Final answer:
The option that is not a major structural classification of hormones is (C) Tryptophan derivatives. Hormones are categorized as lipid-derived, amino acid-derived, and peptide hormones, with tryptophan derivatives falling under amino acid-derived hormones.
Step-by-step explanation:
The correct answer to which of the following is not one of the major structural classifications of hormones is Tryptophan derivatives. Hormones are primarily classified into three main structural classes: lipid-derived (or lipid-soluble) hormones, amino acid-derived hormones, and peptide (or protein) hormones. Lipid-derived hormones, like steroids, are derived from cholesterol and can diffuse across cell membranes due to their lipid solubility. Amino acid-derived hormones are based on the amino acids tyrosine or tryptophan, examples of which include hormones like epinephrine and melatonin.
Peptide hormones consist of polypeptide chains and include hormones such as oxytocin and growth hormone; these hormones are water-soluble and cannot pass through the plasma membrane, therefore their receptors are located on the surface of target cells. Tryptophan derivatives as a class do not exist; instead, individual hormones like melatonin are derived from tryptophan and fall under the amino acid-derived category.