Signaling Molecules

Signaling molecules are necessary for the coordination of cellular responses by serving as ligands and binding to cell receptors.

 

 

Key Points

• Signaling molecules can range from small proteins to small ions and can be hydrophobic, water-soluble, or even a gas.
  
• Hydrophobic signaling molecules (ligands) can diffuse through the plasma membrane and bind to internal receptors.
  
• Water-soluble ligands are unable to pass freely through the plasma membrane due to their polarity and must bind to an extracellular domain of a cell-surface receptor.
  
• Other types of ligands can include gases, such as nitric oxide, which can freely diffuse through the plasma membrane and bind to internal receptors.
  

Terms

• hydrophobic
  lacking an affinity for water; unable to absorb, or be wetted by water
• ligand
  an ion, molecule, or functional group that binds to another chemical entity to form a larger complex.
  
  

  Signaling Molecules

  Produced by signaling cells and the subsequent binding to receptors in target cells, ligands act as chemical signals that travel to the target cells to coordinate responses. The types of molecules that serve as ligands are incredibly varied and range from small proteins to small ions like calcium (Ca²⁺).
  

  Small Hydrophobic Ligands

  Small hydrophobic ligands can directly diffuse through the plasma membrane and interact with internal receptors. Important members of this class of ligands are the steroid hormones. Steroids are lipids that have a hydrocarbon skeleton with four fused rings; different steroids have different functional groups attached to the carbon skeleton. Steroid hormones include the female sex hormone, estradiol, which is a type of estrogen; the male sex hormone, testosterone; and cholesterol, which is an important structural component of biological membranes and a precursor of steriod hormones . Other hydrophobic hormones include thyroid hormones and vitamin D. In order to be soluble in blood, hydrophobic ligands must bind to carrier proteins while they are being transported through the bloodstream.
  
  

  

  Steroid Hormones

  Steroid hormones have similar chemical structures to their precursor, cholesterol. Because these molecules are small and hydrophobic, they can diffuse directly across the plasma membrane into the cell, where they interact with internal receptors.

  Water-Soluble Ligands

  Water-soluble ligands are polar and, therefore, cannot pass through the plasma membrane unaided; sometimes, they are too large to pass through the membrane at all. Instead, most water-soluble ligands bind to the extracellular domain of cell-surface receptors. Cell-surface receptors include: ion-channel, G-protein, and enzyme-linked protein receptors. The binding of these ligands to these receptors results in a series of cellular changes. These water soluble ligands are quite diverse and include small molecules, peptides, and proteins.
  

  Other Ligands

  Nitric oxide (NO) is a gas that also acts as a ligand. It is able to diffuse directly across the plasma membrane; one of its roles is to interact with receptors in smooth muscle and induce relaxation of the tissue. NO has a very short half-life; therefore, it only functions over short distances. Nitroglycerin, a treatment for heart disease, acts by triggering the release of NO, which causes blood vessels to dilate (expand), thus restoring blood flow to the heart.