Figure 2.

Structure of representative endogenous opioids. Colored squares highlight the shared N-terminal sequence, YGGF, in β-endorphin, enkephalins, dynorphin A (1–17), and dynorphin A (1–8). Colored squares in nociceptin and orphanin depict common amino acid positions. All peptides shown have a Phe residue at the fourth position. All peptides shown, except heptadecapeptides, nociceptin, and orphanin FQ2 (sharing a Phe residue), share a Tyr residue at the N-terminus. All peptide structures were drawn at pH = 7 with PepDraw [29], the free web-based software from Tulane University (https://pepdraw.com/ (accessed on 17 May 2023)).