Most medicines work by fitting a protein in your body, the way a key fits a lock. The protein is the medicine's target.
A protein's chain of amino acids folds up into its own 3D form. The folding leaves pockets in its surface, each lined with particular groups in particular places. A medicine works when its molecule slips into one of these pockets and stays there for a while.
Two kinds of protein are the targets of most medicines. Enzymes speed up the chemistry in your cells. Each has an active site, the pocket where its own molecule fits and reacts. A medicine that sits in the active site keeps that molecule out, and the enzyme stops working.
Receptors pick up messages. The smell receptors in your nose are one kind. When the right messenger molecule fits a receptor, the receptor passes a signal into the cell. Some medicines switch a receptor on, the way its messenger does. Others sit in it and block it: antihistamines for hay fever block the receptor for histamine, a messenger that makes your nose run and your eyes itch.
What holds a medicine in place
A medicine holds on to its pocket with the same small pulls that hold molecules to each other:
- hydrogen bonds, where a δ+ hydrogen on an -OH or N–H group points at a lone pair on a δ− oxygen or nitrogen
- the pull between other δ+ and δ− ends of polar bonds, such as the C=O of a ketone
- oily, non-polar parts, such as a benzene ring or a carbon chain, resting against oily patches of the pocket, the way oil gathers with oil
Each pull is weak, so a medicine needs several of them, each in the right place, to hold on well. A molecule's form in 3D and its groups both matter. A molecule of the wrong size or form cannot get its groups close to the matching spots in the pocket. A molecule that fits with a group missing loses a pull and holds on less tightly. Most medicines sit in their pocket for a while, let go, and bind again, the way hydrogen bonds in water keep breaking and forming. The next section shows a medicine that bonds to its enzyme for good.