Sulfa drugs

"Sulfonamides, also known as sulfa drugs, are synthetic antimicrobial agents derived from sulfonic acid. In bacteria, these drugs are competitive inhibitors of para-aminobenzoic acid (PABA), a substrate of the enzyme dihydropteroate synthetase. This reaction is necessary in these organisms for the synthesis of folic acid. Ever since the development of penicillin, there has been a drive to make antibiotics better: better ability to penetrate into infected tissue, better efficacy against a more broad spectrum of bacterial organisms, less potential to harm host tissues. The combination of trimethoprim and sulfa antibiotics has created a very unique method to combat bacteria: the sequential blockade. An essential nutrient used in the synthesis of many important biochemicals is folic acid. Folic acid is made from para-amino benzoic acid (PABA) through a step-by-step process involving two enzymes. The sulfa drug inhibits the first enzyme and trimethoprim inhibits the second enzyme. This double inhibition is called the sequential blockade and produces death of the bacterium whereas either antibiotic alone could only have inhibited bacterial reproduction. Mammal enzymes are far less sensitive to the blockade than bacterial enzymes but what really protects the infected host from the blockade is the simple fact that mammals do not have to manufacture their own folic acid; they can eat it in their diet. "