Sulfa drugs like Bactrim, given for bacterial infections, inhibit bacteria by blocking folic acid synthesis. The precursor molecule of folic acid is para-amino benzoic acid (PABA). Interestingly, PABA has a structure very similar to a sulfa drug. If a sulfa drug is present, the bacterial enzyme will bind the sulfa drug because of structural similarity. This is an example of .

Competititve Inhibition

Explanation:

In competitive inhibtion, an enzyme or protein can tie substrate (forming an ES complex) or inhibitor (EI) but not both (ESI). The competitive inhibitor looks like the substrate and ties to the active or dynamic site of the enzyme.The substrate is in this way kept prevented from binding  to a similar active site. A competitive inhibitor lessens the rate of catalysis by decreasing the extent of enzyme molecules bound to a substrate. At some inhibitor concentration, competitive inhibition can be diminished by expanding the substrate concentration.