut and spliced
together within the gland cell to make the hormone.
Eg. Insulin is produced from proinsulin in beta cells of the islets of Langerhans in the
pancreas
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