Hormones: Types and Their Interaction with Target Cells

Overview of Hormones and Their Functions

Classes of Hormones

  • Hormones are classified based on chemical structure.
  • There are three main types of hormones:
    • Amine hormones
    • Peptide and protein hormones
    • Steroid hormones

1. Amine Hormones

  • Derived from single amino acids.
  • Main amino acids involved:
    • Tryptophan: precursor for melatonin (regulates daily rhythms).
    • Tyrosine: precursor for catecholamines (includes adrenaline, noradrenaline, and dopamine) and thyroid hormones.

Example: Catecholamine Hormones

  • Adrenaline and noradrenaline are examples of amine hormones derived from tyrosine.
    • Structure: simple structural configuration; relatively small compared to other hormone types.

2. Peptide and Protein Hormones

  • Composed of chains of amino acids.
  • Classified into peptides (short chains) and proteins (longer chains).
    • Peptides: typically under 200 amino acids.
    • Proteins: typically more than 200 amino acids.

Examples:

  • Follicle Stimulating Hormone (FSH): a larger protein with two subunits.
  • Human Growth Hormone: a smaller protein (approximately 97 amino acids).

Characteristics:

  • This category represents the largest group of hormones in the body.

3. Steroid Hormones

  • Derived from lipid cholesterol.
  • Includes hormones such as cortisol, estrogen, testosterone, and progesterone.
  • Acids: eicosanoids (e.g., prostaglandins) – though not the focus, these have local effects as paracrine factors without entering the bloodstream.

Transport Note:

  • Steroid hormones are lipid-soluble and require carrier proteins for transport in the blood due to their hydrophobic nature, facilitating their interaction with target cells.

Hormone Interaction with Target Cells

Type of Receptors

  • Receptor Locations:
    • On the cell surface: for amine and peptide hormones.
    • Within the cell: for steroid hormones.

Water-Soluble Hormones (Amine and Peptide)

  1. Action occurs via a first messenger (the hormone itself).
  2. Binds to a receptor on the cell surface.
  3. Triggers intracellular processes using second messengers (like cAMP).
  4. Example of process:
    • Binding activates a G-protein linked process, leading to a cascade of phosphorylation events that ultimately results in a cellular response.

Lipid-Soluble Hormones (Steroids)

  1. Can diffuse directly through the cell membrane.
  2. Binds to its receptor inside the cell (either in the cytoplasm or nucleus).
  3. Forms a hormone-receptor complex that interacts directly with DNA at specific response elements.
  4. Triggers transcription of genes, leading to the formation of new proteins, which mediate the response.

Summary of Process

  • Water-soluble hormones interact externally and use second messengers to invoke a response.
  • Lipid-soluble hormones enter cells and directly modulate gene expression, resulting in longer-term cellular changes.