Biochemical Foundations of Human Vital Activity - Volkov, N.I., Nesen, E.N. 2000

Biochemical Foundations of Human Vital Activity
Hormones as Regulators of Metabolism
Chemical Nature of Hormones

Depending on their chemical Structure, Hormones are mainly divided into three groups: steroid, protein-peptide, and Amino Acid Derivatives (Fig. 47).

Steroid Hormones are synthesized from the lipid Cholesterol. Structurally, steroid hormones differ in their functional groups or protein side chains. They include Hormones of the adrenal cortex (aldosterone and cortisol) and the Gonads (testosterone and estradiol):

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Steroid hormones are lipid-soluble and easily cross Cell membranes. Their receptors are located in the Cytoplasm or Nucleus of target Cells.

Protein-Peptide Hormones are synthesized from Amino Acids and can be either Peptides (ranging from 2 to 50 amino acids) or Polypeptides—Proteins with a complex three-dimensional structure, such as the pancreatic hormone Insulin. Insulin consists of two peptide chains containing a different number of amino acids:

Protein-peptide hormones include those of the Hypothalamus, Pituitary Gland, and Pancreas, as well as thyrocalcitonin from The Thyroid Gland. These hormones are synthesized in advance and stored in the glands. For instance, the hypothalamus synthesizes two structurally similar peptide hormones—oxytocin and vasopressin—which are then transported to the posterior pituitary. These hormones differ by only Two amino acids:

Fig. 47 Classification of hormones according to their chemical nature

Peptide and Protein hormones typically exert their effects on intracellular processes through specific receptors located On the surface membrane of target cells.

Amino acid derivative hormones are synthesized from The amino acid Tyrosine, which in turn is synthesized from the essential amino acid phenylalanine. These include The adrenal medulla hormones norepinephrine and epinephrine, as well as the THYROID HORMONES triiodothyronine (T₃) and thyroxine (T₄). The pathway for the synthesis of adrenal medulla hormones from the amino acids phenylalanine or tyrosine is as follows:

The synthesis of thyroid hormones involves the Iodination of tyrosine residues by free iodine supplied via the bloodstream. This yields the biologically active hormones tetraiodothyronine (thyroxine) and triiodothyronine:

The iodine required for the synthesis of these hormones is a trace element obtained from food and Water. The daily requirement of The Human Body is approximately 70 mg ∙ day^-1.