Question

All statements are true for natural estrogens except:

• A. Contain aromatic A-ring
• B. Contain C-18 steroid nucleus
• C. Have hydroxyl groups at the C-5 and/or C-18 positions
• D. Are synthesized from cholesterol through steroidogenesis.

Hint:

Natural estrogens are C-18 estranes characterized by a fully aromatic A-ring and a phenolic hydroxyl group at C-3. They do not feature hydroxylations at the C-5 or C-18 positions.

Answer 1

The Correct Option is C

Concept: Natural estrogens (such as Estradiol, Estrone, and Estriol) are endogenous steroid hormones responsible for the development and maintenance of the female reproductive system and secondary sexual characteristics. Like all steroid hormones, they are derived from a cyclopentanoperhydrophenanthrene core ring skeleton, but they possess unique structural modifications that differentiate them from androgens, progestins, and corticosteroids. Let us evaluate the accuracy of each statement:
• Option (A): Contain aromatic A-ring. This statement is completely true. Estrogens are unique among endogenous steroid hormones because their A-ring undergoes complete aromatization (forming a benzene-like ring with three alternating double bonds). This planar aromatic ring orientation is required for high-affinity binding to estrogen receptors (\(\text{ER}\alpha\) and \(\text{ER}\beta\)).
• Option (B): Contain C-18 steroid nucleus. This statement is completely true. Estrogens are classified as estranes, which contain 18 carbon atoms. They lack the C-19 angular methyl group located at the C-10 position that is present in androgens (andrastanes, C-19) and progestins/corticosteroids (pregnanes, C-21). They retain only the single angular methyl carbon at position 13, making them C-18 steroids.
• Option (C): Have hydroxyl groups at the C-5 and/or C-18 positions. This statement is INCORRECT. In natural estrogens, the primary hydroxyl group is always located at the C-3 position of the aromatic A-ring, giving it a phenolic character. In estradiol, a second hydroxyl group is located at the C-17 position (\(\beta\)-orientation). Because the A-ring is fully aromatic, carbon 5 (C-5) is involved in a double bond within the ring system and cannot support an isolated hydroxyl substitution. Carbon 18 (C-18) is the single angular methyl carbon atom and is never hydroxylated in natural estrogens.
• Option (D): Are synthesized from cholesterol through steroidogenesis. This statement is completely true. All endogenous steroid hormones are derived from cholesterol. Cholesterol (C-27) is converted to pregnenolone (C-21), then to androgens like testosterone or androstenedione (C-19), and finally aromatized by the enzyme complex aromatase to form estrogens (C-18). Conclusion: Because natural estrogens contain hydroxyl groups at the C-3 and C-17 or C-16 positions rather than at C-5 or C-18, Option (C) is the incorrect statement.