Explain in brief about the synthesis and functions of steroid and peptide hormones.

Steroid Hormone Synthesis

Steroid hormone synthesis is a complex process that occurs primarily in the adrenal cortex, testes, and ovaries. It begins with cholesterol, which undergoes a series of enzymatic transformations to produce different steroid hormones.

• Cholesterol Acquisition: Cells can obtain cholesterol from dietary sources or synthesize it de novo.
• Key Enzymes: Cytochrome P450 enzymes play a crucial role in steroid hormone synthesis, catalyzing various oxidation and hydroxylation reactions.
• Synthesis Pathways: Different pathways lead to the production of various steroid hormones, including glucocorticoids (cortisol), mineralocorticoids (aldosterone), androgens (testosterone), and estrogens (estradiol).
• Cellular Location: Synthesis occurs within the smooth endoplasmic reticulum and mitochondria.

For example, cortisol synthesis in the adrenal cortex involves multiple steps, starting with cholesterol and progressing through pregnenolone, progesterone, and 11-deoxycortisol before finally forming cortisol.

Steroid Hormone Function

Steroid hormones exert their effects by binding to intracellular receptors, which then form complexes that act as transcription factors, regulating gene expression.

• Mechanism of Action: Steroid hormones diffuse across the cell membrane and bind to receptors in the cytoplasm or nucleus.
• Receptor Types: These receptors are typically nuclear receptors, such as the glucocorticoid receptor (GR) and the estrogen receptor (ER).
• Physiological Effects: Steroid hormones regulate a wide range of physiological processes, including metabolism, inflammation, immune function, sexual development, and reproduction.

Peptide Hormone Synthesis

Peptide hormone synthesis occurs in the ribosomes of endocrine cells, involving transcription of mRNA into preprohormones, followed by processing into prohormones and finally, active hormones.

• Transcription & Translation: DNA is transcribed into mRNA, which is then translated into a preprohormone.
• Processing: The preprohormone undergoes proteolytic cleavage to form a prohormone, which is then further processed into the active hormone.
• Packaging & Secretion: Hormones are packaged into secretory vesicles and released via exocytosis.
• Cellular Location: Synthesis occurs in the rough endoplasmic reticulum (RER) and Golgi apparatus.

Insulin, for example, is synthesized as preproinsulin, cleaved to proinsulin, and then processed into active insulin within pancreatic beta cells.

Peptide Hormone Function

Peptide hormones bind to receptors on the cell surface, initiating intracellular signaling cascades that lead to cellular responses.

• Mechanism of Action: Peptide hormones bind to cell surface receptors, activating second messenger systems (e.g., cAMP, calcium).
• Receptor Types: These receptors are typically G protein-coupled receptors (GPCRs) or receptor tyrosine kinases (RTKs).
• Physiological Effects: Peptide hormones regulate a diverse array of physiological processes, including growth, metabolism, reproduction, and stress response.

Comparison of Steroid and Peptide Hormones

Feature	Steroid Hormones	Peptide Hormones
Chemical Structure	Derived from cholesterol	Composed of amino acid chains
Solubility	Lipid-soluble	Water-soluble
Synthesis Location	Smooth ER & Mitochondria	RER & Golgi Apparatus
Receptor Location	Intracellular (cytoplasm/nucleus)	Cell Surface
Mechanism of Action	Gene expression regulation	Second messenger systems
Speed of Action	Relatively slow	Relatively fast