Medicines from Organic Molecules: Shape Fits the Target

0:00How can a small molecule work as a medicine?

0:04For thousands of years, people used willow bark to ease pain and fever.

0:09In the 1800s, chemists extracted a compound called salicin from the bark.

0:15They turned salicin into salicylic acid, which eased pain but irritated the stomach.

0:22Its molecule has a benzene ring, a ring of six carbon atoms, with two functional groups attached.

0:29A functional group is a small group of atoms that controls how a molecule reacts.

0:35These are a hydroxyl group, O H, and a carboxyl group, C O O H.

0:42Chemists then changed the hydroxyl group into an ester group.

0:47In this ester group, a carbon double bonded to an oxygen is joined to the ring through a second oxygen.

0:55The new molecule is acetylsalicylic acid, or A S A. First made pure in 1897, it irritates the stomach less.

1:05In the body, its target is an enzyme, a protein that speeds up a reaction.

1:11This enzyme makes signal molecules involved in pain and fever.

1:16The enzyme does its work in a pocket called the active site, which only the right shape fits well.

1:23A S A has the right shape, so it fits like a key in a lock and blocks it. Fewer signals are made.

1:31Small changes matter. If this group were bigger, the molecule would not fit, so it would not block the enzyme.

1:40So a medicine works when its shape and its functional groups fit its protein target.