Folic acid biosynthesis inhibition

Bacteria require p-aminobenzoic acid to biosyn thesize folic acid a growth factor Structurally sul fanilamide resembles p-aminobenzoic acid and is mistaken for it by the bacteria Folic acid biosynthesis IS inhibited and bacterial growth is slowed suffi ciently to allow the body s natural defenses to effect a cure Because animals do not biosynthesize folic acid but obtain it in their food sulfanilamide halts the growth of bacteria without harm to the host... 

Folic acid is vital for both humans and bacteria. Bacteria synthesize this compound, but humans are unable to synthesize it and, consequently, obtain the necessary amounts from the diet, principally from green vegetables and yeast. This allows selectivity of action. Therefore, sulfa drugs are toxic to bacteria because folic acid biosynthesis is inhibited, whereas they produce little or no ill effects in humans. The structural relationships between carboxylic acids and sulfonic acids that we have observed in rationalizing chemical reactivity are now seen to extend to some biological properties. 

Competitive inhibitors bind to specific groups in the enzyme active site to form an enzyme-inhibitor complex. The inhibitor and substrate compete for the same site, so that the substrate is prevented from binding. This is usually because the substrate and inhibitor share considerable stmctural similarity. Catalysis is diminished because a lower proportion of molecules have a bound substrate. Inhibition can be relieved by increasing the concentration of substrate. Some simple examples are shown below. Thus, sulfanilamide is an inhibitor of the enzyme that incorporates j9-aminobenzoic acid into folic acid, and has antibacterial properties by restricting folic acid biosynthesis in the bacterium (see Box 11.13). Some phenylethylamine derivatives, e.g. phenelzine, provide useful antidepressant drags by inhibiting the enzyme monoamine oxidase. The cA-isomer maleic acid is a powerful inhibitor of the enzyme that utilizes the trans-isomer fumaric acid in the Krebs cycle. 

Another class of drugs that has proven clinically valuable against malaria are the antifolates. Sulfonamides such as pyridoxine and sulfones like dapsone inhibit the plasmodial enzyme dihydropteroate synthetase on the pathway of folic acid biosynthesis (48). The 2,4-... 

These are pyrimidine derivatives and are effective because of differences in susceptibility between the enzymes in humans and in the infective organism. Anticancer agents based on folic acid, e.g. methotrexate, inhibit dihydrofolate reductase, but they are less selective than the antimicrobial agents and rely on a stronger binding to the enzyme than the natural substrate has. They also block pyrimidine biosynthesis. Methotrexate treatment is potentially lethal to the patient, and is usually followed by rescue with folinic acid (A -formyl-tetrahydrofolic acid) to counteract the folate-antagonist action. The rationale is that folinic acid rescues normal cells more effectively than it does tumour cells. 

This substance inhibits the growth of bacteria by interfering with the synthesis of folic acid, 7, which is an essential substance for bacteria and animals alike. However, animals acquire folic acid from a normal diet, whereas bacteria have to synthesize it. Biosynthesis of folic acid is blocked by 4-aminobenzenesulfonamide, probably because of the structural similarity of the sulfonamide to 4-aminobenzoic acid, which is a normal ingredient in the biosynthesis of folic acid. The enzyme system involved apparently substitutes the sulfonamide for... 

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... 

FIGURE 33-2 Folic acid metabolism in bacterial cells. Certain antibacterial drugs [e.g., sulfonamides and trimethoprim] inhibit the dihydrofolate synthetase and reductase enzymes, thus interfering with DNA biosynthesis. 

Trimethoprim (Proloprim, Trimpex) interferes with the bacterial folic acid pathway by inhibiting the dihydrofolate reductase enzyme in susceptible bacteria (see Fig. 33-2). This enzyme converts dihydrofolic acid to tetrahydrofolic acid during the biosynthesis of folic acid cofactors. By inhibiting this enzyme, trimethoprim directly interferes with the production of folic acid cofactors, and subsequent production of vital bacterial nucleic acids is impaired. 

A wide range of compounds also inhibit a number of the enzyme systems that are involved in the biosynthesis of purines and pyrimidines in bacteria. For example, sulphonamide bacteriostatics inhibit dihydropteroate synthetase, which prevents the formation of folic acid in both humans and bacteria. However, although both mammals and bacteria synthesize their folic acid from PABA (Figure 7.12), mammals can also obtain it from their diet. In contrast, trimethoprim specifically inhibits bacterial DHF, which prevents the conversion... 

Tetrahydrofolic acid (THF) is a coenzyme in the synthesis of purine bases and thymidine. These are constituents of DNA and RNA and are required for cell growth and replication. Lack of THF leads to inhibition of cell proliferation. Formation of THF from dihydrofolate (DHF) is catalyzed by the enzyme dihydrofolate reductase. DHF is made from folic acid, a vitamin that cannot be synthesized in the body but must be taken up from exogenous sources. Most bacteria do not have a requirement for folate, because they are capable of synthesizing it-more precisely DHF-ffom precursors. Selective interference with bacterial biosynthesis of THF can be achieved with sulfonamides and trimethoprim. 

The sulfonamides act as competitive enzyme inhibitors and block the biosynthesis of the vitamin folic acid in bacterial cells (Fig. 10.14). They do this by inhibiting the... 

In summary, DNA synthesis requires synthesis of dTMP and the purines adenine and guanine. THF, derived from the vitamin folic acid, is required for the biosynthesis of these nucleotides. Treatment with methotrexate blocks the cell s ability to regenerate THF, leading to inhibition of these biosynthetic pathways. The lack of nucleotides prevents DNA synthesis, and these cancer cells cannot divide without DNA synthesis. Unfortunately, the effects of methotrexate are nonspecific and other rapidly dividing cells such as epithelial cells in the oral cavity, intestine, skin, and blood cells are also inhibited. This leads to the side effects associated with methotrexate (and other cancer chemotherapy drugs) such as mouth sores, low white blood cell counts, stomach upset, hair loss, skin rashes, and itching. 

The sulfonamides are bacteriostatic when administered to humans in achievable doses. They inhibit the enzyme dihydropteroate synthase, an important enzyme needed for the biosynthesis of folic acid derivatives and, ultimately, the thymidine required for DNA. They do this by competing at the active site with... 

A further step in the pathway leading from the pteroates to folic acid and on to DNA bases requires the enzyme dihydrofolate reductase. Exogenous folic acid must be reduced stepwise to dihydrofolic acid and then to tetrahydrofolic acid, an important cofactor essential for supplying a 1-carbon unit in thymidine biosynthesis and, ultimately, for DNA synthesis (Fig. 38.5). The same enzyme also must reduce endogenously produced dihydrofolate. Inhibition of this key... 

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