Mechanism of action of Group I and Group II hormones

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In endocrinology, hormones can be classified into two main groups based on their mechanism of action: Group I hormones, which include steroid hormones, and Group II hormones, which include peptide and amino acid-derived hormones. Let's delve into the mechanisms of action for each group:

Group I Hormones (Steroid Hormones):

  1. Diffusion Across Cell Membrane: Steroid hormones are lipophilic (lipid-soluble) molecules that can passively diffuse across the cell membrane of target cells.

  2. Binding to Intracellular Receptors: Once inside the target cell, steroid hormones bind to specific intracellular receptors, typically located in the cytoplasm or nucleus. These receptors are often transcription factors.

  3. Formation of Hormone-Receptor Complex: Hormone binding induces a conformational change in the receptor, leading to the formation of a hormone-receptor complex.

  4. Transcriptional Regulation: The hormone-receptor complex translocates to the nucleus (if not already there) and binds to specific DNA sequences called hormone response elements (HREs) within the promoter regions of target genes.

  5. Gene Expression Alteration: Binding of the hormone-receptor complex to HREs modulates gene transcription, leading to changes in mRNA synthesis.

  6. Protein Synthesis and Cellular Response: The altered mRNA is translated into proteins, which mediate the cellular response to the steroid hormone. These proteins may include enzymes, transporters, receptors, or structural proteins, depending on the target tissue and hormone.

Group II Hormones (Peptide and Amino Acid-Derived Hormones):

  1. Binding to Cell Surface Receptors: Peptide and amino acid-derived hormones, being water-soluble, cannot cross the cell membrane. Instead, they bind to specific receptors located on the cell surface.

  2. Activation of Signal Transduction Pathways: Hormone binding to its receptor initiates a series of intracellular signaling events, often involving second messengers such as cyclic adenosine monophosphate (cAMP), inositol trisphosphate (IP3), or calcium ions.

  3. Intracellular Responses: Activation of signal transduction pathways leads to various intracellular responses, such as phosphorylation cascades, activation of protein kinases, and alteration of enzyme activity.

  4. Gene Transcription Regulation (Optional): In some cases, the intracellular signaling pathways activated by peptide hormones can also regulate gene transcription indirectly by modulating the activity of transcription factors.

  5. Cellular Responses: The intracellular responses ultimately elicit specific cellular responses, such as changes in enzyme activity, ion transport, gene expression, cell growth, or secretion of other hormones.

Overall, while both Group I and Group II hormones ultimately lead to cellular responses, they do so through distinct mechanisms based on their chemical properties and receptors. These mechanisms enable precise regulation of various physiological processes in the body.

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