Many peptide receptors belong to the G protein-coupled receptor family — the largest class of membrane receptors in the human genome, and the target of a substantial share of approved medicines. When a peptide binds, it changes the receptor’s shape. That change switches on a partner G protein inside the cell, which in turn sets off a chain of internal chemical signals called second messengers.
This is why the exact sequence matters so much. Reviews of receptor pharmacology describe how changing just a few amino acids can alter how tightly the peptide binds, shift which internal pathway it favours, or switch off its activity altogether. It is also why a peptide’s effects cannot be guessed from its family — only from the receptors it actually binds.
