We conducted an exhaustive examination of databases and scholarly literature concerning
Leishmania metabolic pathways. Utilizing the compiled data, we systematically analyzed, arranged, and interlinked the pathways and the corresponding involved proteins, as visualized in
Figure 5. The proteins showcased in
Figure 5 are integral constituents of crucial metabolic pathways, such as glycolysis, PPP, citric acid cycle, among others. They undertake pivotal functions in host infection and immune evasion mechanisms, thus underlining their indispensability for the survival of
Leishmania.[
67] Several proteins exhibit multifaceted roles across distinct metabolic pathways. For instance, Arginase engages in arginine biosynthesis, proline and arginine metabolism, as well as secondary metabolite and antibiotic biosynthesis pathways. [
68]
Furthermore, proteins 3B64, 1LML, and 4P4M are structural macromolecules with diverse roles within the
Leishmania parasite. They do not align with any distinct metabolic pathway and are primarily considered for structural evaluations. It is vital to underscore the importance of these proteins in the context of
Leishmania v, along with their potential implications for future research endeavors aimed at comprehending the parasite’s metabolism and pathogenicity.
Figure 6 also displays correlations between proteins and pathways, concurrently highlighting protein structures and outlining the proteins not presented in
Figure 5. In this illustration, proteins not linked to any specific pathway are denoted with the label "Not included in pathway". Additionally, their EC functions are differentiated by colors; thus, proteins featuring multiple colors denote having two or more EC functions.
4.1. Oxidoreductases (EC 1)
The group of oxidoreductase enzymes identified in this study plays a crucial role in catalyzing oxidation-reduction reactions, where a substrate donates electrons to an electron-withdrawing species, resulting in the substrate being oxidized [
70]. These enzymes are essential for the Glycolysis and Pentose Phosphate Pathways (PPP), which are key metabolic pathways in Trypanosomatids, including
Leishmania parasites [
71].
Glucose and other hexoses are vital nutrients for
Leishmania parasites, and these parasites, whether in the promastigote or amastigote form, can acquire glucose from the extracellular environment or synthesize it through gluconeogenesis [
72]. The promastigote parasites are capable of both glucose uptake and gluconeogenesis, while the amastigote parasites mainly rely on the gluconeogenic pathway for their glucose requirements [
72]. These findings highlight the importance of oxidoreductase enzymes in the metabolic processes of
Leishmania parasites, particularly in relation to glucose metabolism and energy production. Understanding the role of these enzymes and the specific pathways involved can provide valuable insights for developing targeted interventions against
Leishmania infections.
Glyceraldehyde 3-phosphate dehydrogenase from
Leishmania (major) mexicana, represented by PDB IDs 1GYP and 1A7K (see
Figure 7), is an oxidoreductase enzyme that plays a role in both the Pentose Phosphate Pathway and glycolysis metabolic pathways [
71]. This enzyme is a crucial component of glycolysis and is a homotetrameric protein with a molecular weight of 156 kDa. It catalyzes the conversion of glyceraldehyde 3-phosphate (G3P) to 1,3-biphosphoglycerate (1,3-BPG), simultaneously reducing NAD
to NADH using the NAD
cofactor [
73,
74]. Another important enzyme involved in the gluconeogenic process is malate dehydrogenase, represented by PDB ID 4H7P. This enzyme participates in the conversion of oxaloacetate (OAA) to malate and vice versa, utilizing the NAD/NADH coenzyme system [
75,
76]. It is worth noting that 4H7P has two isoforms in eukaryotes, which differ in their subcellular localization and their specificity for the coenzymes NAD (all types of malate dehydrogenases) or NADP (only malate dehydrogenases from chloroplast cells) [
76]. These enzymes, glyceraldehyde 3-phosphate dehydrogenase and malate dehydrogenase, play vital roles in
Leishmania’s metabolic pathways, contributing to the conversion of key substrates and the production of important intermediates in glycolysis and gluconeogenesis.
Dihydrofolate Reductase-Thymidylate Synthase (DHFR-TS), represented by PDB ID 3INV, is an enzyme found in
Leishmania major. It combines the functions of two enzymes, dihydrofolate reductase (DHFR) and thymidylate synthase (TS), in a single polypeptide chain. DHFR-TS is involved in the salvage pathway of folate compounds in trypanosomatid parasites, including
Leishmania. These parasites are unable to synthesize folate de novo and rely on the salvage pathway to obtain folate compounds for their metabolic processes. Folate compounds are crucial for various metabolic pathways in
Leishmania, including nucleic acid and protein biosynthesis [
77,
78].
Pteridine reductase (PTR1), represented by PDB ID 1E92, is an enzyme found in
Trypanosoma cruzi, another member of the Trypanosomatidae family. PTR1 is also involved in the salvage pathway of folate compounds. It plays a crucial role in the metabolism of folate and pteridine derivatives in trypanosomatids, including
Leishmania. The salvage of folate compounds through PTR1 is essential for the normal metabolic processes of
Leishmania parasites, as they are auxotrophic for folate and require these compounds for nucleic acid and protein biosynthesis [
77,
78].
Both DHFR-TS and PTR1 are important enzymes in the salvage pathways of folate compounds in
Leishmania parasites. Their functions are critical for the synthesis of essential biomolecules and the survival of the parasites. Understanding the structure and function of these enzymes can provide valuable insights for the development of therapeutic strategies against
Leishmania infections. The structure of the DHFR enzyme varies significantly between host organisms and parasites, leading to the development of specific DHFR inhibitors known as antifolates. These inhibitors have been utilized as therapeutic agents against parasites, including Leishmania infections [
79].
Dihydroorotate dehydrogenase (DHODH), represented by PDB ID 3GYE, is a flavoprotein enzyme involved in the de novo biosynthesis pathway of pyrimidines. The pyrimidine biosynthesis pathway in Leishmania parasites is crucial for various cellular processes, such as DNA and RNA biosynthesis, protein glycosylation, membrane lipid biosynthesis, and DNA strand break repair [
80]. DHODH enzymes are classified into two major classes: class 1 (A and B) and class 2, based on their subcellular localization. Class 1 proteins are found in the cytosol and are present in Gram-positive bacteria, anaerobic yeast
Saccharomyces cerevisiae, and all trypanosomatid species, including Leishmania parasites [
80,
81]. The protein represented by PDB ID 3GYE belongs to the class 1 DHODH enzymes and catalyzes the oxidation of (S)-dihydroorotate to orotate in a redox reaction [
81]. Understanding the structure and function of DHODH can provide insights into the development of drugs targeting this enzyme, which could be potential therapeutics for Leishmania infections.
Eukaryotic unicellular kinetoplastid flagellates, such as
Trypanosoma and
Leishmania species, possess a unique hypermodified base in their nuclear DNA known as J base or
-D-glucosylhydroxymethyl-uracil [
82,
83,
84]. The presence of the J base is associated with the silencing of telomeric expression sites for variant surface glycoprotein (VSG) genes, except in actively transcribed VSG genes [
82,
83]. The biosynthesis and maintenance of the J base in these parasites rely on the JBP1 protein, which is essential for the growth and survival of
Leishmania parasites. JBP1 contains a DNA-binding JBP1 domain (DB-JBP1), represented by PDB ID 2XSE, that specifically binds to J-DNA. This interaction between DB-JBP1 and J-DNA makes JBP1 a potential target for drug development. Understanding the structure and function of DB-JBP1 and its role in J base biosynthesis and maintenance can provide valuable insights for the development of drugs aimed at disrupting this process and inhibiting the growth and survival of
Leishmania parasites [
85,
86]. Targeting JBP1 could offer new therapeutic strategies for combating
Leishmania infections.
Macrophages employ various defense mechanisms against
Leishmania parasites, including the production of toxic compounds such as peroxynitrite, hydroxyl radicals, hydrogen peroxide, hydroperoxide, superoxide radicals, and others [
87,
88,
89]. These compounds can disrupt the parasite’s metabolism and hinder its survival. However,
Leishmania parasites have developed a trypanothione-mediated hydroperoxide metabolism to counteract the effects of oxidative agents, whether endogenous or exogenous [
90]. Tryparedoxin (represented by PDB ID: 3S9F) and Tryparedoxin peroxidase I (represented by PDB ID: 3TUE) (TXN/TXNPx) are proteins that play a crucial role in reducing macrophage-generated hydroperoxides to water [
91]. These proteins exist in a cytosolic form and are involved in the detoxification pathway, which is essential for the parasite’s survival [
91]. Pseudoperoxidase
L. major (LmPP) (PDB ID: 5VIA) is a heme protein expressed by
Leishmania parasites as a defense mechanism against Reactive Nitrogen Species (RNS) [
92,
93]. Heme peroxidases, such as LmPP, utilize peroxides to oxidize various physiologically important molecules. Ascorbate peroxidase (APX) (PDB ID: 3RIV) is another example of a heme peroxidase and is considered a potential drug target [
92].
Understanding the structure and function of these proteins involved in the detoxification pathways can provide insights for developing novel therapeutic approaches to disrupt the parasite’s defense mechanisms and enhance the efficacy of anti-
Leishmania treatments [
90,
91,
92,
93]. Superoxide dismutase enzyme (FeSODA) (PDB ID: 4F2N) protects the
Leishmania parasite against macrophage toxic radicals. When the amastigote is phagocytized, macrophage cells produce a respiratory burst generating Reactive Oxygen Species (ROS) intermediates, such as hydrogen peroxide, OH
, O
radicals, and peroxynitrite, as part of the macrophage mechanism to fight invasive microorganisms [
94,
95,
96]. Thus, 4F2N acts as the first line of defense against those ROS. This fact makes 4F2N a suitable enzymatic target for
Leishmania drug development [
94].
Trypanothione reductase (TR) (PDB ID: 2YAU) is a homodimeric enzyme extracted from
Leishmania infantum and is essential for parasite survival [
97]. This enzyme catalyzes the reduction of trypanothione by NADPH, protecting the parasite against oxidative damage and toxic heavy metals [
97,
98,
99]. TR reduces trypanothione disulfide (TS
), a bis(
-L-glutamyl-L-cysteinylglycine) spermidine or bis(glutathionyl) spermidine conjugate, to the di-thiol form [T(SH)
] [
98].
The enzyme 9,11-endoperoxide prostaglandin H2 reductase from
L. major (PDB ID: 4F40) is involved in the lipid metabolic pathway, acting through an NADP cofactor [
100,
101]. Sterol 14
-demethylase (CYP51) from
L. infantum (PDB ID: 3L4D) is an enzyme that catalyzes the removal of the 14
-methyl group from sterol precursors. This reaction is essential for membrane cell biosynthesis, specifically, CYP51 is involved in the ergosterol pathway and is believed to be decisive for the survival of
Leishmania parasites [
102]. Understanding the structures and functions of these enzymes involved in parasite defense mechanisms, oxidative stress response, and essential metabolic pathways provides valuable insights for the design of novel therapeutic strategies against
Leishmania infections [
94,
95,
96,
97,
100,
102].
4.2. Transferases Group (EC. 2)
Transferases are enzymes that catalyze reactions involving the transfer of a chemical group from an electron/proton donor substrate to an electron/proton withdrawing substrate [
70]. These proteins play crucial roles in various cellular processes across all kingdoms of life, including DNA repair, RNA editing, and activation of metabolites in catabolic pathways [
104]. In the case of
Leishmania parasites, the purine nucleotide salvage process is vital for their viability and growth, as these parasites are strict purine nucleotide auxotrophs [
105,
106]. To acquire purine nucleotides,
Leishmania parasites employ a protein arsenal and molecular mechanisms [
106,
107]. Adenine phosphoribosyltransferase (APRT) (PDB ID: 1QB7 and 1MZV) belongs to the phosphoribosyltransferase family type I (PRTs) [
108] and is involved in the purine salvage process. APRT catalyzes the conversion of adenine to adenosine-5-monophosphate (AMP) [
106,
107].
Protein fatty acylation is a common form of protein modification, with myristoylation being one of the most prevalent types [
109]. Myristoylation plays roles in targeting proteins to membrane locations, stabilizing protein structures, mediating protein-protein interactions, and substrate activation [
109,
110]. Proteins destined for myristoylation typically start their primary sequence with a Methionine-Glycine (Met-Gly) dipeptide, where the initiator methionine residue is removed by methionine aminopeptidase, and a myristate molecule is linked via an amide bond [
109].
N-myristoyltransferase (NMT) protein (PDB ID: 4UCM) is responsible for the co-translational transfer of myristic acid (myristate) from myristoyl-CoA to the N-terminal glycine of target proteins [
110,
111]. NMT plays a crucial role in Leishmania parasites and is considered a potential drug target [
112]. Understanding the structures and functions of transferases, such as APRT and NMT, involved in purine salvage and protein modification processes, respectively, can provide insights for the development of targeted therapies against Leishmania infections.
The
de novo pyrimidine biosynthesis pathway is a series of six enzymatic steps that lead to the synthesis of Uridine 5’-monophosphate (UMP). The final two steps of this pathway are mediated by the enzymes Orotate Phosphoribosyltransferase (OPRT) and Orotidine 5’-monophosphate decarboxylase (OMPDC) [
113,
114]. In
Leishmania parasites, these two enzymes are expressed as a bifunctional protein known as UMP synthase [
114]. Crystal structures of the bifunctional UMP synthase protein (PDB ID: 3QW4) and the OMP decarboxylase single protein (PDB ID: 3QW3) are available in the PDB database. Trypanothione plays a crucial role in maintaining intracellular redox homeostasis and providing defense against oxidative stress in
Leishmania parasites [
115,
116,
117]. The biosynthesis of trypanothione indirectly depends on the availability of cysteine, and the
de novo biosynthesis pathway of cysteine relies on the enzymes Serine Acetyltransferase and Cysteine Synthase (PDB ID: 4AIR) [
115,
118].
Amastigote
Leishmania parasites survive and proliferate within the phagolysosome vacuole, which exposes them to extreme acidic conditions and various toxic compounds [
119]. These harsh host cell conditions cause high levels of DNA damage in the parasite [
120]. Therefore, DNA Polymerase Beta (PDB ID: 4P4M) plays an essential role in
Leishmania parasites for the maintenance, replication, and recombination of DNA [
120]. This protein is particularly required in the amastigote form of the parasite [
120]. Understanding the structures and functions of enzymes involved in pyrimidine biosynthesis, trypanothione metabolism, and DNA repair can provide insights into the development of targeted therapies against
Leishmania infections.
In
Leishmania parasites, both pyrimidine and purine nucleotides can be obtained through
de novo biosynthesis and salvage pathways [
121]. However, it is important to note that
Leishmania species are incapable of synthesizing purines
de novo [
121].
Thymidine kinase (TK) (PDB ID: 4UXH) is an essential enzyme that initiates the pyrimidine salvage process in
Leishmania parasites [
122]. This enzyme catalyzes the transfer of the
-phosphate group from ATP to 2’-deoxythymidine (dThd), forming thymidine monophosphate (dTMP). TK plays a key role in the parasitization process and is an important target for drug development [
123]. Farnesyl pyrophosphate synthase (FPPS) (PDB ID: 4JZX) is involved in ergosterol synthesis and is responsible for the early steps of isoprene synthesis, as well as the maintenance of lipid bilayer integrity in
Leishmania parasites [
124,
125]. FPPS is considered a potential enzymatic target, as it has been successfully inhibited with bisphosphonates [
124].
UDP-glucose pyrophosphorylase
L. major (UGP) (PDB ID: 4J18) is an enzyme involved in the glycolytic pathway of
Leishmania parasites [
104]. It catalyzes the reversible conversion of glucose-1-phosphate (Glc-1-P) and uridine 5’-triphosphate (UTP) to UDP-glucose (UDP-Glc) and inorganic pyrophosphate (PPi) in the presence of Mg
. UGP plays a role in carbohydrate metabolism and is important for the synthesis of essential glycoconjugates in the parasite [
104]. Understanding the pathways involved in nucleotide biosynthesis and the enzymes associated with these processes can provide valuable insights for the development of targeted therapies against
Leishmania infections.
Aspartate aminotransferase (AAT) (PDB ID: 4H51) is an enzyme that catalyzes the reversible transfer of the
-amino group between aspartate and glutamate [
126]. Specifically, it converts L-aspartate and 2-oxoglutarate to oxaloacetate and L-glutamate. This enzymatic reaction is essential for amino acid metabolism and the synthesis of key molecules in the cell. AAT relies on the pyridoxal phosphate (PLP) cofactor, which plays a crucial role in the catalytic mechanism of the enzyme [
126]. PLP acts as a coenzyme, facilitating the transfer of the amino group between the amino acid substrates. The dependence of AAT on PLP makes it an attractive target for drug development, as inhibiting its activity can disrupt amino acid metabolism and lead to detrimental effects on the parasite. Targeting AAT with specific inhibitors can interfere with the normal functioning of
Leishmania parasites, disrupting their ability to synthesize essential molecules and causing metabolic imbalances. Consequently, AAT represents a potential drug target for the development of novel therapies against
Leishmania infections [
127].
Sulfurtransferases are a diverse family of enzymes that play important roles in sulfur metabolism in both prokaryotes and eukaryotes. One member of this family is 3-mercaptopyruvate sulfurtransferase (PDB ID: 1OKG), which is involved in cysteine metabolism. This enzyme catalyzes the transfer of a sulfur atom from 3-mercaptopyruvate to various sulfur acceptor molecules, polarizing the carboxyl group of 3-mercaptopyruvate through a nucleophilic attack [
128,
129]. Transketolase (PDB ID: 1R9J) is another important enzyme involved in carbohydrate metabolism. It plays a key role in the nonoxidative branch of the pentose phosphate pathway (PP pathway). Transketolase transfers two-carbon glycolaldehyde units from ketose donors to aldose acceptor sugars, facilitating the interconversion of different sugar phosphates [
130]. Pyruvate kinase (PDB ID: 1PKL) is an enzyme involved in the glycolytic pathway, specifically in the final step of glycolysis. It catalyzes the transfer of a phosphate group from phosphoenolpyruvate to adenosine diphosphate (ADP), generating pyruvate and ATP. This reaction is crucial for energy production in cells [
131].
These enzymes, sulfurtransferase, transketolase, and pyruvate kinase, have important roles in various metabolic pathways and are essential for the proper functioning of cells. Understanding their structures and functions provides insights into their mechanisms of action and their potential as targets for drug development and therapeutic interventions.
4.3. Hydrolases Group (EC. 3)
Hydrolases are a class of enzymes that catalyze hydrolysis reactions, where a bond in a substrate molecule is cleaved by the addition of a water molecule, resulting in the formation of two product fragments [
70]. In the case of the
Leishmania parasite, which cannot synthesize purines de novo, the Purine Nucleoside Hydrolase of
L. major (PDB ID: 1EZR) plays a crucial role in salvaging nucleotides from the host [
132]. This enzyme is responsible for hydrolyzing purine nucleosides to release the purine base and ribose or deoxyribose [
132]. Due to its essential role in nucleotide metabolism, Purine Nucleoside Hydrolase is considered a potential drug target for
Leishmania [
132].
Leishmanolysin (gp63 gene) is a glycoprotein expressed on the surface of
Leishmania promastigotes, and its structure is represented by PDB ID: 1LML [
133]. This enzyme plays a critical role in the infection of macrophages by
Leishmania parasites, mediating the degradation of host extracellular matrix proteins and evading the immune response [
133]. Consequently, Leishmanolysin is considered a potential drug target for combating
Leishmania infections [
133]. Peptidases, a family of enzymes, have diverse functions in metabolic pathways, host invasion, and immune evasion by parasites [
134]. Oligopeptidase B (OPB) of
L. major (PDB ID: 2XE4) is a peptidase enzyme known to be an important virulence factor [
135,
136]. It plays a role in the degradation of host proteins and the processing of parasite virulence factors [
135,
136].
Leucyl aminopeptidase (LAP) of
L. major (PDB ID: 5NTH) is another peptidase enzyme involved in the catalysis of N-terminal residues of proteins [
137]. It participates in the hydrolysis of peptide bonds, contributing to the processing and turnover of proteins in
Leishmania [
137]. These hydrolase enzymes, including Purine Nucleoside Hydrolase, Leishmanolysin, Oligopeptidase B, and Leucyl aminopeptidase, have significant roles in the metabolism, virulence, and survival of
Leishmania parasites. Understanding their structures and functions can aid in the development of drugs and interventions targeted against
Leishmania infections.
Tyrosine phosphatase PRL-1 (PDB ID: 3S4O) is primarily secreted by the promastigote form of
Leishmania parasites. However, during the macrophage infection process, the amastigote form produces PRL-1 more efficiently and abundantly, which is crucial for the parasite’s survival [
138]. This enzyme is involved in the dephosphorylation of tyrosine residues in protein substrates, regulating various signaling pathways and cellular processes [
138]. The increased production of PRL-1 by amastigotes suggests its importance in the adaptation and virulence of
Leishmania during the intracellular infection of macrophages [
138].
Arginase (PDB ID: 4ITY) is an enzyme that catalyzes the hydrolysis of arginine to urea and ornithine. It plays a critical role in the first step of polyamine biosynthesis, which is essential for cellular growth and proliferation [
139]. Polyamines are involved in various biological processes, including DNA synthesis, cell division, and differentiation [
139]. In
Leishmania, arginase is crucial for the production of polyamines, which are required for the parasite’s survival and virulence [
139]. Both tyrosine phosphatase PRL-1 and arginase are important enzymes for the survival and adaptation of
Leishmania parasites. They contribute to key cellular processes and are potential targets for the development of interventions against
Leishmania infections.