Figure 35.4: The Three Principal Chemical Classes of Hormones (A) Peptide hormones are composed of strings of amino acids. They include proteins and small polypeptide molecules. Shown here are two proteins, insulin and growth hormone. Each of these proteins consists of two subunits, which are shown in different colors. (B) Steroid hormones are modified from cholesterol molecules and all include a characteristic set of four ring structures, here called the sterol backbone and colored green. They include the corticosteroids produced by the adrenal cortex and the sex steroids produced primarily by the gonads. (C) Amine hormones are small molecules synthesized from single amino acid molecules. Thyroxine and epinephrine are both made from the amino acid tyrosine. The structures in B and C are in abbreviated form; not all carbon (C) atoms are shown.