Lab 11 — Biochemistry
Chirality: Why a Drug Works
A carbon bonded to four different groups comes in two non-superimposable mirror images, like a left and a right hand — these are enantiomers (optical isomers). The proteins in your body (receptors, enzymes) are themselves handed, so typically only one enantiomer fits the pocket and acts as a drug. The mirror image is made of the exact same atoms, yet it may not fit at all, or do something entirely different. Shape and handedness decide the effect.
The R form matches all three points and docks. Switch to S (the mirror image) and the colors no longer line up
The three-point attachment model: the drug binds only when all three of its groups meet the receptor’s three complementary sites. A mirror image, spun any way in the plane, has the opposite handedness and can never satisfy all three at once. The tragic case is thalidomide — one enantiomer is a sedative, the other is teratogenic (and worse, the body interconverts them, so giving just one didn’t help). This is why making a single enantiomer (asymmetric synthesis) is decisive in modern drug design. Life’s own choice of handedness — nearly all L-amino acids and D-sugars — is the same phenomenon.