Humans also need folic acid, but are incapable of synthesizing it. It is a vitamin which must come form diet. O group and the hydrophobic benzene ring in the middle. With a high concentration of sulfa drugs in the system, this PABA analog will out compete PABA for the binding site, leading to inhibition of of the production of folic acid. This, in turn, will lead to the eventual loss of bacterial cell walls, and to bacterial death.
Irreversible inhibitors bind very tightly, some even covalently, to the enzyme.
They can bind either at the active site, or away from the active site in such a way as to prevent substrate binding.