(Description) Figure 7a The folic acid pathway. Trimethoprim prevents the enzyme dihydrofolate reductase reacting with the intermediate compound dihydrofolic acid, thereby blocking the pathway at the point shown.

Week 2: How do antibiotics work?

This figures shows the folic acid pathway. Para-aminobenzoic acid and pteridine are converted into dihydrpteroic acid by dihydorpteroate synthetase. Dihydropteroic acid is then converted to dihydrofolic acid by dihydrofolate synthetase. In the final step of the pathway dihydrofolic acid is converted to tetrahydrofolic acid, the active form of folic acid, by dihydrofolate reductase. Trimethoprim prevents the enzyme dihydrofolate reductase reacting with the intermediate compound dihydrofolic acid, thereby blocking the pathway in the final step.