sentences:  Bacteria and some protozoa are unable to obtain folic acid from the environment, and must instead synthesize it by converting PABA ("para"-aminobenzoate) to dihydropteroate using the enzyme dihydropteroate synthase.Sulfonamides act as a competitive inhibitor; being structurally similar to PABA, they are able to bind to the enzyme's active site and prevent the synthesis of folic acid from progressing.Folic acid is necessary for these organisms to produce nucleic acids (i.e.