Classify hormones and write down the process of steroid hormone biosynthesis. (2024/15 Marks)

Introduction

Hormones are chemical messengers produced by endocrine glands that regulate various physiological processes in the body. They can be classified into different categories based on their chemical structure, including peptide hormones, steroid hormones, and amino acid-derived hormones.

Classification of Hormones

Hormones can be classified based on their chemical structure and solubility:

• Amino Acid-Derived Hormones
  
  • Amine Hormones: Synthesized from single amino acids like tyrosine and tryptophan.
    
    • Examples: Epinephrine, norepinephrine, and thyroid hormones.
  • Peptide and Protein Hormones: Composed of chains of amino acids.
    
    • Examples: Insulin, glucagon, and growth hormone.
• Lipid-Derived Hormones
  
  • Steroid Hormones: Derived from cholesterol and characterized by a four-ring carbon structure.
    
    • Examples: Cortisol, aldosterone, estrogen, progesterone, and testosterone.
• Fatty Acid-Derived Hormones
  
  • Eicosanoids: Derived from arachidonic acid, a fatty acid.
    
    • Examples: Prostaglandins and leukotrienes.

Process of Steroid Hormone Biosynthesis

• Cholesterol Uptake and Transport
  
  • Cholesterol, the precursor for all steroid hormones, is either synthesized de novo or acquired from circulating lipoproteins.
  • Transported into mitochondria by the Steroidogenic Acute Regulatory (StAR) protein.
• Conversion to Pregnenolone: Within mitochondria, cholesterol undergoes side-chain cleavage by the enzyme cytochrome P450scc (CYP11A1), resulting in pregnenolone.
• Formation of Progesterone: Pregnenolone is converted to progesterone by the enzyme 3β-hydroxysteroid dehydrogenase.
• Branching into Specific Steroid Pathways
  
  • Progesterone serves as a precursor for various steroid hormones:
    
    • Glucocorticoids (e.g., cortisol): Involves hydroxylation reactions mediated by enzymes like 17α-hydroxylase and 21-hydroxylase.
    • Mineralocorticoids (e.g., aldosterone): Requires actions of 21-hydroxylase and aldosterone synthase.
    • Androgens (e.g., testosterone): Involves conversion to androstenedione and subsequent reduction.
    • Estrogens (e.g., estradiol): Aromatization of androgens by the enzyme aromatase.

Conclusion

Steroid hormones play a crucial role in regulating various physiological processes in the body. The biosynthesis of steroid hormones involves the conversion of cholesterol into pregnenolone, which is then further metabolized into specific steroid hormones through a series of enzymatic reactions.