**2.7 Cysteine synthase (CS, O-acetylserine sulfhydrylase, OASS, EC 2.5.1.47)**

Cysteine biosynthesis is a potential target for antileishmanial drug development. The structure of *L*. *major* cysteine synthase was revealed in 2012 by Fyfe et al. [145]. Cyclic imide derivatives were identified with a multitarget profile including TOPOI, *N*-myristoyltransferase, cyclophilin, and CS enzymes using *in silico* approach and *L. amazonensis* activity of the compounds were reported [146].

### **2.8 Oligopeptidase B (OPB, EC 3.4.21.83)**

It was found out that a high level of serine protease activity was expressed by *L. donovani*, which was explained by an increase in OPB enzyme activity [147]. The crystal structure of *L*. *major* OPB was revealed in 2010 by McLuskey et al. [148]. Epoxy-α-lapachone was shown activity on both promastigote and amastigote forms of *L. amazonensis* in a study exploring natural compounds as potential antileishmanial agents. Moreover, this activity was associated with serine proteinase inhibitory activity of epoxy-α-lapachone in the same study [149]. Peptidic structure ShPI-I (Kunitz-type protease inhibitor from the sea anemone *Stichodactyla helianthus*) was shown to be a potent inhibitor of *L. amazonensis* serine proteases [150].

#### **2.9 Superoxide dismutase (SOD, EC 1.15.1.1)**

SOD enzyme was found in *L. tropica* by Meshnick and Eaton and it was suggested that the enzyme may be containing iron (Fe) which causes a difference from its host's enzymes which is linked to a copper or zinc atom [151]. Later, molecular isolation and characterization of Fe containing SOD cDNAs of *L. chagasi* were reported in 1997 [152] and the 3D structure of Fe-dependent superoxide dismutases (FeSODs) from *L*. *major* was reported [153].

In a study, imidazole-containing phthalazine derivatives were found to be potent inhibitors of Fe-SOD with antileishmanial properties. Additionally, the tested compounds were selective toward parasite Fe-SOD over human CuZn-SOD [154]. Arylamine Mannich base derivatives, known to be effective against *Trypanosoma cruzi*, were exhibited remarkable activity against *Leishmania* species. The mechanism of action of these compounds was linked to their potent Fe-SOD inhibition [155].

2-Iminothiazole derivatives [156], scorpiand-like azamacrocycles [157, 158], pyrazole-containing polyamine macrocycles [159], natural product momordicatin [ethyl 2-(4-hydroxybutyl)benzoate] [160], imidazole or pyrazole-based benzo [g]

#### *Toward New Antileishmanial Compounds: Molecular Targets for Leishmaniasis Treatment DOI: http://dx.doi.org/10.5772/intechopen.101132*

phthalazine derivatives [161], triphenyl tin salicylanilide thiosemicarbazone [162], Se containing aromatics and heteroaromatic compounds [163], ruthenium complexes with purine analogs [164], fisetin—a flavanoid anolog [57] and dialkyl pyrazole-3,5-dicarboxylates [165] were reported as SOD inhibitors exhibiting antileishmanial activity in the literature.

### **2.10 Nitroreductases (NTR, EC 1.7.1.16)**

Nitroreductase enzymes catalyze the reduction of nitro/nitroaromatic compounds. Based on oxygen sensitivity, NTRs are divided into two groups: NTR1 is oxygen-insensitive and functions via a series of two-electron reductions, NTR2 is oxygen-sensitive and mediated a one-electron reduction [166]. NTR1 enzyme is found mainly in bacteria and absent in most eukaryotes. Keeping this in mind, *L*. *major* NTR1 (*Lm*NTR) was characterized and identified as a potential drug target for leishmaniasis [167].

It was reported that aziridinyl nitrobenzamide compounds [168], nitroquinolinone derivatives [169], 3-nitro-2-(phenylsulfonylmethyl) imidazo[1,2-a]pyridine derivatives [170], and nitro-heteroaryl nitrone derivatives [172] are NTR inhibitors with antileishmanial effects.

#### **2.11 Nucleoside hydrolases (NH, EC 3.2.2.1)**

Koszalka and Krenitsky, separated and purified three nucleoside hydrolases from promastigotes of *L. donovani*—purine 2′-deoxyribonucleosidase, purine ribonucleosidase, and pyrimidine ribonucleosidase [172]. Then, the X-Ray structure and amino acid sequence of nucleoside hydrolase from *L. major* was revealed alongside its several nanomolar transition state inhibitors [39].

Augustyns's research group design and synthesize various compounds and tested against IAG-NH (inosine-adenosine-guanosine nucleoside hydrolase) from *Tabanus vivax*. In contrast to promising enzyme activity of the compounds, antileishmanial activity of the compounds hasn't been investigated [41, 173, 174]. Freitas et al. also tested immucillin derivatives against *L. donovani*, *L. inf. Chagasi* and *L. amazonensis* parasites [175].

It was found out that hydroxychromenone and tetrahydrocyclohexanecarboxylic acid fragments could bind to the enzyme in a fragment-based analysis on *Ld*NH using saturation transfer difference (STD) NMR spectroscopy [176].

In a recent study, a natural product from Brazilian flora, flavonoids, and proanthocyanidins, with antileishmanial activity screened against *Ld*NH and described as an inhibitor of *Ld*NH [43, 177].

Interestingly, *Ld*NH (NH36) is the main area of interest for human recombinant vaccine-based studies and phase I trial of nucleoside hydrolase NH36 of *L. donovani*, the main antigen of the Leishmune® vaccine, and the sterol 24-c-methyltransferase (SMT) from *L. infantum* is in progress [178].

#### **2.12 Cysteine proteases**

There are two cysteine protease genes from *L. major*—one is structurally similar to the cathepsin L (CatL) family and the other is similar to the cathepsin B (CatB) family of cysteine proteases. These cysteine protease enzymes were isolated and sequenced by Sakanari et al. [179].

It is reported that aziridine-2,3-dicarboxylate [180], natural products flavone derivatives [181], trans-aziridine-2,3-dicarboxylate derivatives [182] organotellurane RF07 and palladacycle complex [183–185], and dipeptidyl enoates [186] exhibit antileishmanial effect and inhibit cysteine proteases.

### **2.13 Glyceraldehyde-3-phosphate dehydrogenase (GAPDH, EC 1.2.1.12)**

GAPDH activity was detected in two cell compartments of *Leishmania mexicana* promastigotes [187]. Then, the crystal structure of *L. mexicana* GAPDH in complex with inhibitors was reported to the literature [188].

Although GAPDH enzyme is found in *Leishmania sp*., it is an attractive target for the development of novel antitrypanosomatid agents rather than antileishmanial compounds.

#### **2.14 Dihydroorotate dehydrogenase (DHODH, EC 1.3.5.2)**

DHODH enzyme catalyzes the stereoselective oxidation of (S)-dihydroorotate (DHO) to orotate (ORO) in the *de novo* pyrimidine biosynthetic pathway. The structure of *L*. *major* DHODH was revealed by X-ray diffraction analysis [189]. It was reported that natural compounds from Asteraceae species could inhibit *Lm*DHODH by Chibli et al., though the antileishmanial effect of the compounds has not been evaluated [190].

#### **2.15 Methionyl-tRNA synthetase (MetRS, EC 6.1.1.10)**

Considering the structure of *L*. *major* MetRS, the difference in human cytosolic and mitochondrial MetRS and near the ATP- and methionine-binding regions of *Lm*MetRS promises selectivity for MetRS inhibitors [191].

DDD806905, a known *Tb*MetRS inhibitor, tested against *Ld*MetRS and showed antileishmanial effect upon *Leishmania* axenic amastigote yet, it has not shown efficacy in an animal model of leishmaniasis due to high protein binding as well as sequestration of this dibasic compound into acidic compartments [192]. Researchers have characterized a new series of *Ld*MetRS inhibitors bearing 4,6-diamino-substituted pyrazolopyrimidine core that target a previously undefined, allosteric binding site in the enzyme recently [193].

#### **2.16 Phosphodiesterases (PDE, EC 3.1.4.17)**

Phosphodiesterases control the cellular concentration of the second messengers cAMP and cGMP that are key regulators of several physiological processes.

A correlation between cAMP concentration in *Leishmania* cells and proliferation and transformation is demonstrated. By the addition of phosphodiesterase inhibitors to the culture medium, the intracellular level of cAMP was increased [194].

Crystal structure of the *L*. *major* phosphodiesterase *Lmj*PDEB1, one of the five PDE encoding genes, was reported in 2007 [195].

Isoxazolo[3,4-d]pyridazinone analogs were reported to inhibit PDE extracted from *L*. *mexicana* [196]. Later, it was reported that triphenyl-substituted imidazole compound exhibits *in vitro* antileishmanial and PDE inhibitor activity. Moreover, there was a correlation between *in vitro* antileishmanial activity and cAMP content [197].

#### **2.17 Squalene synthase (SQS, SSN, E.C. 2.5.1.21)**

SQS enzyme catalyzes the first step in sterol biosynthesis. Cloning, expression, and purification of a catalytically active recombinant squalene synthase of *L. donovani* (*Ld*SSN) [198].

Biphenylazabicyclooctanol, biphenylquiniclidine, and quiniclidine derivatives possessing *Lm*SQS inhibitory activity have shown antileishmanial effects against

*Toward New Antileishmanial Compounds: Molecular Targets for Leishmaniasis Treatment DOI: http://dx.doi.org/10.5772/intechopen.101132*

*L. amazonensis*, therefore, SQS might serve as a potential target for antileishmanial drug discovery [199–201].
