Protein hormones are water-soluble.
Protein hormones, unlike steroid hormones, cannot readily pass through the cell membrane because they are water-soluble (hydrophilic). Their water solubility is a key characteristic determining their mechanism of action. Since the cell membrane is primarily composed of lipids, water-soluble substances have difficulty traversing it directly.
How Water Solubility Affects Protein Hormone Action:
Because they can't enter the cell directly, protein hormones bind to receptors located on the surface of the target cell's plasma membrane. This binding initiates a cascade of events inside the cell, often involving second messengers like cyclic AMP (cAMP) or calcium ions (Ca2+). This cascade amplifies the signal and leads to the specific physiological response dictated by the hormone.
Key Characteristics of Protein Hormones:
- Water-Soluble: Easily dissolved in water-based environments, such as blood.
- Extracellular Receptors: Bind to receptors on the cell membrane.
- Second Messenger Systems: Often utilize second messengers to amplify the signal inside the cell.
- Rapid Action: Typically elicit rapid but short-lived responses.
Examples of Protein Hormones:
- Insulin
- Growth hormone
- Prolactin
- Follicle-stimulating hormone (FSH)
- Luteinizing hormone (LH)
In summary, the water solubility of protein hormones dictates that they bind to cell surface receptors, initiating intracellular signaling cascades to produce their effects.
