It is very important from the point of view of drug development and its mechanism of action. Since most of the natural (biological) environment consists of enantiomeric molecules (aminoacids, nucleosides, carbohydrates and phospholipids are chiral molecules) it makes sense that drugs developed are also chiral.
Frequently only one stereoisomer is active, and sometimes the other one is toxic (the current policies of FDA in drug approval is that the inactive enantiomer in the racemic drug has to be shown to be devoid of any toxicity or undesired side-effects).
A drug upon administration undergoes a series of steps before exerting its activity. At each step the structure of the drug and hence its chirality influences the further metabolism.
The reason for chiral recognition by drug receptors is a three-point interaction of the agonist or substrate with the receptor or enzyme active site, respectively.
Example: Only the (-) enantiomer of epinephrine has the OH group in the binding site, and therefore has a much more potent pressor activity.
The D(-) lactoyl choline is hydrolyzed much more slowly than the L(+)-isomer due to favorable binding of the OH group in the latter case.
Likewise, cis/trans isomers of cyclic compounds, or Z/E isomers of alkenes are also expected to have different binding potency and therefore also different biological activity.
