This is very cool. The TL;DR is buried in the middle of the article:
> The team found that within the structure of the interlocked molecules, the [odor receptor, one of a "family of proteins known as G-protein-coupled receptors (GPCRs)" - gumby] had trapped propionate within a small pocket. When they enlarged the pocket, the receptor lost much of its sensitivity to propionate and to another small molecule that normally activates it. The tweaked receptor preferred larger odor molecules, which confirmed that the size and chemistry of the binding pocket tunes the receptor to detect only a narrow set of molecules.
> The structural analysis also uncovered a small, flexible loop atop the receptor, which locks down like a lid over the pocket once an odor molecule binds inside it. The discovery suggests that this highly variable looping piece may contribute to our ability to detect diverse chemistry, according to Manglik.
> The team found that within the structure of the interlocked molecules, the [odor receptor, one of a "family of proteins known as G-protein-coupled receptors (GPCRs)" - gumby] had trapped propionate within a small pocket. When they enlarged the pocket, the receptor lost much of its sensitivity to propionate and to another small molecule that normally activates it. The tweaked receptor preferred larger odor molecules, which confirmed that the size and chemistry of the binding pocket tunes the receptor to detect only a narrow set of molecules.
> The structural analysis also uncovered a small, flexible loop atop the receptor, which locks down like a lid over the pocket once an odor molecule binds inside it. The discovery suggests that this highly variable looping piece may contribute to our ability to detect diverse chemistry, according to Manglik.