This figure shows the underlying competitive mechanism by which trimethoprim inhibits dihydrofolate reductase. Part (a) shows the mechanism in the absence of trimethoprim represented as an equation in which dihydrofolic acid (in green) and dihydrofolate reductase (in yellow labelled enzyme) interact so that dihydrofolic acid binds to the active site of the dihydrofolate reductase. Part (b) shows the mechanism in the presence of trimethoprim represented as an equation. Trimethoprim (in red) competes with dihydrofolic acid (green) to bind to the enzyme (represented in yellow). When trimethoprim binds to the enzyme dihydrofolic acid is unable to bind and the pathway is interrupted.