Hormones are chemical messengers that carry information from the sensory cells which detect changes in the environment to a target cell and initiate the production of enzymes that catalyze the reactions involved in responding to these changes in the environment.
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Hormones can be classified based on their chemical structure in to two groups; Amino acid/protein-based hormones and steroid hormones. Amino acid based hormones are derived from one Amino acids while protein based hormones are derived from a chain of amino acids that have linked up to form a protein. Amino acid based hormones are derived from the modification of the structure of an amino acid such as tryptophan or tyrosine by mechanisms such as the removal of a carboxyl group leaving the amine group. An example of a hormone synthesized from tryptophan is melatonin, which is involved in the regulation of circadian rhythm and is produced in the pineal gland while examples of hormones synthesized from tyrosine are epinephrine, norepinephrine and dopamine. Epinephrine and norepinephrine are involved in the fight or flight response in response to danger while dopamine is involved in the control of many functions of the nervous system from movement to lactation.
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Protein based hormones are basically amino acid based since a protein is made up of polypeptides which are made up of peptides which are made up of amino acids. Synthesis is by basic protein synthesis where De-oxy-Ribonucleic acid (DNA) is transcribed in to messenger Ribonucleic Acid (m RNA) which is translated in to an amino acid chain. Examples of protein based hormones are Anti-Diuretic Hormone which is produced by the pituitary gland and is involved in fluid balance and Atrial-Natriuretic Peptide which is produced by the heart and is involved in regulating blood pressure.
The second class of hormones would be steroid based hormones which are derived from lipid cholesterol and are characterized by interlocking carbohydrates. Examples of hormones that are steroid based are aldosterone, produced in the adrenal cortex and involved in osmoregulation, testosterone and estrogen produced in the gonads and involved in the development of male and female sexual characteristics respectively.
Hormone chemical classification determines a variety of factors regarding its function including; half-life, and the mechanism that will be involved in the activation of cellular response at the target site. Steroid based hormones have a longer half-life than amino acid/protein based hormones (60-90 minutes) while amine/protein hormones have a half-life of one minute. Activation of the target site will also be determined by the chemical structure of the hormone and in the case of protein/peptide/amino acid based hormones, the structure of the receptor protein. Due to the lipophilic nature of steroid hormones, they diffuse through the cell membrane, bind to the receptor protein, activating the DNA segment which undergoes transcription to messenger RNA which is translated to an enzyme that acts as a catalyst in the reactions that lead to the regulation of cellular physiologic activity. On the other hand, the hydrophilic nature of amine based and protein based hormones does not allow them to diffuse through the cell membrane and instead they have bind to receptor proteins on the membrane which stimulate the production of second messengers such as cyclic AMP and inositol triphosphate (IP3) depending on the structure of the receptor protein. For instance, G-protein membrane bound protein activates adenylate cyclase which catalyzes the production of cyclic AMP from ATP while IP3 triggers release of CA2+ from the endoplasmic reticulum. The CA2+ and cyclic AMP produced then activate enzymes involved in the generation of the cellular changes.