What is a characteristic function of protein hormones?

Protein hormones are characterized by their mechanism of action, which primarily involves binding to specific receptors on the surface of target cells rather than entering the cells themselves. This is due to their molecular structure; being larger and hydrophilic, they cannot cross the lipid bilayer of the cell membrane. Instead, they exert their effects by initiating a signal transduction pathway upon binding to receptors located on the cell surface. This interaction typically leads to a cascade of biochemical events inside the cell that ultimately alters cellular function.

While protein hormones are often synthesized and stored in endocrine glands, this is not a defining characteristic of how they function within the body. Additionally, while some hormones may influence inflammatory responses, it does not define protein hormones as a whole. Lastly, protein hormones do not serve as energy sources; energy storage and metabolism are more closely associated with carbohydrates and lipids. Therefore, the aspect of protein hormones not entering cells is crucial to understanding their role in physiology.