Cytochrome b5 from Giardia lamblia

Giardia duodenalis flavohemoglobin is a target of 5-nitroheterocycle and benzimidazole compounds acting as enzymatic inhibitors or subversive substrates

Giardia duodenalis causes giardiasis in humans, companion, livestock and wild animals. Control of infection involves drugs as benzimidazoles (e.g., albendazole, ABZ) and 5-nitroheterocyclics [5-NHs: metronidazole (MTZ), furazolidone (FZD), nitazoxanide (NTZ)] as first-line agents. During infection, Giardia is exposed to immune and pro-oxidant host responses involving nitric oxide (NO). In Giardia, NO is detoxified by a flavohemoglobin (gFlHb), a heme-containing enzyme which is absent in mammals. gFlHb has NO dioxygenase and NADH oxidase activities converting NO into nitrate and producing a superoxide anion (O2•-) that causes oxidative stress and parasite death. The modulation of gFlHb activities may provide novel approaches for treatment of giardiasis. We investigated the capacity of selected benzimidazole-2-carbamates (BZCs: ABZ, oxibendazole, nocodazole), non-BZCs (thiabendazole), an ehtylphenylcarbamate (LQM-996) and 5-NHs (MTZ, NTZ, FZD and some derivatives) to bind to recombinant gFlHb at the heme group, modifying NADH consumption activity and/or inducing ROS production. Of these, BZCs and NTZ bind to heme and increased O2•- production (i.e. caused enzyme subversion), whereas MTZ binds to heme but inhibited NADH consumption. LQM-996 decreased NADH consumption and two out of four NTZ derivatives altered NADH oxidase activity. In silico docking and molecular dynamics studies suggested the interaction of distinct drug moieties in ABZ and NTZ with gFlHb sites involved in NADH and NO catalysis. These findings provide new insights on gFlHb as a novel target of BZCs, MTZ and NTZ, and provides a useful platform to assess the compounds binding capacity to gFlHb prior to experimental and clinical trials in giardiasis.

Assessment of Drug Activities against Giardia Using Hyperspectral Raman Microscopy

This study demonstrates the capability of Raman microscopy for detecting structural differences in Giardia cells exposed to different drugs and incubation times. While metronidazole (MTZ) visibly affects the cells by inducing extracellular vesicle releases of toxic iron intermediates and modified triple-bond moieties, oseltamivir (OSM) alters the phenylalanine and lipid structures. Modifications in the heme protein environment and the transformation of iron from ferric to ferrous observed for both drug treatments are more notable for MTZ. Different contents and amounts of vesicle excretion are detected for 24 h or 48 h with MTZ incubation. At a shorter drug exposure, releases of altered proteins, glycogen, and phospholipids dominate. Agglomerates of transformed iron complexes from heme proteins and multiple-bond moieties prevail at 48 h of treatment. No such vesicle releases are present in the case of OSM usage. Drug incorporations into the cells and their impact on the plasma membrane and the dynamics of lipid raft confirmed by confocal fluorescence microscopy reveal a more destructive extent by OSM, corroborating the Raman results. Raman microscopy provides a broader understanding of the multifaceted factors and mechanisms responsible for giardiasis treatment or drug resistance by enabling a label-free, simultaneous monitoring of structural changes at the cellular and molecular levels.

Giardia duodenalis: Flavohemoglobin is involved in drug biotransformation and resistance to albendazole

Giardia duodenalis causes giardiasis, a major diarrheal disease in humans worldwide whose treatment relies mainly on metronidazole (MTZ) and albendazole (ABZ). The emergence of ABZ resistance in this parasite has prompted studies to elucidate the molecular mechanisms underlying this phenomenon. G. duodenalis trophozoites convert ABZ into its sulfoxide (ABZSO) and sulfone (ABZSOO) forms, despite lacking canonical enzymes involved in these processes, such as cytochrome P450s (CYP450s) and flavin-containing monooxygenases (FMOs). This study aims to identify the enzyme responsible for ABZ metabolism and its role in ABZ resistance in G. duodenalis. We first determined that the iron-containing cofactor heme induces higher mRNA expression levels of flavohemoglobin (gFlHb) in Giardia trophozoites. Molecular docking analyses predict favorable interactions of gFlHb with ABZ, ABZSO and ABZSOO. Spectral analyses of recombinant gFlHb in the presence of ABZ, ABZSO and ABZSOO showed high affinities for each of these compounds with Kd values of 22.7, 19.1 and 23.8 nM respectively. ABZ and ABZSO enhanced gFlHb NADH oxidase activity (turnover number 14.5 min-1), whereas LC-MS/MS analyses of the reaction products showed that gFlHb slowly oxygenates ABZ into ABZSO at a much lower rate (turnover number 0.01 min-1). Further spectroscopic analyses showed that ABZ is indirectly oxidized to ABZSO by superoxide generated from the NADH oxidase activity of gFlHb. In a similar manner, the superoxide-generating enzyme xanthine oxidase was able to produce ABZSO in the presence of xanthine and ABZ. Interestingly, we find that gFlHb mRNA expression is lower in albendazole-resistant clones compared to those that are sensitive to this drug. Furthermore, all albendazole-resistant clones transfected to overexpress gFlHb displayed higher susceptibility to the drug than the parent clones. Collectively these findings indicate a role for gFlHb in ABZ conversion to its sulfoxide and that gFlHb down-regulation acts as a passive pharmacokinetic mechanism of resistance in this parasite.

A cytosolic ferredoxin-independent hydrogenase possibly mediates hydrogen uptake in Trichomonas vaginalis

Trichomonads, represented by the highly prevalent sexually transmitted human parasite Trichomonas vaginalis, are anaerobic eukaryotes with hydrogenosomes in the place of the standard mitochondria. Hydrogenosomes form indispensable FeS-clusters, synthesize ATP, and release molecular hydrogen as a waste product. Hydrogen formation is catalyzed by [FeFe] hydrogenase, the hallmark enzyme of all hydrogenosomes found in various eukaryotic anaerobes. Eukaryotic hydrogenases were originally thought to be exclusively localized within organelles, but today few eukaryotic anaerobes are known that possess hydrogenase in their cytosol. We identified a thus-far unknown hydrogenase in T. vaginalis cytosol that cannot use ferredoxin as a redox partner but can use cytochrome b5 as an electron acceptor. Trichomonads overexpressing the cytosolic hydrogenase, while maintaining the carbon flux through hydrogenosomes, show decreased excretion of hydrogen and increased excretion of methylated alcohols, suggesting that the cytosolic hydrogenase uses the hydrogen gas as a source of reducing power for the reactions occurring in the cytoplasm and thus accounts for the overall redox balance. This is the first evidence of hydrogen uptake in a eukaryote, although further work is needed to confirm it. Assembly of the catalytic center of [FeFe] hydrogenases (H-cluster) requires the activity of three dedicated maturases, and these proteins in T. vaginalis are exclusively localized in hydrogenosomes, where they participate in the maturation of organellar hydrogenases. Despite the different subcellular localization of cytosolic hydrogenase and maturases, the H-cluster is present in the cytosolic enzyme, suggesting the existence of an alternative mechanism of H-cluster assembly.

Reduction potential and heme-pocket polarity in low potential cytochrome b5 of Giardia intestinalis

Although it lacks mitochondria and the ability to synthesize heme, the protozoan parasite Giardia intestinalis encodes several heme proteins. This includes four members of the cytochrome b5 family, three of which are of similar size to mammalian cytochromes b5 but with reduction potentials that are 140 to 180mV lower. While no structures have yet been determined for any of these proteins, homology modeling points to an increase in heme pocket polarity as a reason for their low potentials. To test this we measured the reduction potentials of four mutants of Giardia cytochrome b5 isotype-I (gCYTB5-I) in which polar residues at two candidate positions (C84, Y51) in the heme pocket were changed to nonpolar ones (C84A, C84F; Y51L, Y51F). All mutants were expressed with comparable levels of heme incorporation and had UV-visible spectra consistent with low spin bis-histidyl coordination. These mutations increased the reduction potential by 18 to 57mV and highlight the influence of C84, which is a residue unique to gCYTB5-I and whose mutation to alanine caused the largest increase. The influence of these two residues plus that of Y61 reported previously accounts for much of the reduction potential difference between gCYTB5-I and microsomal cytochrome b5. A complementary triple mutant of the latter with the hydrophilic residues found in gCYTB5-I bound heme less effectively but nonetheless had a reduction potential that was 135mV lower than wild type.

Heme proteins of Giardia intestinalis

Among the few organisms that cannot make the iron cofactor heme, some nonetheless possess heme proteins. This includes the protozoan parasite Giardia intestinalis, which encodes five known heme proteins: a flavohemoglobin and four members of the cytochrome b5 family. Giardia flavohemoglobin closely resembles those of the Enterobacteriaceae in structure and function, acting as a nitric oxide dioxygenase that is induced when trophozoites are exposed to reactive nitrogen species. The Giardia cytochromes b5 are soluble proteins having relatively low reduction potentials and lack several features that are expected to promote rapid electron transfer with redox partners. Only one potential electron donor, and no electron acceptors, have yet been identified in the Giardia genome, and the roles of these cytochromes are presently unknown. The answer may lie in the sequences that flank the heme-binding core of these proteins which could serve to localize them within the cell through reversible post-translational modifications and to promote specific protein-protein interactions.

Giardia intestinalis incorporates heme into cytosolic cytochrome b₅

The anaerobic intestinal pathogen Giardia intestinalis does not possess enzymes for heme synthesis, and it also lacks the typical set of hemoproteins that are involved in mitochondrial respiration and cellular oxygen stress management. Nevertheless, G. intestinalis may require heme for the function of particular hemoproteins, such as cytochrome b5 (cytb5). We have analyzed the sequences of eukaryotic cytb5 proteins and identified three distinct cytb5 groups: group I, which consists of C-tail membrane-anchored cytb5 proteins; group II, which includes soluble cytb5 proteins; and group III, which comprises the fungal cytb5 proteins. The majority of eukaryotes possess both group I and II cytb5 proteins, whereas three Giardia paralogs belong to group II. We have identified a fourth Giardia cytb5 paralog (gCYTb5-IV) that is rather divergent and possesses an unusual 134-residue N-terminal extension. Recombinant Giardia cytb5 proteins, including gCYTb5-IV, were expressed in Escherichia coli and exhibited characteristic UV-visible spectra that corresponded to heme-loaded cytb5 proteins. The expression of the recombinant gCYTb5-IV in G. intestinalis resulted in the increased import of extracellular heme and its incorporation into the protein, whereas this effect was not observed when gCYTb5-IV containing a mutated heme-binding site was expressed. The electrons for Giardia cytb5 proteins may be provided by the NADPH-dependent Tah18-like oxidoreductase GiOR-1. Therefore, GiOR-1 and cytb5 may constitute a novel redox system in G. intestinalis. To our knowledge, G. intestinalis is the first anaerobic eukaryote in which the presence of heme has been directly demonstrated.