Some drugs, known collectively as agonists, can be thought of as molecular switches - if the molecule fits the active site of the receptor, the biological response is switched on. But so-called partial agonists pose a problem for this simple model. A partial agonist is a compound that elicits less than a full biological response on binding to its target, even when it occupies all the available binding sites. How can this be? Reporting on page 722 of this issue, Lape et al provide an answer for partial agonists that bind to two structurally related channels - the glycine receptor and the muscle nico-tinic receptor. The binding of a full agonist to these receptors causes the channels to be open almost constantly, so that the maximum possible current in the channel is observed as ions flow through.
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