Bacterial synthesis of folic acid

Sulfonamides are structural analogs of PABA that competitively inhibit bacterial synthesis of folic acid (see p. 371). Because purine synthesis requires THF as a coenzyme, the sulfa drugs slow down this pathway in bacteria. 

Sulfonamides were the first group of chemotherapeutic agents used for the prevention or treatment of bacterial infections in humans. Sulfonamides (e.g., sulfisoxazole) act by inhibiting bacterial synthesis of folic acid, a chemical required for synthesis of nucleic acid and protein. These drugs competitively inhibit the first step in the synthesis of folic acid—the conversion of para-aminobenzoic acid into dihydrofolic acid. Because humans absorb preformed folic acid from food, sulfonamide inhibition has only a minimal effect on hiunan cells. 

Both of these drugs interfere with bacterial synthesis of folic acid and the production of nucleotides. Sulfamethoxazole decreases the enzymatic conversion of PABA, and trimethoprim decreases the activity of dihydrofolate reductase. Thus, the actions are synergistic (see Figure p 2621. 

The first synthetic antibiotics were the sulfonamides (right). As analogues of p-ami-nobenzoic acid, these affect the synthesis of folic acid, an essential precursor of the coenzyme THF (see p. 108). Transport antibiotics (top center) have the properties of ion channels (see p. 222). When they are deposited in the plasma membrane, it leads to a loss of ions that damages the bacterial cells. 

Thus sulfonamides are bacteriostatic drugs that inhibit bacterial growth by interfering with the microbial synthesis of folic acid. More specifically, sulfonamides block the biosynthetic pathway of folic acid synthesis, thus competitively inhibiting the transformation of p-aminobenzoic acid to folic acid (mediated by the enzyme dihydropteroate synthetase), which allows them to be considered as antimetabolites. 

Both the sulfonamides and trimethoprim interfere with bacterial folate metabolism. For purine synthesis tetrahydrofolate is required. It is also a cofactor for the methylation of various amino acids. The formation of dihydrofolate from para-aminobenzoic acid (PABA) is catalyzed by dihydropteroate synthetase. Dihydrofolate is further reduced to tetrahydrofolate by dihydrofolate reductase. Micro organisms require extracellular PABA to form folic acid. Sulfonamides are analogues of PABA. They can enter into the synthesis of folic acid and take the place of PABA. They then competitively inhibit dihydrofolate synthetase resulting in an accumulation of PABA and deficient tetrahydrofolate formation. On the other hand trimethoprim inhibits dihydrofolate... 

Mecfianism of Action An antibiotic that is a competitive antagonist of para-ami-nobenzoicacid (PABA) it prevents normal bacterial utilization of PABA for synthesis of folic acid. Therapeutic Effect Inhibits bacterial growth. 

Mectianism of Action Interferes with synthesis of folic acid that bacteria require for growth by inhibition of para-aminobenzoic acid metabolism. Therapeutic Effect Prevents further bacterial growth. 

Sulfa drugs have a close structural resemblance to PABA. When taken by a person suffering from a bacterial infection, a sulfa drug is transformed by the body to the compound sulfanilamide, which attaches to the bacterial receptor sites designed for PABA, as shown in Figure 14.7, thereby preventing the synthesis of folic acid. Without folic acid, the bacteria soon die. The patient, however, because he or she receives folic acid from the diet, lives on. 

Another group of inhibitors prevents nucleotide biosynthesis indirectly by depleting the level of intracellular tetrahydrofolate derivatives. Sulfonamides are structural analogs of p-aminobenzoic acid (fig. 23.19), and they competitively inhibit the bacterial biosynthesis of folic acid at a step in which p-aminobenzoic acid is incorporated into folic acid. Sulfonamides are widely used in medicine because they inhibit growth of many bacteria. When cultures of susceptible bacteria are treated with sulfonamides, they accumulate 4-carboxamide-5-aminoimidazole in the medium, because of a lack of 10-formyltetrahydrofolate for the penultimate step in the pathway to IMP (see fig. 23.10). Methotrexate, and a number of related compounds inhibit the reduction of dihydrofolate to tetrahydrofolate, a reaction catalyzed by dihydrofolate reductase. These inhibitors are structural analogs of folic acid (see fig. 23.19) and bind at the catalytic site of dihydrofolate reductase, an enzyme catalyzing one of the steps in the cycle of reactions involved in thymidylate synthesis (see fig. 23.16). These inhibitors therefore prevent synthesis of thymidylate in replicating... 

The sulfonamides include sulfadiazine, sulfamethizole, and similar agents (see Table 33-4). Sulfonamides interfere with bacterial nucleic acid production by disrupting folic acid synthesis in susceptible bacteria. Sulfonamide drugs are structurally similar to PABA, which is the substance used in the first step of folic acid synthesis in certain types of bacteria (see Fig. 33-2). Sulfonamides either directly inhibit the enzyme responsible for PABA utilization or become a substitute for PABA, which results in the abnormal synthesis of folic acid. In either case, folic acid synthesis is reduced, and bacterial nucleic acid synthesis is impaired. 

Unfortunately, there are few pure examples of true selective toxicity. Perhaps the best is penicillin. The therapeutic specificity of this antibiotic is based upon the qualitative difference between bacterial cell wall synthesis and mammalian cell membrane synthesis. Synthesis of the former can be inhibited by penicillin while the latter is unaffected. Thus, penicillin is one of the few examples of a drug that can actually cure an illness. A similar example involves the sulfa drugs, which interfere with the synthesis of folic acid, used in nucleic acid formation, in bacteria. While bacteria must synthesize their own folic acid, mammalian cells utilize dietary, preformed folic acid and are not susceptible to interference with its formation. 

Figure 1.12. Stractures of the sulfonamide drag prontosil rubrum , its antibacterially active metabolite sulfanilamide, and the bacterial metabolite p-Aminobenzoic acid. Sulfanilamide acts as an antimetabolite (i.e., competitive inhibitor) in the synthesis of folic acid, of which aminobenzoic acid is a component...

Pyrimethamine and trimethoprim reversibly inhibit the second step in the synthesis of folic acid by inhibiting the enzyme dihydrofolate reductase, which catalyzes the reduction of dihydrofolic acid to tetrahydrofolic acid. The trimethoprim-binding affinity is much stronger for the bacterial enzyme than the corresponding mammalian enzyme, which produces selective toxicity. A powerful synergism exists between either pyrimethamine or trimethoprim and sulfonamides (e g., sulfemethoxazole and trimethoprim) because of sequential blockage of the same biosynthetic pathway. 

Sulfonamides act by competitively antagonizing bacterial use of p-aminobenzoic acid, which is important in the synthesis of folic acid. Therefore, organisms dependent on self-... 

Sulfamethoxazole like other sulfonamides is bacteriostatic. It acts by inhibiting formation of tetrahydrofolic acid from PABA, thus preventing bacterial cell synthesis of folic acid (see Figure 90). 

Chemicals that inhibit the growth of microbes are called antimetabolites. The sulfanilamides are antimetabolites for those bacteria that require p-aminobenzoic acid. The sulfanilamides apparently inhibit those enzymatic steps of the bacteria that are involved in the synthesis of folic acid. The bacterial enzymes are apparently unable to distinguish between a molecule of a sulfanilamide and a molecule of p-aminobenzoic acid thus, sulfanilamide inhibits the bacterial enzyme. Because the microorganism is unable to synthesize enough folic acid when sulfanilamide is present, it dies. Humans are unaffected by sulfanilamide therapy because we derive our folic acid from dietary sources (folic acid is a vitamin) and do not synthesize it from p-aminobenzoic acid. 

Pharmacology Aminosalicylic acid is bacteriostatic against Mycobacterium tuberculosis. It inhibits the onset of bacterial resistance to streptomycin and isoniazid. The mechanism of action has been postulated to be inhibition of folic acid synthesis (but without potentiation with antifolic compounds) or inhibition of synthesis of the cell wall component, mycobactin, thus reducing iron uptake by M. tuberculosis. 

B. Overproduction (A) of PABA is one of the resistance mechanisms of sulfonamides. Changes in the synthesis of DNA gyrases (B) is a well-described mechanism for quinolone resistance. Plasmid-mediated resistance (C) does not occur with quinolones. An active efflux system for transport of drug out of the cell has been described for quinolone resistance, but it is not plasmid mediated. Inhibition of structural blocks (D) in bacterial cell wall synthesis is a basic mechanism of action of p-lactam antibiotics. Inhibition of folic acid synthesis (E) by blocking different steps is the basic mechanism of action of sulfonamides. 

The mode of action of the sulfonamides as antagonists of 4-aminobenzoic acid (PAB) is well documented, as is the effect of physicochemical properties of the sulfonamide molecule, e.g. pK, on potency (B-81MI10802). Sulfonamides compete with PAB in the biosynthesis of folic acid (44), a vital precursor for several coenzymes found in all living cells. Mammalian cells cannot synthesize folic acid (44), and rely on its uptake as an essential vitamin. However, bacteria depend on its synthesis from pteridine precursors, hence the selective toxicity of sulfonamides for bacterial cells. Sulfonamides may compete with PAB at an enzyme site during the assembly of folic acid (44) or they may deplete the pteridine supply of the cell by forming covalently-bonded species such as (45) or they may replace PAB as an enzyme substrate to generate coupled products such as (46) which are useless to the cell. 

Several antibacterial drugs inhibit bacterial nucleic acid synthesis by inhibiting the production of folic acid.17 Folic acid serves as an enzymatic cofactor in a number of reactions, including synthesis of bacterial nucleic acids and certain essential amino acids. The pathway for synthesis of these folic acid cofactors is illustrated in Figure 33-2. Certain antibacterial drugs block specific steps in the folate pathway, thus impairing the production of this enzymatic cofactor and ultimately impairing the production of nucleic acids and... 

Mechanism of Action. Pyrimethamine blocks the production of folic acid in susceptible protozoa by inhibiting the function of the dihydrofolate reductase enzyme. Folic acid helps catalyze the production of nucleic and amino acids in these parasites. Therefore, this drug ultimately impairs nucleic acid and protein synthesis by interfering with folic acid production. The action of sulfadoxine and other sulfonamide antibacterial agents was discussed in Chapter 33. These agents also inhibit folic acid synthesis in certain bacterial and protozoal cells. 

A closer look at these events reveals that bacteria synthesize folic acid using several enzymes, including one called dihydropteroate synthetase, which catalyzes the attachment of p-aminobenzoic acid to a pteridine ring system. When sulfanilamide is present it competes with the p-amino-benzoic acid (note the structural similarity) for the active site on the enzyme. This activity makes it a competitive inhibitor. Once this site is occupied on the enzyme, folic acid synthesis stops and bacterial growth stops. Folic acid can also be synthesized in the laboratory. ... 

Folic acid is a pteridine derivative (rings A and B constitute the pteridine heterocyclic system) synthesized by bacteria from GTP, p-aminobenzoic acid, and glutamic acid. Accordingly. the structure of folic acid is compased of three moieties the pteridine moiety derived from GTP. the p-aminobcnzoic acid moiety, and the glutamic acid moiety. (Antibacterial sulfonamides [see Chapter 8 compete with p-aminobenzoic acid and, thereby, interfere with bacterial folic acid synthesis.) Humans cannot synthesize folic acid. 

Co-trimoxazole (Septrin) is a well-known combination of a sulfonamide (sulfamethoxazole) with trimethoprim. This combination inhibits enzymes at two points of folic acid (32.2) utilisation - the sulfonamide inhibits incorporation of p-aminobenzoic acid during bacterial folic acid synthesis, and trimethoprim inhibits its conversion into tetrahydrofolate. The overall result is synergistic, i.e. there is a greater activity than the sum of the two components. 

Sulphonamides inhibit an enzyme, dihydropteroate synthatase, used in the production of folic acid in many bacterial cells. Folic acid is the starting point for a cofactor necessary for DNA synthesis in both bacterial and human cells. However, in susceptible bacterial cells, folic acid must be synthesized, whereas in humans it is provided preformed in the diet. 

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