Dihydropteroate, synthase

Ligases (syn. synthases) Dihydropteroate synthase Sulphonamides (inhibitors)... 

These organisms are typically devoid of dihydrofolate reductase. However, a recent study showed that the yii/P gene of Helicobacter pylori a bifunctional dihydropteroate synthase/dihydropteroate reductase... 

In E. coli GTP cyclohydrolase catalyzes the conversion of GTP (33) into 7,8-dihydroneoptetin triphosphate (34) via a three-step sequence. Hydrolysis of the triphosphate group of (34) is achieved by a nonspecific pyrophosphatase to afford dihydroneopterin (35) (65). The free alcohol (36) is obtained by the removal of residual phosphate by an unknown phosphomonoesterase. The dihydroneoptetin undergoes a retro-aldol reaction with the elimination of a hydroxy acetaldehyde moiety. Addition of a pyrophosphate group affords hydroxymethyl-7,8-dihydroptetin pyrophosphate (37). Dihydropteroate synthase catalyzes the condensation of hydroxymethyl-7,8-dihydropteroate pyrophosphate with PABA to furnish 7,8-dihydropteroate (38). Finally, L-glutamic acid is condensed with 7,8-dihydropteroate in the presence of dihydrofolate synthetase. 

Overproduction of the chromosomal genes for the dihydrofolate reductase (DHFR) and the dihydroptero-ate synthase (DHPS) leads to a decreased susceptibility to trimethoprim and sulfamethoxazol, respectively. This is thought to be the effect of titrating out the antibiotics. However, clinically significant resistance is always associated with amino acid changes within the target enzymes leading to a decreased affinity of the antibiotics. 

Mouillon, J.M. et al., Folate synthesis in higher-plant mithocondria coupling between the dihydropterin pyrophosphokinase and the dihydropteroate synthase activities, Biochem. J., 363, 313, 2002. 

The answer is c. (Hardman, pp 1058-1059. Katzung, pp 793-795.) Trimethoprim inhibits dihydro folic acid reductase. Sulfamethoxazole inhibits p-aminobenzoic acid (PABA) from being incorporated into folic acid by competitive inhibition of dihydropteroate synthase. Either action inhibits the synthesis of tetrahydrofolic acid. 

Dihydrojasmone, 14 598 Dihydrolinalool, 24 502 Dihydromyrcene, 24 487, 488 Dihydromyrcenol, 24 487, 488, 495 Dihydroperoxides geminal, 18 455-456 thermal decomposition of, 18 455 Di(hydroperoxyalkyl) peroxides, 18 459 Dihydropteroate synthase (DHPS),... 

Genes encoding phosphotransferases confer resistance to streptomycin Genes encoding a drug-resistant dihydropteroate synthase enzyme required for folate biosynthesis confer resistance to sulfonamide Tetracycline... 

Mammals must obtain their tetrahydrofolate requirements from their diet, but microorganisms are able to synthesize this material. This offers scope for selective action and led to the use of sulfanilamide and other antibacterial sulfa drugs, compounds that competitively inhibit the biosynthetic enzyme (dihydropteroate synthase) that incorporates p-aminobenzoic acid into the structure (see Box 7.23). 

DIHYDROOROTATE DEHYDROGENASE DIHYDROOROTATE OXIDASE DIHYDROPTERIDINE REDUCTASE DIHYDROPTEROATE SYNTHASE DIHYDROPYRIMIDINASE DIHYDROURACIL DEHYDROGENASE Dihydroxyacetone kinase,... 

Both sulfonamides and trimethoprim (not a sulfonamide) sequentially interfere with folic acid synthesis by bacteria. Folic acid functions as a coenzyme in the transfer of one-carbon units required for the synthesis of thymidine, purines, and some amino acids and consists of three components a pteridine moiety, PABA, and glutamate (Fig. 44.1). The sulfonamides, as structural analogues, competitively block PABA incorporation sulfonamides inhibit the enzyme dihydropteroate synthase, which is necessary for PABA to be incorporated into dihydropteroic acid, an intermediate compound in the formation of folinic acid. Since the sulfonamides reversibly block the synthesis of folic acid, they are bacteriostatic drugs. Humans cannot synthesize folic acid and must acquire it in the diet thus, the sulfonamides selectively inhibit microbial growth. 

E) Structural changes in dihydropteroate synthase and overproduction of PABA. 

Inhibit folic acid synthesis by competitive inhibition of dihydropteroate synthase. 

Mammalian cells (and some bacteria) lack the enzymes required for folate synthesis from PABA and depend on exogenous sources of folate therefore, they are not susceptible to sulfonamides. Sulfonamide resistance may occur as a result of mutations that (1) cause overproduction of PABA, (2) cause production of a folic acid-synthesizing enzyme that has low affinity for sulfonamides, or (3) impair permeability to the sulfonamide. Dihydropteroate synthase with low sulfonamide affinity is often encoded on a plasmid that is transmissible and can disseminate rapidly and widely. Sulfonamide-resistant dihydropteroate synthase mutants also can emerge under selective pressure. 

Pyrimethamine and proguanil selectively inhibit plasmodial dihydrofolate reductase, a key enzyme in the pathway for synthesis of folate. Sulfonamides and sulfones inhibit another enzyme in the folate pathway, dihydropteroate synthase. As described in Chapter 46 and shown in Figure 46-2, combinations of inhibitors of these two enzymes provide synergistic activity. 

In many areas, resistance to folate antagonists and sulfonamides is common for P falciparum and less common for P vlvax. Resistance is due primarily to mutations in dihydrofolate reductase and dihydropteroate synthase, with increasing numbers of mutations leading to increasing levels of resistance. At present, resistance seriously limits the efficacy of sulfadoxine-pyrimethamine (Fansidar) for the treatment of malaria in most areas, but in Africa most parasites exhibit only moderate resistance, such that antifolates appear to continue to offer preventive efficacy against malaria. Because different mutations may mediate resistance to different agents, cross-resistance is not uniformly seen. 

Intracellular protozoa of the phylum Apicomplexa such as plasmodium, toxoplasma, and eimeria have long been known to respond to sulfonamides and sulfones. This has led to the assumption that Apicomplexa must synthesize their own folate in order to survive. The reaction of 2-amino-4-hydroxy-6-hydroxymethyl-dihydropteridine diphosphate with />aminobenzoate to form 7,8-dihydropteroate has been demonstrated in cell-free extracts of the human malaria parasite Plasmodium falciparum. 2-Amino-4-hydroxy-6-hydroxymethyl-dihydropteridine pyrophosphokinase and 7,8-dihydropteroate synthase have also been identified. Sulfathiazole, sulfaguanidine, and sulfanilamide act as competitive inhibitors of p-aminobenzoate. It has not been possible to demonstrate dihydrofolate synthase activity in the parasites, which raises the possibility that 7,8-dihydropteroate may have substituted for dihydrofolate in malaria parasites. Similar lack of recognition of folate as substrate was also observed in the dihydrofolate reductase of Eimeria tenella, a parasite of chickens. 

The gene encoding 7,8-dihydropteroate synthase was cloned from P falciparum and found to encode a bifunctional enzyme that includes the pyrophosphokinase at the amino terminal of the protein. Discrepancies were observed in the sequences of 7,8-dihydropteroate synthase portion of the genes from sulfadoxine-sensitive versus sulfadoxine-resistant P falciparum, thus confirming that this enzyme is the target for the antimalarial sulfonamide drugs. 

The sulfones and sulfonamides synergize with the inhibitors of dihydrofolate reductase, and the combinations have been effective in controlling malaria, toxoplasmosis, and coccidiosis. Fansidar, a combination of sulfadoxine and pyrimethamine, has been successful in controlling some strains of chloroquine-resistant Plasmodium falciparum malaria (see Chapter 53 Antiprotozoal Drugs). However, reports of Fansidar resistance have increased in recent years. New inhibitors effective against the sulfonamide-resistant 7,8-dihydropteroate synthase are needed. 

The pharmacologic properties of parasite 7,8-dihydropteroate synthases may differ from those of the bacterial enzymes. For instance, metachloridine and 2-ethoxy-p-aminobenzoate are both ineffective against sulfonamide-sensitive bacteria, but the former has antimalarial activity and the latter is effective against infection by the chicken parasite Eimeria acervulina both activities can be reversed by p-aminobenzoate. 

HG Vinnicombe, JP Derrick. Dihydropteroate synthase from Streptococcus pneumoniae. characterization of substrate binding order and sulfonamide inhibition. Bio-chem Biophys Res Commun 258 752-757, 1999. 

Finally, sulfonamides can interfere with intermediary metabolism. Because of their structural similarity to para-aminobenzoic acid (PABA), they can function as competitive inhibitors for dihydropteroate synthase. The result is interruption of microbial synthesis of folic acid by blocking formation of the folic acid precursor dihydropteroic acid. Sensitive microorganisms are those that must synthesize their own folic acid. Conversely, resistant bacteria and normal mammalian cells are unaffected since they do not synthesize folic acid but use the preformed vitamin. 

Figure 10.2. Biosynthesisoffolicacidandtetrahydrobiopterin.GTPcyclohydrolasel.EC 3.5.4.16 dihydropteroate synthase, EC 2.5.1.15 pyruvoyl-tetrahydrobiopterin synthase, EC 4.6.1.10 and sepiapterin reductase, EC 1.1.1.153.

An example of prediction results for sulfathiazole is shown in Figure 6.6. This substance was found in SAR Base and was excluded from the SAR Base on prediction of its activity spectrum. The known (contained in SAR Base of PASS version 2007) activity spectrum includes the following activities antibacterial, antibiotic, dihydropteroate synthase inhibitor, iodide peroxidase inhibitor. In Figure 6.6 the predicted activity spectrum includes 65 of 374 pharmacological effects, 176 of 2755 molecular mechanisms, 7 of 50 side effects and toxicity, 11 of 121 metabolism terms at default Pa> Pi cutting points. All activities included in the SAR Base are predicted with Pa> Pi- The activity of as... 

Dihydropteroate synthase inhibitor Iodide peroxidase inhibitor... 

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