While one molecule could fit snugly in the active site from the front, for example, the mirror image might fit just as well coming in from the rear. Some other attribute of the active site determines whether it specifically binds one or the other mirror image. Sometimes it is a constraint, such as an obstacle preventing binding from one direction. Often it is a fourth interaction in the active site, for example with a metal ion, that turns the encounter into a four-point landing. This shows you can have at least two.
Selection is dependent on the active site environment of the protein.
This new way of looking at protein interactions would significantly affect drug designers, who, Koshland says, may needlessly be discarding good drug candidates because of this misunderstanding. Juggling three-dimensional computer models of enzymes or receptors, drug designers typically engineer small molecules to fit just so into a particular active site, with such precision that even the mirror image wouldn't fit. The goal is to switch a receptor on or off or disable an enzyme in hopes of stopping some disease process. However, assuming only one of two mirror images of a drug will bind strongly, when in fact both may bind to the active site, can lead to confusing assays and possible rejection of a promising drug candidate. People are going to have to revise their thinking.