IMP dehydrogenase

Although the stmctures of ribavirin and selenazofutin are similar, they appear to exert their antiviral action at different enzyme sites along the same biochemical pathway. Selenazofutin forms the nicotinamide adenosiae dinucleotide (NAD) analogue, which inhibits IMP dehydrogenase by binding ia place of the NAD cofactor, and hence this potent reduction of guanylate pools is responsible for the antiviral effect of selenazofutin. 

We saw in Chapter 3 that bisubstrate reactions can conform to a number of different reaction mechanisms. We saw further that the apparent value of a substrate Km (KT) can vary with the degree of saturation of the other substrate of the reaction, in different ways depending on the mechanistic details. Hence the determination of balanced conditions for screening of an enzyme that catalyzes a bisubstrate reaction will require a prior knowledge of reaction mechanism. This places a necessary, but often overlooked, burden on the scientist to determine the reaction mechanism of the enzyme before finalizing assay conditions for HTS purposes. The importance of this mechanistic information cannot be overstated. We have already seen, in the examples of methotrexate inhibition of dihydrofolate, mycophenolic acid inhibiton of IMP dehydrogenase, and epristeride inhibition of steroid 5a-reductase (Chapter 3), how the [5]/A p ratio can influence one s ability to identify uncompetitive inhibitors of bisubstrate reactions. We have also seen that our ability to discover uncompetitive inhibitors of such reactions must be balanced with our ability to discover competitive inhibitors as well. 

In the second control mechanism, exerted at a later stage, an excess of GMP in the cell inhibits formation of xanthylate from inosinate by IMP dehydrogenase, without affecting the formation of AMP (Fig. 22-35). Conversely, an accumulation of adenylate inhibits formation of adenylosuccinate by adenylosuccinate synthetase, without affecting the biosynthesis of GMP. In the third mechanism, GTP is required in the conversion of IMP to AMP (Fig. 22-34, step (T)), whereas ATP is required for conversion of IMP to GMP (step (4)), a reciprocal arrangement that tends to balance the synthesis of the two ribonucleotides. 

Chen, J. L., Gerwick, W. H., Schatzman, R., and Laney, M., Isorawsonol and related IMP dehydrogenase inhibitors from the tropical green alga Avrainvillea rawsonii, J. Nat. Prod., 57, 947, 1994. 

The conversion of IMP to AMP and GMP is shown in Figure 10.8. Note that GTP is required for the biosynthesis of AMP and ATP for GMP biosynthesis, and high-ATP levels channel the conversion of IMP to GMP. In addition, IMP dehydrogenase is inhibited by GMP. We therefore have additional loci for controlling cellular concentrations of purine nucleotides by regulating the fate of IMP. The conversion of nucleoside monophosphates to di- and triphosphates is discussed later. 

The enzymatic activity of amido phosphoribosyltransferase (P-Rib-PP— PR A) is low and flux through the de novo pathway in vivo is regulated by the end-products, AMP, IMP and GMP. Inhibition of reaction 1 by dihydrofolate polyglutamates would signal the unavailability of /V1()-formyl tetrahydrofolate, required as a substrate at reactions 3 and 9 of the pathway. The purine pathway is subject to further regulation at the branch point from IMP XMP is a potent inhibitor of IMP cyclohydrolase (FAICAR—> IMP), AMP inhibits adenylosuccinate synthetase (IMP—> sAMP) and GMP inhibits IMP dehydrogenase (IMP— XMP). 

Fig. 15-16 The de novo purine biosynthetic pathway. Rib-5-P, ribose 5-phosphate P-Rib-PP, 5-phosphoribosyl 1-pyrophosphate PRA, 5-phosphoribosylamine IO-CHO-FH4, /Vl0-formyl tetrahydrofolate GAR, glycineamide ribotide FGAR. /V-formylglycineamide ribotide FGAM, /V-formylglycineamidine ribotide AIR, 5-aminoimidazole ribotide CAIR, 4-carboxy-5-aminoimidazole ribotide SAICAR, iV-succino-5-aminoimidazole-4-carboxamide ribotide AICAR, 5-aminoimidazole-4-carboxamide ribotide FAICAR, 5-formamidoimidazole-4-carboxamide ribotide sAMP, /V-succino-AMP. Enzymes (1) amido phosphoribosyltransferase (2) GAR synthetase (3) GAR transformylase (4) FGAM synthetase (5) AIR synthetase (6) AIR carboxylase (7) SAICAR synthetase (8) adenylosuecinase (9) AICAR transformylase (10) IMP cyclohydrolase (11) sAMP synthetase (12) adenylosuecinasc (13) IMP dehydrogenase (14) GMP synthetase.

Kohler GA, White T, Agabian N Overexpression of a cloned IMP dehydrogenase gene of Candida albicans confers resistance to the specific inhibitor mycophenolic acid. J Bacteriol 1997 179 2331-2338. 

G. D. Markham, Monovalent Cation Activation of IMP Dehydrogenase, Inosine Monophosphate Dehydrogenase A Major Therapeutic Target , K. W. Pankiewicz and B. M. Goldstein eds., ACS Symposium, series 839, Washington, DC, 2003, p. 169. 

AMP is a competitive inhibitor (see "Enzymes Catalysis and Kinetics" Lecture) of Adenylosuccinate Synthetase, GMP competitively inhibits IMP Dehydrogenase. Note GTP is required for AMP syndesis and ATP is required for GMP synthesis, hence there is coordinated regulation of these nucleotides. 

Feedback regulation of the de novo pathway of purine biosynthesis. Solid lines represent metabolic pathways, and broken lines represent sites of feedback regulation. , Stimulatory effect , inhibitory effect. Regulatory enzymes A, PRPP synthetase B, amidophosphoribosyltransferase C, adenylosuccinate synthetase D, IMP dehydrogenase. 

Mycophenolic acid and ribavarin monophosphate inhibit IMP dehydrogenase and hence GMP synthesis. 

Thioguanine is similar to 6-mercaptopurine in its action. The most active form is 6-thio-GMP, which inhibits guanylate kinase and, at higher concentrations, IMP dehydrogenase. Thio-IMP and thio-GMP also inhibit PRPP amidotransferase. 

Monophosphorylated form inhibits IMP dehydrogenase triphosphate inhibits viral RNA polymerase and end-capping of viral RNA. 

Amantadine blocks the attachment, penetration, and uncoating of influenza virus A zanamivir and oseltamivir inhibit influenza viruses A and B neuraminidase, promoting viral dumping and deceasing the chance of penetration. Ribavirin becomes phosphorylated and inhibits IMP dehydrogenase and RNA polymerase. It is used to treat respiratory syncytial virus, influenza A and B, Lassa fever,... 

Inhibitors for the foregoing enzymes have been furnished elsewhere (in Appendix D of Hoffman, 1999), with the exception of the enzyme UDP kinase. Interestingly, the substance known as nicotinamide, a vitamin, shows up as an inhibitor for IMP dehydrogenase. 

IMP is a branch point between synthesis of GMP and AMP (Figure 22.6). IMP is acted upon by adenylosuccinate synthetase in AMP biosynthesis and by IMP dehydrogenase in GMP biosynthesis. 

The reaction is part of the de novo biosynthetic pathway of purines. IMP dehydrogenase is allosterically inhibited by GMP, the end product of the pathway. 

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