HOW PEPTIDE HORMONES BIND TO CELLS
Nonsteroid peptide hormones are unable to cross the plasma membrane. How are these hormones able to generate the desired response in the target cell? The hormone triggers the production of intracellular molecules known as second messengers, which are responsible for the subsequent response to the hormone. The most common second messenger in animal cells is cyclic AMP (cAMP). Other second messengers include diacylglycerol (DAG) and inositol trisphosphate (IP₃).

Although the mechanisms leading to the production of second messengers differ, there are some important similarities. First, the hormone binds to a receptor in the membrane of the target cell. The hormone-receptor complex then traverses the membrane until it meets with a G protein. When the G protein is contacted by the hormone-receptor complex, the GDP molecule is exchanged for a GTP molecule. The G protein is now able to trigger the production of either cAMP or DAG and IP₃.