Keeping the cofactor content oligomeric state subcellular lo

Keeping the cofactor content, oligomeric state, subcellular localization, and membrane association in view, DHODH has been broadly divided into two classes, class 1 and class 2 [26], [27]. In many species it is present to either cytoplasm (Class 1) or to the inner mitochondrial membrane (or plasma membrane for bacteria) with the catalytic domain oriented towards the inner membrane space (Class 2). The latter one has been found to show detergent-dependent kinetics [28]. Human as well as Plasmodium contain Class 2 mitochondrial enzymes. Class 1 enzymes are soluble cytosolic enzyme and possess cysteine as the catalytic base. This class has been further subdivided into 1A and 1B on the basis of the cofactors involved. The class 1A are homodimeric proteins using fumarate as electron acceptor [29], [30]. It has a subunit encode by PyrDB gene and is found in Lactococcuslactis,[30]Enterococcus faecalis, Saccharomyces cerevisiae[29], [31] and in some eukaryotic parasites. [32]The class 1B unlike 1A is heterodimeric in nature containing catalytic subunits encoded by PyrDB and PyrK genes. The PyrK polypeptides is of the ferreredoxin reductase super family. It uses NAD+ as electron acceptor [33] and a [2Fe-2S] cluster as cofactors. It is absent in gram positive bacteria. L. lactis[34] and E. faecalis[35] contain both class 1A and class 1B DHODH while as of Bacillus[36] and Clostridium[37] possess only class 1A DHODH. The class 2 DHODH is membrane associated protein that uses quinones as electron acceptors. This is found in the cytoplasmic membrane of E. coli and Helicobactr pylori and in the inner mitochondrial membrane of most of the eukaryotes [38].
A spectrum of 2-(3-alkoxy-1H-pyrazol-1-yl) pyrimidine based hDHODH inhibitors (Fig. 14a) have recently been reported to constitute notable antiviral properties than the drugs available in market like brequinar and Teriflunomide [74]. A group of thiazole based hDHODH inhibitors (Fig. 14b) has been recently reported which showed double digit nanomolar activity. These compounds were reported to have significant effect against inflammation in vivo and drug candidates for the treatment of rheumatoid arthritis and significantly alleviated foot swelling in a dose-dependent manner in vivo study [75]. Carbonyl group containing hDHODH inhibitors (Fig. 14c) were found to act as immunosuppressive agents. Its carbonyl part forms hydrogen bond interaction with the water molecule (W444) or hydrophobic residues Gln47 and Arg136. The 4-phenylthiazole moiety fits into the pocket of the enzyme through hydrophobic effects and van der Waals' interaction with Leu42, Met43, Ala59, Phe62, and Leu359 residues. Among these compounds, IOX 1 14d possess a great B-cell inhibitory activity with IC50 value 1.78 μM. Compound 14e was reported to be a significant inhibitor with respect to ubiquinone binding [75].
Inhibitors of bacterial, coccidian of fungi DHODH DHODH inhibitors for various disease causing organism like fungi, bacteria and viruses have also been a matter of concern for the researchers. Hydantoins (Fig. 15a) as DHODH inhibitors were reported to exhibit antibacterial activity especially aganist Clostridium oroticum.[76] Thiadiazolidinedione based bacterial DHODH inhibitors, like compound 15b were synthesized and their property of reducing bacterial growth was confirmed [77]. A number of amide substituted pyrazole derivatives, such as compound 15c were reported to inhibit the ubiquinone binding site of H. pylori DHODH [78]. A tricarbonyl containing compound, for example 15d which also act as Michael acceptor, inhibit bacterial DHODH and thereby shows antibacterial activity [79]. Compound 15e has antifungal effect and inhibit Aspergillus nidulans DHODH [80]. Quinazolinones such as compound 15f also inhibit fungal DHODH [81].
Conclusion A better way to arrest the growth and development of malarial parasite is to target a unique pathway of parasite or a pathway that is highly different from the host. DHODH is an important enzyme involved in the electron transport chain of plasmodium parasite. Survival of the parasite is wholly dependent on DHODH as it has a pivotal role in the de novo pyrimidine biosynthesis. To control the last step of a pathway is more appropriate than the initial steps, since DHODH is the enzyme required at the final stage of the pyrimidine biosynthesis and parasite has no alternative to this pathway. So the inhibition of this enzyme may provide a complete cure to malaria. Various inhibitors of DHODH have been developed to curb malaria. Traizolopyrimidine core structure based inhibitors had a good degree of success. These inhibitors are highly species selective for PfDHODH and have no effect on its closely related species of the same family. Inhibitors containing electron negative atom attached to aromatic rings possess good inhibitors capacity. Among these, fluorine containing molecules have better binding and inhibiting power. This suggest the active site of the enzyme may contain positively charged amino acid residues. Molecules containing small groups like cyclopropyle also facilitated this inhibition. Completely different binding modes for the same inhibitor with hDHODH and PfDHODH were observed, also supports the need of species selective inhibitor development. The active site variation of the two species can be fruitful to target PfDHODH exclusively to eradicate malaria.