Nonsteroid hormones, which include proteins, peptides, and amines, exert their effects by binding with specific receptor molecules on the target cell membrane. Each receptor molecule has a blinding site, to which the hormone binds, and an activity site, which interacts with membrane proteins. The binding of hormones and membrane proteins to the receptor alter the function of enzymes, which ultimately has various effects on biochemical activities that take place within the cell. In this case, the hormone acts as the first messenger, whereas the biochemicals that induce cellular changes are referred to as second messengers. Ultimately, the changes that occur within the cell are recognized as a response to the hormone.
