Purification and kinetic characterization of recombinant alternative oxidase from Trypanosoma brucei brucei

Alternative Oxidase: From Molecule and Function to Future Inhibitors

In the respiratory chain of the majority of aerobic organisms, the enzyme alternative oxidase (AOX) functions as the terminal oxidase and has important roles in maintaining metabolic and signaling homeostasis in mitochondria. AOX endows the respiratory system with flexibility in the coupling among the carbon metabolism pathway, electron transport chain (ETC) activity, and ATP turnover. AOX allows electrons to bypass the main cytochrome pathway to restrict the generation of reactive oxygen species (ROS). The inhibition of AOX leads to oxidative damage and contributes to the loss of adaptability and viability in some pathogenic organisms. Although AOXs have recently been identified in several organisms, crystal structures and major functions still need to be explored. Recent work on the trypanosome alternative oxidase has provided a crystal structure of an AOX protein, which contributes to the structure-activity relationship of the inhibitors of AOX. Here, we review the current knowledge on the development, structure, and properties of AOXs, as well as their roles and mechanisms in plants, animals, algae, protists, fungi, and bacteria, with a special emphasis on the development of AOX inhibitors, which will improve the understanding of respiratory regulation in many organisms and provide references for subsequent studies of AOX-targeted inhibitors.

Filamentous Fungi-Derived Orsellinic Acid-Sesquiterpene Meroterpenoids: Fungal Sources, Chemical Structures, Bioactivities, and Biosynthesis

Fungi-derived polyketide-terpenoid hybrids are important meroterpenoid natural products that possess diverse structure scaffolds with a broad spectrum of bioactivities. Herein, we focus on an ever-increasing group of meroterpenoids, orsellinic acid-sesquiterpene hybrids comprised of biosynthetic start unit orsellinic acid coupling to a farnesyl group or/and its modified cyclic products. The review entails the search of China National Knowledge Infrastructure (CNKI), Web of Science, Science Direct, Google Scholar, and PubMed databases up to June 2022. The key terms include "orsellinic acid", "sesquiterpene", "ascochlorin", "ascofuranone", and "Ascochyta viciae", which are combined with the structures of "ascochlorin" and "ascofuranone" drawn by the Reaxys and Scifinder databases. In our search, these orsellinic acid-sesquiterpene hybrids are mainly produced by filamentous fungi. Ascochlorin was the first compound reported in 1968 and isolated from filamentous fungus Ascochyta viciae (synonym: Acremonium egyptiacum; Acremonium sclerotigenum); to date, 71 molecules are discovered from various filamentous fungi inhabiting in a variety of ecological niches. As typical representatives of the hybrid molecules, the biosynthetic pathway of ascofuranone and ascochlorin are discussed. The group of meroterpenoid hybrids exhibits a broad arrange of bioactivities, as highlighted by targeting hDHODH (human dihydroorotate dehydrogenase) inhibition, antitrypanosomal, and antimicrobial activities. This review summarizes the findings related to the structures, fungal sources, bioactivities, and their biosynthesis from 1968 to June 2022.

Respiratory complex I in mitochondrial membrane catalyzes oversized ubiquinones

NADH-ubiquinone (UQ) oxidoreductase (complex I) couples electron transfer from NADH to UQ with proton translocation in its membrane part. The UQ reduction step is key to triggering proton translocation. Structural studies have identified a long, narrow, tunnel-like cavity within complex I, through which UQ may access a deep reaction site. To elucidate the physiological relevance of this UQ-accessing tunnel, we previously investigated whether a series of oversized UQs (OS-UQs), whose tail moiety is too large to enter and transit the narrow tunnel, can be catalytically reduced by complex I using the native enzyme in bovine heart submitochondrial particles (SMPs) and the isolated enzyme reconstituted into liposomes. Nevertheless, the physiological relevance remained unclear because some amphiphilic OS-UQs were reduced in SMPs but not in proteoliposomes, and investigation of extremely hydrophobic OS-UQs was not possible in SMPs. To uniformly assess the electron transfer activities of all OS-UQs with the native complex I, here we present a new assay system using SMPs, which were fused with liposomes incorporating OS-UQ and supplemented with a parasitic quinol oxidase to recycle reduced OS-UQ. In this system, all OS-UQs tested were reduced by the native enzyme, and the reduction was coupled with proton translocation. This finding does not support the canonical tunnel model. We propose that the UQ reaction cavity is flexibly open in the native enzyme to allow OS-UQs to access the reaction site, but their access is obstructed in the isolated enzyme as the cavity is altered by detergent-solubilizing from the mitochondrial membrane.

Xenotopic expression of alternative oxidase (AOX) to study mechanisms of mitochondrial disease

The mitochondrial respiratory chain or electron transport chain (ETC) facilitates redox reactions which ultimately lead to the reduction of oxygen to water (respiration). Energy released by this process is used to establish a proton electrochemical gradient which drives ATP formation (oxidative phosphorylation, OXPHOS). It also plays an important role in vital processes beyond ATP formation and cellular metabolism, such as heat production, redox and ion homeostasis. Dysfunction of the ETC can thus impair cellular and organismal viability and is thought to be the underlying cause of a heterogeneous group of so-called mitochondrial diseases. Plants, yeasts, and many lower organisms, but not insects and vertebrates, possess an enzymatic mechanism that confers resistance to respiratory stress conditions, i.e., the alternative oxidase (AOX). Even in cells that naturally lack AOX, it is autonomously imported into the mitochondrial compartment upon xenotopic expression, where it refolds and becomes catalytically engaged when the cytochrome segment of the ETC is blocked. AOX was therefore proposed as a tool to study disease etiologies. To this end, AOX has been xenotopically expressed in mammalian cells and disease models of the fruit fly and mouse. Surprisingly, AOX showed remarkable rescue effects in some cases, whilst in others it had no effect or even exacerbated a condition. Here we summarize what has been learnt from the use of AOX in various disease models and discuss issues which still need to be addressed in order to understand the role of the ETC in health and disease.

Alternative respiratory oxidases to study the animal electron transport chain

Oxidative phosphorylation is a common process to most organisms in which the main function is to generate an electrochemical gradient across the inner mitochondrial membrane (IMM) and to make energy available to the cell. However, plants, many fungi and some animals maintain non-energy conserving oxidases which serve as a bypass to coupled respiration. Namely, the alternative NADH:ubiquinone oxidoreductase NDI1, present in the complex I (CI)-lacking Saccharomyces cerevisiae, and the alternative oxidase, ubiquinol:oxygen oxidoreductase AOX, present in many organisms across different kingdoms. In the last few years, these alternative oxidases have been used to dissect previously indivisible processes in bioenergetics and have helped to discover, understand, and corroborate important processes in mitochondria. Here, we review how the use of alternative oxidases have contributed to the knowledge in CI stability, bioenergetics, redox biology, and the implications of their use in current and future research.

Targeting the alternative oxidase (AOX) for human health and food security, a pharmaceutical and agrochemical target or a rescue mechanism?

Some of the most threatening human diseases are due to a blockage of the mitochondrial electron transport chain (ETC). In a variety of plants, fungi, and prokaryotes, there is a naturally evolved mechanism for such threats to viability, namely a bypassing of the blocked portion of the ETC by alternative enzymes of the respiratory chain. One such enzyme is the alternative oxidase (AOX). When AOX is expressed, it enables its host to survive life-threatening conditions or, as in parasites, to evade host defenses. In vertebrates, this mechanism has been lost during evolution. However, we and others have shown that transfer of AOX into the genome of the fruit fly and mouse results in a catalytically engaged AOX. This implies that not only is the AOX a promising target for combating human or agricultural pathogens but also a novel approach to elucidate disease mechanisms or, in several cases, potentially a therapeutic cure for human diseases. In this review, we highlight the varying functions of AOX in their natural hosts and upon xenotopic expression, and discuss the resulting need to develop species-specific AOX inhibitors.

Antitrypanosomal properties of Anogeissus leiocarpa extracts and their inhibitory effect on trypanosome alternative oxidase

Background

African trypanosomiasis is a protozoan disease with huge socio-economic burden to sub-Saharan African exceeding US$4.6 annual loss. To mitigate the incidence of trypanosomal drug resistance, efforts are geared towards discovery of molecules, especially from natural products, with potential to inhibit important molecular target (trypanosome alternative oxidase, TAO) in trypanosomes that are critical to their survival.

Method

Crude methanol extract of Anogeissus leiocarpa was subjected to in vitro bioassay-guided antitrypanosomal assay to identify the most active extract with trypanocidal activity. The most active extract was run on a column chromatography yielding five fractions, F1-F5. The fractions were assayed for inhibitory effect on TAO. The most promising TAO inhibitor was subjected to antitrypanosomal evaluation by trypanosome count, drug incubation infectivity test (DIIT) and in vivo studies. Gas chromatography-mass spectrometry (GC-MS) was used to identify and quantify phytochemical constituents of the potential TAO-inhibiting fraction.

Results

Ethyl acetate extract (EtOAc) significantly (p<0.05) produced trypanocidal effect and was the most active extract. Of the five fractions, only F4 significantly (p<0.05) inhibited TAO compared to the control. F4 completely immobilised the trypanosomes up to 0.5 μg/μl, yielding an EC₅₀ of 0.024 μg/μl compared to the 0.502 μg/μl of diminazene aceturate positive control group. The DIIT showed that F4 was significantly (p<0.05) potent up to 0.1 μg/μl. F4 significantly (p<0.05) suppressed parasite multiplication in systemic circulation of the treated rats and significantly (p<0.05) maintained high PCV when compared to the 5% DMSO group. Furthermore, F4 significantly (p<0.05) lowered serum concentrations of malondialdehyde. Phytoconstituents identified by the GC-MS include tetradecene; cetene; 3-(benzylthio) acrylic acid, methyl ester; 1-octadecene; 9-heptadecanone; hexadecanoic acid, methyl ester; dibutyl phthalate; eicosene; octadecenoic acid, methyl ester; oleic acid; 2-methyl-Z,Z-3,13-octadecadienol; 1-docosene; 3-phenylthiane, s-oxide; phenol, 3-methyl; phthalic acid, di(2-propylpentyl) ester and 1,4-benzenedicarboxylic acid, bis (2-ethylhexyl) ester.

Conclusion

F4 from EtOAc contains six carbohydrates (9.58%), two free fatty acids (6.48%), five fatty acid esters (27.73%), two aromatic compounds (50.63%) and one organosulphide (5.61%). It inhibited TAO and demonstrated antitrypanosomal effects.

Genetically encoded tools for measuring and manipulating metabolism

Over the past few years, we have seen an explosion of novel genetically encoded tools for measuring and manipulating metabolism in live cells and animals. Here, we will review the genetically encoded tools that are available, describe how these tools can be used and outline areas where future development is needed in this fast-paced field. We will focus on tools for direct measurement and manipulation of metabolites. Metabolites are master regulators of metabolism and physiology through their action on metabolic enzymes, signaling enzymes, ion channels and transcription factors, among others. We hope that this Perspective will encourage more people to use these novel reagents or even join this exciting new field to develop novel tools for measuring and manipulating metabolism.

Partially Purified Leaf Fractions of Azadirachta indica Inhibit Trypanosome Alternative Oxidase and Exert Antitrypanosomal Effects on Trypanosoma congolense

Introduction

Trypanosomiasis is a neglected disease of humans and livestock caused by single-celled flagellated haemo-protozoan parasites belonging to the genus Trypanosoma.

Purpose

Widespread resistance to trypanocidal drugs creates urgent need for new, more effective drugs with potential to inhibit important trypanosome molecular targets.

Methods

Nine column chromatographic, partially purified leaf fractions of Azadirachta indica (AIF) were subjected to trypanosome alternative oxidase (TAO) inhibition assay using ubiquinol oxidase assay. The potent TAO inhibitors were evaluated for trypanocidal activities against T. congolense in rat model using in vitro, ex vivo, and in vivo assays. Complete cessation or reduction in parasite motility was scored from 0 (no parasite) to 6 (greater than or equal to 6 × 10⁷ trypanosomes/milliliter of blood), and was used to evaluate the efficacy of in vitro treatments.

Results

Only AIF1, AIF2, and AIF5 significantly inhibited TAO. AIF1 and AIF5 produced significant, dose-dependent suppression of parasite motility reaching score zero within 1 h with EC₅₀ of 0.005 and 0.004 µg/µL, respectively, while trypanosome-laden blood was still at score six with an EC₅₀ of 44,086 µg/µL. Mice inoculated with the concentrations at scores 0 and 1 (1–2 moribund parasites) at the end of the experiment did not develop parasitaemia. The two fractions significantly (p < 0.05) lowered parasite burden, with the AIF5 exhibiting highest in vivo trypanocidal effects. Packed cell volume was significantly higher in AIF1 (p < 0.05) and AIF5 (p < 0.001) groups compared to DMSO-treated group. Only AIF5 significantly (p < 0.05) lowered malondialdehyde.

Conclusion

AIF1 and AIF5 offer prospects for the discovery of TAO inhibitor(s).

Structural and Biophysical Characterization of Purified Recombinant Arabidopsis thaliana's Alternative Oxidase 1A (rAtAOX1A): Interaction With Inhibitor(s) and Activator

In higher plants, alternative oxidase (AOX) participates in a cyanide resistant and non-proton motive electron transport pathway of mitochondria, diverging from the ubiquinone pool. The physiological significance of AOX in biotic/abiotic stress tolerance is well-documented. However, its structural and biophysical properties are poorly understood as its crystal structure is not yet revealed in plants. Also, most of the AOX purification processes resulted in a low yield/inactive/unstable form of native AOX protein. The present study aims to characterize the purified rAtAOX1A protein and its interaction with inhibitors, such as salicylhydroxamic acid (SHAM) and n-propyl gallate (n-PG), as well as pyruvate (activator), using biophysical/in silico studies. The rAtAOX1A expressed in E. coli BL21(DE3) cells was functionally characterized by monitoring the respiratory and growth sensitivity of E. coli/pAtAOX1A and E. coli/pET28a to classical mitochondrial electron transport chain (mETC) inhibitors. The rAtAOX1A, which is purified through affinity chromatography and confirmed by western blotting and MALDI-TOF-TOF studies, showed an oxygen uptake activity of 3.86 μmol min^-1 mg^-1 protein, which is acceptable in non-thermogenic plants. Circular dichroism (CD) studies of purified rAtAOX1A revealed that >50% of the protein content was α-helical and retained its helical absorbance signal (ellipticity) at a wide range of temperature and pH conditions. Further, interaction with SHAM, n-PG, or pyruvate caused significant changes in its secondary structural elements while retaining its ellipticity. Surface plasmon resonance (SPR) studies revealed that both SHAM and n-PG bind reversibly to rAtAOX1A, while docking studies revealed that they bind to the same hydrophobic groove (Met191, Val192, Met195, Leu196, Phe251, and Phe255), to which Duroquinone (DQ) bind in the AtAOX1A. In contrast, pyruvate binds to a pocket consisting of Cys II (Arg174, Tyr175, Gly176, Cys177, Val232, Ala233, Asn294, and Leu313). Further, the mutational docking studies suggest that (i) the Met195 and Phe255 of AtAOX1A are the potential candidates to bind the inhibitor. Hence, this binding pocket could be a 'potential gateway' for the oxidation-reduction process in AtAOX1A, and (ii) Arg174, Gly176, and Cys177 play an important role in binding to the organic acids like pyruvate.

Comparison of the Kinetic Parameters of Alternative Oxidases From Trypanosoma brucei and Arabidopsis thaliana-A Tale of Two Cavities

The alternative oxidase (AOX) is widespread in plants, fungi, and some protozoa. While the general structure of the AOX remains consistent, its overall activity, sources of kinetic activation and their sensitivity to inhibitors varies between species. In this study, the recombinant Trypanosoma brucei AOX (rTAO) and Arabidopsis thaliana AOX1A (rAtAOX1A) were expressed in the Escherichia coli ΔhemA mutant FN102, and the kinetic parameters of purified AOXs were compared. Results showed that rTAO possessed the highest V _max and K _m for quinol-1, while much lower V _max and K _m were observed in the rAtAOX1A. The catalytic efficiency (k _cat/K _m) of rTAO was higher than that of rAtAOX1A. The rTAO also displayed a higher oxygen affinity compared to rAtAOX1A. It should be noted that rAtAOX1a was sensitive to α-keto acids while rTAO was not. Nevertheless, only pyruvate and glyoxylate can fully activate Arabidopsis AOX. In addition, rTAO and rAtAOX1A showed different sensitivity to AOX inhibitors, with ascofuranone (AF) being the best inhibitor against rTAO, while colletochlorin B (CB) appeared to be the most effective inhibitor against rAtAOX1A. Octylgallate (OG) and salicylhydroxamic acid (SHAM) are less effective than the other inhibitors against protist and plant AOX. A Caver analysis indicated that the rTAO and rAtAOX1A differ with respect to the mixture of polar residues lining the hydrophobic cavity, which may account for the observed difference in kinetic and inhibitor sensitivities. The data obtained in this study are not only beneficial for our understanding of the variation in the kinetics of AOX within protozoa and plants but also contribute to the guidance for the future development of phytopathogenic fungicides.

Kinetic characterisation and inhibitor sensitivity of Candida albicans and Candida auris recombinant AOX expressed in a self-assembled proteoliposome system

Candidemia caused by Candida spp. is a serious threat in hospital settings being a major cause of acquired infection and death and a possible contributor to Covid-19 mortality. Candidemia incidence has been rising worldwide following increases in fungicide-resistant pathogens highlighting the need for more effective antifungal agents with novel modes of action. The membrane-bound enzyme alternative oxidase (AOX) promotes fungicide resistance and is absent in humans making it a desirable therapeutic target. However, the lipophilic nature of the AOX substrate (ubiquinol-10) has hindered its kinetic characterisation in physiologically-relevant conditions. Here, we present the purification and expression of recombinant AOXs from C. albicans and C. auris in a self-assembled proteoliposome (PL) system. Kinetic parameters (Km and Vmax) with respect to ubiquinol-10 have been determined. The PL system has also been employed in dose-response assays with novel AOX inhibitors. Such information is critical for the future development of novel treatments for Candidemia.

Weak O2 binding and strong H2O2 binding at the non-heme diiron center of trypanosome alternative oxidase

Alternative oxidase (AOX) catalyzes the four-electron reduction of dioxygen to water as an additional terminal oxidase, and the catalytic reaction is critical for the parasite to survive in its bloodstream form. Recently, the X-ray crystal structure of trypanosome alternative oxidase (TAO) complexed with ferulenol was reported and the molecular structure of the non-heme diiron center was determined. The binding of O₂ was a unique side-on type compared to other iron proteins. In order to characterize the O₂ binding state of TAO, the O₂ binding states were searched at a quantum mechanics/molecular mechanics (QM/MM) theoretical level in the present study. We found that the most stable O₂ binding state is the end-on type, and the binding states of the side-on type are higher in energy. Based on the binding energies and electronic structure analyses, O₂ binds very weakly to the TAO iron center (ΔE =6.7 kcal mol^-1) in the electronic state of Fe(II)…OO, not in the suggested charge transferred state such as the superoxide state (Fe(III)OO· ^-) as seen in hemerythrin. Coordination of other ligands such as water, Cl^-, CN^-, CO, N₃^- and H₂O₂ was also examined, and H₂O₂ was found to bind most strongly to the Fe(II) site by ΔE = 14.0 kcal mol^-1. This was confirmed experimentally through the measurement of ubiquinol oxidase activity of TAO and Cryptosporidium parvum AOX which was found to be inhibited by H₂O₂ in a dose-dependent and reversible manner.

Molecular characterization and gene expression modulation of the alternative oxidase in a scuticociliate parasite by hypoxia and mitochondrial respiration inhibitors

Philasterides dicentrarchi is a marine benthic microaerophilic scuticociliate and an opportunistic endoparasite that can infect and cause high mortalities in cultured turbot (Scophthalmus maximus). In addition to a cytochrome pathway (CP), the ciliate can use a cyanide-insensitive respiratory pathway, which indicates the existence of an alternative oxidase (AOX) in the mitochondrion. Although AOX activity has been described in P. dicentrarchi, based on functional assay results, genetic evidence of the presence of AOX in the ciliate has not previously been reported. In this study, we conducted genomic and transcriptomic analysis of the ciliate and identified the AOX gene and its corresponding mRNA. The AOX gene (size 1,106 bp) contains four exons and three introns that generate an open reading frame of 915 bp and a protein with a predicted molecular weight of 35.6 kDa. The amino acid (aa) sequence of the AOX includes an import signal peptide targeting the mitochondria and the protein is associated with the inner membrane of the mitochondria. Bioinformatic analysis predicted that the peptide is a homodimeric glycoprotein, although monomeric forms may also appear under native conditions, with EXXH motifs associated with the diiron active centers. The aa sequences of the AOX of different P. dicentrarchi isolates are highly conserved and phylogenetically closely related to AOXs of other ciliate species, especially scuticociliates. AOX expression increased significantly during infection in the host and after the addition of CP inhibitors. This confirms the important physiological roles of AOX in respiration under conditions of low levels of O₂ and in protecting against oxidative stress generated during infection in the host.

Kinetic and structural characterisation of the ubiquinol-binding site and oxygen reduction by the trypanosomal alternative oxidase

The alternative oxidase (AOX) is a monotopic di‑iron carboxylate protein which acts as a terminal respiratory chain oxidase in a variety of plants, fungi and protists. Of particular importance is the finding that both emerging infectious diseases caused by human and plant fungal pathogens, the majority of which are multi-drug resistant, appear to be dependent upon AOX activity for survival. Since AOX is absent in mammalian cells, AOX is considered a viable therapeutic target for the design of specific fungicidal and anti-parasitic drugs. In this work, we have mutated conserved residues within the hydrophobic channel (R96, D100, R118, L122, L212, E215 and T219), which crystallography has indicated leads to the active site. Our data shows that all mutations result in a drastic reduction in V_max and catalytic efficiency whilst some also affected the K_m for quinol and oxygen. The extent to which mutation effects inhibitor sensitivity was also investigated, with mutation of R118 and T219 leading to a complete loss of inhibitor potency. However, only a slight reduction in IC₅₀ values was observed when R96 was mutated, implying that this residue is less important in inhibitor binding. In silico modelling has been used to provide insight into the reason for such changes, which we suggest is due to disruptions in the proton transfer network, resulting in a reduction in overall reaction kinetics. We discuss our results in terms of the structural features of the ubiquinol binding site and consider the implications of such findings on the nature of the catalytic cycle. SIGNIFICANCE: The alternative oxidase is a ubiquinol oxidoreductase enzyme that catalyses the oxidation of ubiquinol and the reduction of oxygen to water. It is widely distributed amongst the plant, fungal and parasitic kingdoms and plays a central role in metabolism through facilitating the turnover of the TCA cycle whilst reducing ROS production.

Biochemical characterization and inhibition of the alternative oxidase enzyme from the fungal phytopathogen Moniliophthora perniciosa

Moniliophthora perniciosa is a fungal pathogen and causal agent of the witches' broom disease of cocoa, a threat to the chocolate industry and to the economic and social security in cocoa-planting countries. The membrane-bound enzyme alternative oxidase (MpAOX) is crucial for pathogen survival; however a lack of information on the biochemical properties of MpAOX hinders the development of novel fungicides. In this study, we purified and characterised recombinant MpAOX in dose-response assays with activators and inhibitors, followed by a kinetic characterization both in an aqueous environment and in physiologically-relevant proteoliposomes. We present structure-activity relationships of AOX inhibitors such as colletochlorin B and analogues which, aided by an MpAOX structural model, indicates key residues for protein-inhibitor interaction. We also discuss the importance of the correct hydrophobic environment for MpAOX enzymatic activity. We envisage that such results will guide the future development of AOX-targeting antifungal agents against M. perniciosa, an important outcome for the chocolate industry.

Bioenergetic consequences from xenotopic expression of a tunicate AOX in mouse mitochondria: Switch from RET and ROS to FET

Electron transfer from all respiratory chain dehydrogenases of the electron transport chain (ETC) converges at the level of the quinone (Q) pool. The Q redox state is thus a function of electron input (reduction) and output (oxidation) and closely reflects the mitochondrial respiratory state. Disruption of electron flux at the level of the cytochrome bc₁ complex (cIII) or cytochrome c oxidase (cIV) shifts the Q redox poise to a more reduced state which is generally sensed as respiratory stress. To cope with respiratory stress, many species, but not insects and vertebrates, express alternative oxidase (AOX) which acts as an electron sink for reduced Q and by-passes cIII and cIV. Here, we used Ciona intestinalis AOX xenotopically expressed in mouse mitochondria to study how respiratory states impact the Q poise and how AOX may be used to restore respiration. Particularly interesting is our finding that electron input through succinate dehydrogenase (cII), but not NADH:ubiquinone oxidoreductase (cI), reduces the Q pool almost entirely (>90%) irrespective of the respiratory state. AOX enhances the forward electron transport (FET) from cII thereby decreasing reverse electron transport (RET) and ROS specifically when non-phosphorylating. AOX is not engaged with cI substrates, however, unless a respiratory inhibitor is added. This sheds new light on Q poise signaling, the biological role of cII which enigmatically is the only ETC complex absent from respiratory supercomplexes but yet participates in the tricarboxylic acid (TCA) cycle. Finally, we delineate potential risks and benefits arising from therapeutic AOX transfer.

Oversized ubiquinones as molecular probes for structural dynamics of the ubiquinone reaction site in mitochondrial respiratory complex I

NADH-quinone oxidoreductase (complex I) couples electron transfer from NADH to quinone with proton translocation across the membrane. Quinone reduction is a key step for energy transmission from the site of quinone reduction to the remotely located proton-pumping machinery of the enzyme. Although structural biology studies have proposed the existence of a long and narrow quinone-access channel, the physiological relevance of this channel remains debatable. We investigated here whether complex I in bovine heart submitochondrial particles (SMPs) can catalytically reduce a series of oversized ubiquinones (OS-UQs), which are highly unlikely to transit the narrow channel because their side chain includes a bulky "block" that is ∼13 Å across. We found that some OS-UQs function as efficient electron acceptors from complex I, accepting electrons with an efficiency comparable with ubiquinone-2. The catalytic reduction and proton translocation coupled with this reduction were completely inhibited by different quinone-site inhibitors, indicating that the reduction of OS-UQs takes place at the physiological reaction site for ubiquinone. Notably, the proton-translocating efficiencies of OS-UQs significantly varied depending on their side-chain structures, suggesting that the reaction characteristics of OS-UQs affect the predicted structural changes of the quinone reaction site required for triggering proton translocation. These results are difficult to reconcile with the current channel model; rather, the access path for ubiquinone may be open to allow OS-UQs to access the reaction site. Nevertheless, contrary to the observations in SMPs, OS-UQs were not catalytically reduced by isolated complex I reconstituted into liposomes. We discuss possible reasons for these contradictory results.

Discovery of trypanocidal coumarins with dual inhibition of both the glycerol kinase and alternative oxidase of Trypanosoma brucei brucei

African trypanosomiasis, sleeping sickness in humans or nagana in animals, is a potentially fatal neglected tropical disease and a threat to 65 million human lives and 100 million small and large livestock animals in sub-Saharan Africa. Available treatments for this devastating disease are few and have limited efficacy, prompting the search for new drug candidates. Simultaneous inhibition of the trypanosomal glycerol kinase (TGK) and trypanosomal alternative oxidase (TAO) is considered a validated strategy toward the development of new drugs. Our goal is to develop a TGK-specific inhibitor for coadministration with ascofuranone (AF), the most potent TAO inhibitor. Here, we report on the identification of novel compounds with inhibitory potency against TGK. Importantly, one of these compounds (compound 17) and its derivatives (17a and 17b) killed trypanosomes even in the absence of AF. Inhibition kinetics revealed that derivative 17b is a mixed-type and competitive inhibitor for TGK and TAO, respectively. Structural data revealed the molecular basis of this dual inhibitory action, which, in our opinion, will aid in the successful development of a promising drug to treat trypanosomiasis. Although the EC₅₀ of compound 17b against trypanosome cells was 1.77 µM, it had no effect on cultured human cells, even at 50 µM.-Balogun, E. O., Inaoka, D. K., Shiba, T., Tsuge, C., May, B., Sato, T., Kido, Y., Nara, T., Aoki, T., Honma, T., Tanaka, A., Inoue, M., Matsuoka, S., Michels, P. A. M., Watanabe, Y.-I., Moore, A. L., Harada, S., Kita, K. Discovery of trypanocidal coumarins with dual inhibition of both the glycerol kinase and alternative oxidase of Trypanosoma brucei brucei.

Coenzyme Q2 is a universal substrate for the measurement of respiratory chain enzyme activities in trypanosomatids

The measurement of respiratory chain enzyme activities is an integral part of basic research as well as for specialized examinations in clinical biochemistry. Most of the enzymes use ubiquinone as one of their substrates. For current in vitro measurements, several hydrophilic analogues of native ubiquinone are used depending on the enzyme and the workplace. We tested five readily available commercial analogues and we showed that Coenzyme Q₂ is the most suitable for the measurement of all tested enzyme activities. Use of a single substrate in all laboratories for several respiratory chain enzymes will improve our ability to compare data, in addition to simplifying the stock of chemicals required for this type of research.

Insights into the ubiquinol/dioxygen binding and proton relay pathways of the alternative oxidase

The alternative oxidase (AOX) is a monotopic diiron carboxylate protein which catalyzes the four-electron reduction of dioxygen to water by ubiquinol. Although we have recently determined the crystal structure of Trypanosoma brucei AOX (TAO) in the presence and absence of ascofuranone (AF) derivatives (which are potent mixed type inhibitors) the mechanism by which ubiquinol and dioxygen binds to TAO remain inconclusive. In this article, ferulenol was identified as the first competitive inhibitor of AOX which has been used to probe the binding of ubiquinol. Surface plasmon resonance reveals that AF is a quasi-irreversible inhibitor of TAO whilst ferulenol binding is completely reversible. The structure of the TAO-ferulenol complex, determined at 2.7 Å, provided insights into ubiquinol binding and has also identified a potential dioxygen molecule bound in a side-on conformation to the diiron center for the first time.

Alternative oxidase inhibitors: Mitochondrion-targeting as a strategy for new drugs against pathogenic parasites and fungi

The alternative oxidase (AOX) is a ubiquitous terminal oxidase of plants and many fungi, catalyzing the four-electron reduction of oxygen to water alongside the cytochrome-based electron transfer chain. Unlike the classical electron transfer chain, however, the activity of AOX does not generate adenosine triphosphate but has functions such as thermogenesis and stress response. As it lacks a mammalian counterpart, it has been investigated intensely in pathogenic fungi. However, it is in African trypanosomes, which lack cytochrome-based respiration in their infective stages, that trypanosome alternative oxidase (TAO) plays the central and essential role in their energy metabolism. TAO was validated as a drug target decades ago and among the first inhibitors to be identified was salicylhydroxamic acid (SHAM), which produced the expected trypanocidal effects, especially when potentiated by coadministration with glycerol to inhibit anaerobic energy metabolism as well. However, the efficacy of this combination was too low to be of practical clinical use. The antibiotic ascofuranone (AF) proved a much stronger TAO inhibitor and was able to cure Trypanosoma vivax infections in mice without glycerol and at much lower doses, providing an important proof of concept milestone. Systematic efforts to improve the SHAM and AF scaffolds, aided with the elucidation of the TAO crystal structure, provided detailed structure-activity relationship information and reinvigorated the drug discovery effort. Recently, the coupling of mitochondrion-targeting lipophilic cations to TAO inhibitors has dramatically improved drug targeting and trypanocidal activity while retaining target protein potency. These developments appear to have finally signposted the way to preclinical development of TAO inhibitors.

Selective Cytotoxicity of Dihydroorotate Dehydrogenase Inhibitors to Human Cancer Cells Under Hypoxia and Nutrient-Deprived Conditions

Human dihydroorotate dehydrogenase (HsDHODH) is a key enzyme of pyrimidine de novo biosynthesis pathway. It is located on the mitochondrial inner membrane and contributes to the respiratory chain by shuttling electrons to the ubiquinone pool. We have discovered ascofuranone (1), a natural compound produced by Acremonium sclerotigenum, and its derivatives are a potent class of HsDHODH inhibitors. We conducted a structure–activity relationship study and have identified functional groups of 1 that are essential for the inhibition of HsDHODH enzymatic activity. Furthermore, the binding mode of 1 and its derivatives to HsDHODH was demonstrated by co-crystallographic analysis and we show that these inhibitors bind at the ubiquinone binding site. In addition, the cytotoxicities of 1 and its potent derivatives 7, 8, and 9 were studied using human cultured cancer cells. Interestingly, they showed selective and strong cytotoxicity to cancer cells cultured under microenvironment (hypoxia and nutrient-deprived) conditions. The selectivity ratio of 8 under this microenvironment show the most potent inhibition which was over 1000-fold higher compared to that under normal culture condition. Our studies suggest that under microenvironment conditions, cancer cells heavily depend on the pyrimidine de novo biosynthesis pathway. We also provide the first evidence that 1 and its derivatives are potential lead candidates for drug development which target the HsDHODH of cancer cells living under a tumor microenvironment.

Mitochondrial Supercomplexes Do Not Enhance Catalysis by Quinone Channeling

Mitochondrial respiratory supercomplexes, comprising complexes I, III, and IV, are the minimal functional units of the electron transport chain. Assembling the individual complexes into supercomplexes may stabilize them, provide greater spatiotemporal control of respiration, or, controversially, confer kinetic advantages through the sequestration of local quinone and cytochrome c pools (substrate channeling). Here, we have incorporated an alternative quinol oxidase (AOX) into mammalian heart mitochondrial membranes to introduce a competing pathway for quinol oxidation and test for channeling. AOX substantially increases the rate of NADH oxidation by O2 without affecting the membrane integrity, the supercomplexes, or NADH-linked oxidative phosphorylation. Therefore, the quinol generated in supercomplexes by complex I is reoxidized more rapidly outside the supercomplex by AOX than inside the supercomplex by complex III. Our results demonstrate that quinone and quinol diffuse freely in and out of supercomplexes: substrate channeling does not occur and is not required to support respiration.

Inhibition of trypanosome alternative oxidase without its N-terminal mitochondrial targeting signal (ΔMTS-TAO) by cationic and non-cationic 4-hydroxybenzoate and 4-alkoxybenzaldehyde derivatives active against T. brucei and T. congolense

African trypanosomiasis is a neglected parasitic disease that is still of great public health relevance, and a severe impediment to agriculture in endemic areas. The pathogens possess certain unique metabolic features that can be exploited for the development of new drugs. Notably, they rely on an essential, mitochondrially-localized enzyme, Trypanosome Alternative Oxidase (TAO) for their energy metabolism, which is absent in the mammalian hosts and therefore an attractive target for the design of safe drugs. In this study, we cloned, expressed and purified the physiologically relevant form of TAO, which lacks the N-terminal 25 amino acid mitochondrial targeting sequence (ΔMTS-TAO). A new class of 32 cationic and non-cationic 4-hydroxybenzoate and 4-alkoxybenzaldehyde inhibitors was designed and synthesized, enabling the first structure-activity relationship studies on ΔMTS-TAO. Remarkably, we obtained compounds with enzyme inhibition values (IC₅₀) as low as 2 nM, which were efficacious against wild type and multidrug-resistant strains of T. brucei and T. congolense. The inhibitors 13, 15, 16, 19, and 30, designed with a mitochondrion-targeting lipophilic cation tail, displayed trypanocidal potencies comparable to the reference drugs pentamidine and diminazene, and showed no cross-resistance with the critical diamidine and melaminophenyl arsenical classes of trypanocides. The cationic inhibitors 15, 16, 19, 20, and 30 were also much more selective (900 - 344,000) over human cells than the non-targeted neutral derivatives (selectivity >8-fold). A preliminary in vivo study showed that modest doses of 15 and 16 reduced parasitaemia of mice infected with T. b. rhodesiense (STIB900). These compounds represent a promising new class of potent and selective hits against African trypanosomes.

African trypanosomiasis: Synthesis & SAR enabling novel drug discovery of ubiquinol mimics for trypanosome alternative oxidase

African trypanosomiasis is a parasitic disease affecting 5000 humans and millions of livestock animals in sub-Saharan Africa every year. Current treatments are limited, difficult to administer and often toxic causing long term injury or death in many patients. Trypanosome alternative oxidase is a parasite specific enzyme whose inhibition by the natural product ascofuranone (AF) has been shown to be curative in murine models. Until now synthetic methods to AF analogues have been limited, this has restricted both understanding of the key structural features required for binding and also how this chemotype could be developed to an effective therapeutic agent. The development of 3 amenable novel synthetic routes to ascofuranone-like compounds is described. The SAR generated around the AF chemotype is reported with correlation to the inhibition of T. b. brucei growth and corresponding selectivity in cytotoxic assessment in mammalian HepG2 cell lines. These methods allow access to greater synthetic diversification and have enabled the synthesis of compounds that have and will continue to facilitate further optimisation of the AF chemotype into a drug-like lead.

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Synthesis of ascofuranone like inhibitors of trypanosome alternative oxidase.

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Structure activity relationships of trypanosome alternative oxidase inhibitors.

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Correlation of trypanosome alternative oxidase inhibition and T. b. brucei growth.

Structural insights into the alternative oxidases: are all oxidases made equal?

The alternative oxidases (AOXs) are ubiquinol-oxidoreductases that are members of the diiron carboxylate superfamily. They are not only ubiquitously distributed within the plant kingdom but also found in increasing numbers within the fungal, protist, animal and prokaryotic kingdoms. Although functions of AOXs are highly diverse in general, they tend to play key roles in thermogenesis, stress tolerance (through the management of radical oxygen species) and the maintenance of mitochondrial and cellular energy homeostasis. The best structurally characterised AOX is from Trypanosoma brucei In this review, we compare the structure of AOXs, created using homology modelling, from many important species in an attempt to explain differences in activity and sensitivity to AOX inhibitors. We discuss the implications of these findings not only for future structure-based drug design but also for the design of novel AOXs for gene therapy.

Conjugates of 2,4-Dihydroxybenzoate and Salicylhydroxamate and Lipocations Display Potent Antiparasite Effects by Efficiently Targeting the Trypanosoma brucei and Trypanosoma congolense Mitochondrion

We investigated a chemical strategy to boost the trypanocidal activity of 2,4-dihydroxybenzoic acid (2,4-DHBA)- and salicylhydroxamic acid (SHAM)-based trypanocides with triphenylphosphonium and quinolinium lipophilic cations (LC). Three series of LC conjugates were synthesized that were active in the submicromolar (5a-d and 10d-f) to low nanomolar (6a-f) range against wild-type and multidrug resistant strains of African trypanosomes (Trypanosoma brucei brucei and T. congolense). This represented an improvement in trypanocidal potency of at least 200-fold, and up to >10 000-fold, compared with that of non-LC-coupled parent compounds 2,4-DHBA and SHAM. Selectivity over human cells was >500 and reached >23 000 for 6e. Mechanistic studies showed that 6e did not inhibit the cell cycle but affected parasite respiration in a dose-dependent manner. Inhibition of trypanosome alternative oxidase and the mitochondrial membrane potential was also studied for selected compounds. We conclude that effective mitochondrial targeting greatly potentiated the activity of these series of compounds.

Heterologous expression of the Crassostrea gigas (Pacific oyster) alternative oxidase in the yeast Saccharomyces cerevisiae

Alternative oxidase (AOX) is a terminal oxidase within the inner mitochondrial membrane (IMM) present in many organisms where it functions in the electron transport system (ETS). AOX directly accepts electrons from ubiquinol and is therefore capable of bypassing ETS Complexes III and IV. The human genome does not contain a gene coding for AOX, so AOX expression has been suggested as a gene therapy for a range of human mitochondrial diseases caused by genetic mutations that render Complex III and/or IV dysfunctional. An effective means of screening mutations amenable to AOX treatment remains to be devised. We have generated such a tool by heterologously expressing AOX from the Pacific oyster (Crassostrea gigas) in the yeast Saccharomyces cerevisiae under the control of a galactose promoter. Our results show that this animal AOX is monomeric and is correctly targeted to yeast mitochondria. Moreover, when expressed in yeast, Pacific oyster AOX is a functional quinol oxidase, conferring cyanide-resistant growth and myxothiazol-resistant oxygen consumption to yeast cells and isolated mitochondria. This system represents a high-throughput screening tool for determining which Complex III and IV genetic mutations in yeast will be amenable to AOX gene therapy. As many human genes are orthologous to those found in yeast, our invention represents an efficient and cost-effective way to evaluate viable research avenues. In addition, this system provides the opportunity to learn more about the localization, structure, and regulation of AOXs from animals that are not easily reared or manipulated in the lab.

A Self-Assembled Respiratory Chain that Catalyzes NADH Oxidation by Ubiquinone-10 Cycling between Complex I and the Alternative Oxidase

Complex I is a crucial respiratory enzyme that conserves the energy from NADH oxidation by ubiquinone‐10 (Q₁₀) in proton transport across a membrane. Studies of its energy transduction mechanism are hindered by the extreme hydrophobicity of Q₁₀, and they have so far relied on native membranes with many components or on hydrophilic Q₁₀ analogues that partition into membranes and undergo side reactions. Herein, we present a self‐assembled system without these limitations: proteoliposomes containing mammalian complex I, Q₁₀, and a quinol oxidase (the alternative oxidase, AOX) to recycle Q₁₀H₂ to Q₁₀. AOX is present in excess, so complex I is completely rate determining and the Q₁₀ pool is kept oxidized under steady‐state catalysis. The system was used to measure a fully‐defined K_M value for Q₁₀. The strategy is suitable for any enzyme with a hydrophobic quinone/quinol substrate, and could be used to characterize hydrophobic inhibitors with potential applications as pharmaceuticals, pesticides, or fungicides.

Tuning the Lewis acid phenol ortho-prenylation as a molecular diversity tool

A diversity-oriented approach for the synthesis of various structurally different prenylated alcohols from readily accessible and common precursors was developed. With varying approaches, this article describes some successful examples of a Friedel-Crafts alkylation using methoxyphenols and different prenyl alcohols (geraniol and (E,E)-farnesol). We demonstrated that just by varying the stoichiometry of the Lewis acid used, the course of the reaction can be shifted to produce the alkylated or the cyclized product. Eighteen unique products were obtained with good isolated yields by direct alkylation with or without a consecutive π-cationic cyclization.

Alternative oxidase: distribution, induction, properties, structure, regulation, and functions

The respiratory chain in the majority of organisms with aerobic type metabolism features the concomitant existence of the phosphorylating cytochrome pathway and the cyanide- and antimycin A-insensitive oxidative route comprising a so-called alternative oxidase (AOX) as a terminal oxidase. In this review, the history of AOX discovery is described. Considerable evidence is presented that AOX occurs widely in organisms at various levels of organization and is not confined to the plant kingdom. This enzyme has not been found only in Archaea, mammals, some yeasts and protists. Bioinformatics research revealed the sequences characteristic of AOX in representatives of various taxonomic groups. Based on multiple alignments of these sequences, a phylogenetic tree was constructed to infer their possible evolution. The ways of AOX activation, as well as regulatory interactions between AOX and the main respiratory chain are described. Data are summarized concerning the properties of AOX and the AOX-encoding genes whose expression is either constitutive or induced by various factors. Information is presented on the structure of AOX, its active center, and the ubiquinone-binding site. The principal functions of AOX are analyzed, including the cases of cell survival, optimization of respiratory metabolism, protection against excess of reactive oxygen species, and adaptation to variable nutrition sources and to biotic and abiotic stress factors. It is emphasized that different AOX functions complement each other in many instances and are not mutually exclusive. Examples are given to demonstrate that AOX is an important tool to overcome the adverse aftereffects of restricted activity of the main respiratory chain in cells and whole animals. This is the first comprehensive review on alternative oxidases of various organisms ranging from yeasts and protists to vascular plants.

The plastid terminal oxidase: its elusive function points to multiple contributions to plastid physiology

Plastids have retained from their cyanobacterial ancestor a fragment of the respiratory electron chain comprising an NADPH dehydrogenase and a diiron oxidase, which sustain the so-called chlororespiration pathway. Despite its very low turnover rates compared with photosynthetic electron flow, knocking out the plastid terminal oxidase (PTOX) in plants or microalgae leads to severe phenotypes that encompass developmental and growth defects together with increased photosensitivity. On the basis of a phylogenetic and structural analysis of the enzyme, we discuss its physiological contribution to chloroplast metabolism, with an emphasis on its critical function in setting the redox poise of the chloroplast stroma in darkness. The emerging picture of PTOX is that of an enzyme at the crossroads of a variety of metabolic processes, such as, among others, the regulation of cyclic electron transfer and carotenoid biosynthesis, which have in common their dependence on the redox state of the plastoquinone pool, set largely by the activity of PTOX in darkness.

Expression and crystallization of the plant alternative oxidase

The alternative oxidase (AOX) is an integral monotopic membrane protein located on the inner surface of the inner mitochondrial membrane. Branching from the traditional respiratory chain at the quinone pool, AOX is responsible for cyanide-resistant respiration in plants and fungi, heat generation in thermogenic plants, and survival of parasites, such as Trypanosoma brucei, in the human host. A recently solved AOX structure provides insight into its active site, thereby facilitating rational phytopathogenic and antiparasitic drug design. Here, we describe expression of recombinant AOX using two different expression systems. Purification protocols for the production of highly pure and stable AOX protein in sufficient quantities to facilitate further kinetic, biophysical, and structural analyses are also described.

Functional expression of the Acanthamoeba castellanii alternative oxidase in Escherichia coli; regulation of the activity and evidence for Acaox gene function

To evidence Acanthamoeba castellanii alternative oxidase (AcAOX) gene product function, we studied alterations in the levels of mRNA and protein and AcAOX activity during growth in amoeba batch culture. Moreover, heterologous expression of AcAOX in AOX-deficient Escherichia coli confirmed by the protein immunodetection and functional studies was performed. Despite the presence of native bo and bd quinol oxidases in E. coli membrane, from which the latter is known to be cyanide-resistant, functional expression of AcAOX in E. coli conferred cyanide-resistant benzohydroxamate-sensitive respiration on the bacteria. Moreover, AcAOX activity in transformed bacteria was stimulated by GMP and inhibited by ATP, indicating that AcAOX is regulated by mutual exclusion of purine nucleotides, which was previously demonstrated in the mitochondria of A. castellanii.

Purification and characterisation of recombinant DNA encoding the alternative oxidase from Sauromatum guttatum

The alternative oxidase (AOX) is a non-protonmotive ubiquinol oxidase that is found in mitochondria of all higher plants studied to date. Structural and functional characterisation of this important but enigmatic plant diiron protein has been hampered by an inability to obtain sufficient native protein from plant sources. In the present study recombinant SgAOX (rSgAOX), overexpressed in a ΔhemA-deficient Escherichia coli strain (FN102), was solubilized from E. coli membranes and purified to homogeneity in a stable and highly active form. The kinetics of ubiquinol-1 oxidation by purified rSgAOX showed typical Michaelis-Menten kinetics (K(m) of 332 μM and Vmax of 30 μmol(-1) min(-1) mg(-1)), a turnover number 20 μmol s(-1) and a remarkable stability. The enzyme was potently inhibited not only by conventional inhibitors such as SHAM and n-propyl gallate but also by the potent TAO inhibitors ascofuranone, an ascofuranone-derivative colletochlorin B and the cytochrome bc1 inhibitor ascochlorin. Circular dichroism studies revealed that AOX was approximately 50% α-helical and furthermore such studies revealed that rSgAOX and rTAO partially retained the helical absorbance signal even at 90 °C (58% and 64% respectively) indicating a high conformational stability. It is anticipated that highly purified and active AOX and its mutants will facilitate investigations into the structure and reaction mechanisms of AOXs through the provision of large amounts of purified protein for crystallography and contribute to further progress of the study on this important plant terminal oxidase.

Characterization of two mitochondrial flavin adenine dinucleotide-dependent glycerol-3-phosphate dehydrogenases in Trypanosoma brucei

Glycerol-3-phosphate dehydrogenases (G3PDHs) constitute a shuttle that serves for regeneration of NAD(+) reduced during glycolysis. This NAD-dependent enzyme is employed in glycolysis and produces glycerol-3-phosphate from dihydroxyacetone phosphate, while its flavin adenine dinucleotide (FAD)-dependent homologue catalyzes a reverse reaction coupled to the respiratory chain. Trypanosoma brucei possesses two FAD-dependent G3PDHs. While one of them (mitochondrial G3PDH [mtG3PDH]) has been attributed to the mitochondrion and seems to be directly involved in G3PDH shuttle reactions, the function of the other enzyme (putative G3PDH [putG3PDH]) remains unknown. In this work, we used RNA interference and protein overexpression and tagging to shed light on the relative contributions of both FAD-G3PDHs to overall cellular metabolism. Our results indicate that mtG3PDH is essential for the bloodstream stage of T. brucei, while in the procyclic stage the enzyme is dispensable. Expressed putG3PDH-V5 was localized to the mitochondrion, and the data obtained by digitonin permeabilization, Western blot analysis, and immunofluorescence indicate that putG3PDH is located within the mitochondrion.

Structure of the trypanosome cyanide-insensitive alternative oxidase

In addition to haem copper oxidases, all higher plants, some algae, yeasts, molds, metazoans, and pathogenic microorganisms such as Trypanosoma brucei contain an additional terminal oxidase, the cyanide-insensitive alternative oxidase (AOX). AOX is a diiron carboxylate protein that catalyzes the four-electron reduction of dioxygen to water by ubiquinol. In T. brucei, a parasite that causes human African sleeping sickness, AOX plays a critical role in the survival of the parasite in its bloodstream form. Because AOX is absent from mammals, this protein represents a unique and promising therapeutic target. Despite its bioenergetic and medical importance, however, structural features of any AOX are yet to be elucidated. Here we report crystal structures of the trypanosomal alternative oxidase in the absence and presence of ascofuranone derivatives. All structures reveal that the oxidase is a homodimer with the nonhaem diiron carboxylate active site buried within a four-helix bundle. Unusually, the active site is ligated solely by four glutamate residues in its oxidized inhibitor-free state; however, inhibitor binding induces the ligation of a histidine residue. A highly conserved Tyr220 is within 4 Å of the active site and is critical for catalytic activity. All structures also reveal that there are two hydrophobic cavities per monomer. Both inhibitors bind to one cavity within 4 Å and 5 Å of the active site and Tyr220, respectively. A second cavity interacts with the inhibitor-binding cavity at the diiron center. We suggest that both cavities bind ubiquinol and along with Tyr220 are required for the catalytic cycle for O2 reduction.

Diversity of parasite complex II

Parasites have developed a variety of physiological functions necessary for completing at least part of their life cycles in the specialized environments of surrounding the parasites in the host. Regarding energy metabolism, which is essential for survival, parasites adapt to the low oxygen environment in mammalian hosts by using metabolic systems that are very different from those of the hosts. In many cases, the parasite employs aerobic metabolism during the free-living stage outside the host but undergoes major changes in developmental control and environmental adaptation to switch to anaerobic energy metabolism. Parasite mitochondria play diverse roles in their energy metabolism, and in recent studies of the parasitic nematode, Ascaris suum, the mitochondrial complex II plays an important role in anaerobic energy metabolism of parasites inhabiting hosts by acting as a quinol-fumarate reductase. In Trypanosomes, parasite complex II has been found to have a novel function and structure. Complex II of Trypanosoma cruzi is an unusual supramolecular complex with a heterodimeric iron-sulfur subunit and seven additional non-catalytic subunits. The enzyme shows reduced binding affinities for both substrates and inhibitors. Interestingly, this structural organization is conserved in all trypanosomatids. Since the properties of complex II differ across a wide range of parasites, this complex is a potential target for the development of new chemotherapeutic agents. In this regard, structural information on the target enzyme is essential for the molecular design of drugs. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.

Unraveling the heater: new insights into the structure of the alternative oxidase

The alternative oxidase is a membrane-bound ubiquinol oxidase found in the majority of plants as well as many fungi and protists, including pathogenic organisms such as Trypanosoma brucei. It catalyzes a cyanide- and antimycin-A-resistant oxidation of ubiquinol and the reduction of oxygen to water, short-circuiting the mitochondrial electron-transport chain prior to proton translocation by complexes III and IV, thereby dramatically reducing ATP formation. In plants, it plays a key role in cellular metabolism, thermogenesis, and energy homeostasis and is generally considered to be a major stress-induced protein. We describe recent advances in our understanding of this protein's structure following the recent successful crystallization of the alternative oxidase from T. brucei. We focus on the nature of the active site and ubiquinol-binding channels and propose a mechanism for the reduction of oxygen to water based on these structural insights. We also consider the regulation of activity at the posttranslational and retrograde levels and highlight challenges for future research.

Trypanosome alternative oxidase, a potential therapeutic target for sleeping sickness, is conserved among Trypanosoma brucei subspecies

Trypanosoma brucei rhodesiense and T. b. gambiense are known causes of human African trypanosomiasis (HAT), or "sleeping sickness," which is deadly if untreated. We previously reported that a specific inhibitor of trypanosome alternative oxidase (TAO), ascofuranone, quickly kills African trypanosomes in vitro and cures mice infected with another subspecies, non-human infective T. b. brucei, in in vivo trials. As an essential factor for trypanosome survival, TAO is a promising drug target due to the absence of alternative oxidases in the mammalian host. This study found TAO expression in HAT-causing trypanosomes; its amino acid sequence was identical to that in non-human infective T. b. brucei. The biochemical understanding of the TAO including its 3 dimensional structure and inhibitory compounds against TAO could therefore be applied to all three T. brucei subspecies in search of a cure for HAT. Our in vitro study using T. b. rhodesiense confirmed the effectiveness of ascofuranone (IC(50) value: 1 nM) to eliminate trypanosomes in human infective strain cultures.

Crystallization and preliminary crystallographic analysis of cyanide-insensitive alternative oxidase from Trypanosoma brucei brucei

Cyanide-insensitive alternative oxidase (AOX) is a mitochondrial membrane protein and a non-proton-pumping ubiquinol oxidase that catalyzes the four-electron reduction of dioxygen to water. In the African trypanosomes, trypanosome alternative oxidase (TAO) functions as a cytochrome-independent terminal oxidase that is essential for survival in the mammalian host; hence, the enzyme is considered to be a promising drug target for the treatment of trypanosomiasis. In the present study, recombinant TAO (rTAO) overexpressed in haem-deficient Escherichia coli was purified and crystallized at 293 K by the hanging-drop vapour-diffusion method using polyethylene glycol 400 as a precipitant. X-ray diffraction data were collected at 100 K and were processed to 2.9 A resolution with 93.1% completeness and an overall R(merge) of 9.5%. The TAO crystals belonged to the orthorhombic space group I222 or I2(1)2(1)2(1), with unit-cell parameters a = 63.11, b = 136.44, c = 223.06 A. Assuming the presence of two rTAO molecules in the asymmetric unit (2 x 38 kDa), the calculated Matthews coefficient (V(M)) was 3.2 A(3) Da(-1), which corresponds to a solvent content of 61.0%. This is the first report of a crystal of the membrane-bound diiron proteins, which include AOXs.