Enzyme

An enzyme is a protein that accelerates a specific chemical reaction in the body, emerging from the process unchanged and ready to act again. Most enzymes are highly selective — each one recognises a narrow class of molecules and catalyses one type of transformation.

In pharmacology, enzymes matter because they govern how drugs are metabolised: broken into inactive fragments, converted to active compounds, or made water-soluble enough to be excreted. The enzymes responsible are found mainly in the liver and the gut wall, and their activity determines how much of a dose reaches the bloodstream, how long it stays active, and what other molecules it becomes along the way.

How it works · its role

Enzymes work through an active site: a precisely shaped pocket on the protein surface where the target molecule — called the substrate — binds. Once bound, the enzyme holds the substrate in a position that makes the reaction far easier to complete, then releases the modified product and resets.

The most important enzyme family in drug metabolism is the cytochrome P450 enzymes (CYP), housed mainly in liver cells. CYP3A4 and CYP2D6 alone handle a large proportion of all clinically used drugs, typically by adding or exposing chemical groups that the body can then clear. A separate enzyme, monoamine oxidase (MAO), breaks down monoamine neurotransmitters — dopamine, serotonin, norepinephrine — and the compounds that resemble them.

Genetic variation matters here. Differences in the CYP2D6 gene mean some individuals metabolise certain drugs several times faster or slower than average, shifting the same dose from therapeutic to sub-effective or toxic.

Relevance to substances & effects

Enzyme activity shapes nearly every pharmacokinetic property a substance page reports. A drug broken down quickly by CYP enzymes has a shorter duration and lower peak; one that is metabolised slowly, or that is itself produced by metabolism from an inactive parent compound (a prodrug), behaves very differently.

Several substance classes act by blocking enzymes directly. Monoamine oxidase inhibitors shut down the enzyme that clears monoamines — which is why they carry serious interaction risks with serotonergic drugs, stimulants, and certain foods. Substances that inhibit CYP enzymes can raise blood levels of anything else processed by the same pathway, turning a moderate dose of a second drug into an effectively larger one.

Some substances induce enzymes rather than inhibit them, accelerating their own breakdown or that of co-administered drugs. Understanding which enzymes are in play is the foundation of the interaction data shown across this encyclopedia.