October 2006 Molecule of the Month by David Goodsell
Keywords: monooxygenase activity, drug metabolic process, oxidation reduction, metabolic detoxication
If you have a headache and take a drug to block the pain, you'll notice that the effects of the drug wear off in a few hours. This happens because you have a powerful detoxification system that finds unusual chemicals, like drugs, and flushes them out of your body. This system fights all sorts of unpleasant chemicals that we eat and breathe, including drugs, poisonous compounds in plants, carcinogens formed during cooking, and environmental pollutants. The cytochrome p450 enzymes are our first line of defense in this chemical battle.
The cytochrome p450 enzymes find unusual molecules and add oxygen atoms to them. In most cases, this has the effect of making the molecule more soluble in water, and thus, easier to flush out of the body. The added oxygen also provides a ready handle for other detoxifying enzymes to take hold and further modify, and destroy, these toxic molecules. This task of adding oxygen is chemically tricky, and cytochrome p450 enzymes use a powerful molecular tool to perform the reaction: an iron atom in a heme group (described in more detail later).
Cytochrome p450 enzymes are found in all organisms. Each organism builds several different enzymes, each of which act on a different selection of molecules. Typically, bacteria make about 20 different forms of these enzymes and we produce about 60. Plants often make hundreds of different forms. This is because plants make unusual pigments and exotic toxins to protect themselves. Many of the reactions needed to make these molecules are performed by specialized cytochrome p450 enzymes. For more information on cytochrome p450 from a genomics persective, take a look at the Protein of the Month at the European Bioinformatics Institute.
A Double-edged Sword
The molecule shown here is CYP3A4 (PDB entries
2j0d), the cytochrome p450
that plays the major role in drug detoxification in your body. It has been estimated that
this enzyme acts on about half of known drugs. For instance, it modifies the antibiotic
erythromycin, shown at the bottom in blue. It also detoxifies such diverse drugs as
codeine, diazepam (Valium), paclitaxel (Taxol), and several anti-HIV drugs.
In some cases, however, the reaction performed by cytochrome p450 enzymes can cause more harm than good. For example, CYP3A4 is partially responsible for the toxicity of large doses of acetaminophen (Tylenol). The modified form of acetaminophen is dangerously reactive, but it is normally cleared away quickly by other detoxifying enzymes. But with large doses, the reactive intermediate can build up to dangerous levels.