Wondering why the human brain should have receptor sites for alkaloids from the opium poppy led to the discovery of a family of natural painkillers, the endorphins (from endogenous morphines). These substances are oligopeptides, containing from 5 to 30 amino acids.
The two 5-peptide examples are called enkephalins, from the Greek kephale, meaning "head". These have four of the five amino acids the same: Tyr-Gly-Gly-Phe, tyrosine-glycine-glycine-phenylalanine. One terminates in a leucine, and is known as Leu-enkephalin; the other terminates in a methionine, and is called Met-enkephalin.
Here are ball-and-stick representations:
(The yellow rod in each picture is a hydrogen bond between an amide NH and a C=O that maintains the folded structure of the peptide.)
Notice how much the left-hand portion of each molecule looks like the left-hand portion of morphine: the flat disc-like benzene ring with an OH (the tyrosine residue) and the N upward to the right. It is not hard to imagine how morphine could fit into a receptor designed for these peptides.
Both enkephalins, when synthesized in the laboratory, turn out to be powerful painkillers. Unfortunately, both are readily hydrolyzed by the body's peptidase enzymes. The search is on for a way to stabilize these compounds while retaining their activity.