Denovo synthesis

In acute and chronic urinary tract infection, the combination of trimethoprim and sulfamethoxazole (Bactrim, Septra) exerts a truly synergistic effect on bacteria. The sulfonamide inhibits the utilization of p-amino-benzoic acid in the synthesis of folic acid (Figure 2.3), whereas trimethoprim, by inhibiting dihydrofolic acid reductase, blocks the conversion of dihydrofolic acid to tetrahydrofolic acid, which is essential to bacteria in the denovo synthesis of purines, pyrimidines, and certain amino acids. Because mammalian organisms do not synthesize folic acid and therefore need it as a vitamin in their daily diets, trimethoprim-sulfamethoxazole does not interfere with the metabolism of mammalian cells. 

See also Figure 4.3, Figure 4.2, DeNovo Pyrimidine Nucleotide Metabolism, Nucleotide Salvage Synthesis, Pyrimidine Catabolism... 

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