There are many phrases that express this idea. See the link.
"Receptor sites are like baseball mits waiting to catch a baseball - in this case oestrogen. The action of genistein is to 'fool' the receptor sites, making them ..."
"Receptor sites are like little antennae on the cell that receive specific cytokines like a lock fits into a key"
There are different types of parking spaces: regular, handicapped, motorcycles and maybe large ones for trucks and RVs. Receptor sites come in many different flavors that are specific for their particular ligand.
METAMIA is a free database of analogy and metaphor. Anyone can contribute or search. The subject matter can be anything. Science is popular, but poetry is encouraged. The goal is to integrate our fluid muses with the stark literalism of a relational database. Metamia is like a girdle for your muses, a cognitive girdle.