Steroids are fascinating molecules with a unique structure and diverse functions in the body. From sex hormones to stress regulators, these lipids play crucial roles in our physiology and development.

The steroid family includes testosterone, estradiol, and synthetic contraceptives. Their complex biosynthesis starts with cholesterol and involves intricate enzymatic reactions. Understanding steroids is key to grasping hormone function and medical applications.

Structure and Characteristics of Steroids

Structure of steroid molecules

• Steroids are lipids derived from the tetracyclic hydrocarbon gonane consists of four fused rings: three cyclohexane rings (A, B, and C) and one cyclopentane ring (D)
• Rings labeled A, B, C, and D, and carbon atoms numbered from 1 to 17 allows for consistent nomenclature and identification of functional group positions
• Stereochemistry of ring fusions: trans fusion between rings A and B, cis fusion between rings B and C, and between rings C and D results in a flat, planar structure
• Variety of functional groups attached to gonane skeleton such as methyl groups at C-10 and C-13, alkyl side chain at C-17, and hydroxyl, ketone, or alkene groups at various positions imparts unique biological activities
• Limited conformational flexibility due to fused ring system: rings B and C locked in rigid chair conformation, while rings A and D have some flexibility allowing for half-chair or envelope conformations

Classes and Functions of Steroid Hormones

Classes of steroid hormones

• Sex hormones regulate development and maintenance of male and female characteristics
  • Androgens (testosterone) promote male characteristics, synthesized primarily in testes
  • Estrogens (estradiol) promote female characteristics, synthesized primarily in ovaries
  • Progestogens (progesterone) regulate menstrual cycle and maintain pregnancy, synthesized in ovaries and placenta
• Adrenocortical hormones regulate metabolism, immune response, stress response, and salt/water balance
  • Glucocorticoids (cortisol) regulate metabolism, immune response, and stress response, synthesized in adrenal cortex
  • Mineralocorticoids (aldosterone) regulate salt and water balance and blood pressure, synthesized in adrenal cortex

Specific Natural and Synthetic Steroids

Roles of natural vs synthetic steroids

• Testosterone (androgen steroid hormone)
  1. Structure: gonane skeleton with hydroxyl group at C-17 and ketone at C-3
  2. Functions: development of male secondary sexual characteristics, muscle growth, and bone density
• Estradiol (estrogen steroid hormone)
  1. Structure: gonane skeleton with hydroxyl groups at C-3 and C-17, and aromatic A ring
  2. Functions: development of female secondary sexual characteristics, regulation of menstrual cycle, and maintenance of bone density
• Oral contraceptives (synthetic steroid hormones) prevent pregnancy
  • Combination pills contain synthetic estrogen (ethinylestradiol) and progestin (norethindrone)
    1. Ethinylestradiol: gonane skeleton with ethynyl group at C-17 and hydroxyl group at C-3
    2. Norethindrone: gonane skeleton with ethynyl group at C-17 and ketone at C-3
  • Functions: suppress ovulation, thicken cervical mucus, and alter endometrium to prevent implantation

Steroid Biosynthesis and Regulation

Steroid biosynthesis

• Cholesterol serves as the precursor molecule for all steroid hormones
• Biosynthesis involves a series of enzymatic reactions that modify the cholesterol structure
• Aromatase enzyme plays a crucial role in converting androgens to estrogens by aromatizing the A ring

Steroid action and regulation

• Steroid hormones exert their effects by binding to nuclear receptors
• Nuclear receptors act as transcription factors, regulating gene expression when activated by steroid binding
• Anabolic steroids, synthetic derivatives of testosterone, promote muscle growth and are sometimes misused in sports