Structural and Mutational Analysis of Trypanosoma brucei Prostaglandin H2 Reductase Provides Insight into the Catalytic Mechanism of Aldo-ketoreductases

Enzyme-Inspired Assembly: Incorporating Multivariate Interactions to Optimize the Host-Guest Configuration for High-Speed Enantioselective Catalysis

To achieve a rapid asymmetry conversion, the substrate objects suffer from accelerated kinetic velocity and random rotation at the cost of selectivity. Inspired by natural enzymes, optimizing the host-guest configuration will realize the high-performance enantioselective conversion of chemical reactions. Herein, multivariate binding interactions were introduced into the 1D channel of a chiral catalyst to simulate the enzymatic action. An imidazolium group was used to electrophilically activate the C═O unit of a ketone substrate, and the counterion binds the hydrogen donor isopropanol. This binding effect around the catalytic center produces strong stereo-induction, resulting in high conversion (99.5% yield) and enantioselectivity (99.5% ee) for the asymmetric hydrogenation of biomass-derived acetophenone. In addition, the turnover frequency of the resulting catalyst (5160 h^-1 TOF) is more than 58 times that of a homogeneous Ru-TsDPEN catalyst (88 h^-1 TOF) under the same condition, which corresponds to the best performance reported till date among all existing catalysts for the considered reaction.

Functional characterization of an aldose reductase (bmALD1) obtained from the silkworm Bombyx mori

We describe a new member of the aldo-keto reductase (AKR) superfamily in the silkworm Bombyx mori. On the basis of its amino acid sequence and phylogenetic tree, this AKR belongs to the AKR1B family and has been designated as bmALD1. In the current study, recombinant bmALD1 was overexpressed, purified to homogeneity and kinetically characterized. We discovered that bmALD1 uses NADPH as a coenzyme to reduce carbonyl compounds such as DL-glyceraldehyde, glucose and 2-nonenal. No NADH-dependent activity was detected. To the best of our knowledge, bmALD1 is only the third AKR characterized in silkworm which, given its substrate specificity, could play a major role in glucose metabolism and antioxidant reactions. Our data provide an increased understanding of insect AKR function.

A role for trypanosomatid aldo-keto reductases in methylglyoxal, prostaglandin and isoprostane metabolism

Trypanosomatid parasites are the infectious agents causing Chagas disease, visceral and cutaneous leishmaniasis and human African trypanosomiasis. Recent work of others has implicated an aldo-keto reductase (AKR) in the susceptibility and resistance of Trypanosoma cruzi to benznidazole, a drug used to treat Chagas disease. Here, we show that TcAKR and homologues in the related parasites Trypanosoma brucei and Leishmania donovani do not reductively activate monocyclic (benznidazole, nifurtimox and fexinidazole) or bicyclic nitro-drugs such as PA-824. Rather, these enzymes metabolise a variety of toxic ketoaldehydes, such as glyoxal and methylglyoxal, suggesting a role in cellular defence against chemical stress. UPLC-QToF/MS analysis of benznidazole bioactivation by T. cruzi cell lysates confirms previous reports identifying numerous drug metabolites, including a dihydro-dihydroxy intermediate that can dissociate to form N-benzyl-2-guanidinoacetamide and glyoxal, a toxic DNA-glycating and cross-linking agent. Thus, we propose that TcAKR contributes to benznidazole resistance by the removal of toxic glyoxal. In addition, three of the four enzymes studied here display activity as prostaglandin F_2α synthases, despite the fact that there are no credible cyclooxygenases in these parasites to account for formation of the precursor PGH₂ from arachidonic acid. Our studies suggest that arachidonic acid is first converted non-enzymatically in parasite lysates to (PGH₂-like) regioisomers by free radical-mediated peroxidation and that AKRs convert these lipid peroxides into isoprostanes, including prostaglandin F_2α and 8-iso-prostaglandin F_2α.

Bioactive lipids in Trypanosoma cruzi infection

Chagas disease is caused by Trypanosoma cruzi, a protozoan parasite. Chagas disease remains a serious health problem in large parts of Mexico and Central and South America, where it is a major cause of morbidity and mortality. This disease is being increasingly recognized in non-endemic regions due to immigration. Heart disease develops in 10-30% of infected individuals. It is increasingly clear that parasite- and host-derived bioactive lipids potently modulate disease progression. Many of the changes that occur during acute and chronic Chagas disease can be accounted for by the effects of arachidonic acid (AA)-derived lipids such as leukotrienes, lipoxins, H(P)ETEs, prostaglandins (PGs) and thromboxane. During the course of infection with T. cruzi, changes in circulating levels of AA metabolites are observed. Antagonism of PG synthesis with cyclooxygenase (COX) inhibitors has both beneficial and adverse effects. Treatment with COX inhibitors during acute infection may result in increased parasite load and mortality. However, treatment instituted during chronic infection may be beneficial with no increase in mortality and substantial improvement with cardiac function. Recently, T. cruzi infection of mice deficient in AA biosynthetic enzymes for various pathways has yielded more insightful data than pharmacological inhibition and has highlighted the potential deleterious effects of inhibitors due to "off-target" actions. Using COX-1 null mice, it was observed that parasite biosynthesis is dependent upon host metabolism, that the majority of TXA(2) liberated during T. cruzi infection is derived from the parasite and that this molecule may act as a quorum sensor to control parasite growth/differentiation. Thus, eicosanoids present during acute infection may act as immunomodulators aiding the transition to, and maintenance of, the chronic stage of the disease. It is also likely that the same mediators that initially function to ensure host survival may later contribute to cardiovascular damage. Collectively, the eicosanoids represent a new series of targets for therapy in Chagas disease with defined potential therapeutic windows in which to apply these agents for greatest effect. A deeper understanding of the mechanism of action of non-steroidal anti-inflammatory drugs may provide clues to the differences between host responses in acute and chronic T. cruzi infection.

Crystallization and preliminary X-ray diffraction analysis of mouse prostaglandin F2α synthase, AKR1B3

Aldo-keto reductase 1B3 (AKR1B3) catalyzes the NADPH-dependent reduction of prostaglandin H(2) (PGH(2)), which is a common intermediate of various prostanoids, to form PGF(2α). AKR1B3 also reduces PGH(2) to PGD(2) in the absence of NADPH. AKR1B3 produced in Escherichia coli was crystallized in complex with NADPH by the sitting-drop vapour-diffusion method. The crystal was tetragonal, belonging to space group P4(1)2(1)2 or P4(3)2(1)2, with unit-cell parameters a = b = 107.62, c = 120.76 Å. X-ray diffraction data were collected to 2.4 Å resolution at 100 K using a synchrotron-radiation source.

Structural insight into the stereoselective production of PGF2α by Old Yellow Enzyme from Trypanosoma cruzi

Old yellow enzyme (OYE) is an NADPH oxidoreductase capable of reducing a variety of compounds. It contains flavin mononucleotide (FMN) as a prosthetic group. A ternary complex structure of OYE from Trypanosoma cruzi (TcOYE) with FMN and one of the substrates, p-hydroxybenzaldehyde, shows a striking movement around the active site upon binding of the substrate. From a structural comparison of other OYE complexed with 12-oxophytodienoate, we have constructed a complex structure with another substrate, prostaglandin H(2) (PGH(2)), to provide a proposed stereoselective reaction mechanism for the reduction of PGH(2) to prostaglandin F(2α) by TcOYE.

Catalytic mechanism of the primary human prostaglandin F2α synthase, aldo-keto reductase 1B1--prostaglandin D2 synthase activity in the absence of NADP(H)

Aldo-keto reductase 1B1 and 1B3 (AKR1B1 and AKR1B3) are the primary human and mouse prostaglandin F(2α) (PGF(2α)) synthases, respectively, which catalyze the NADPH-dependent reduction of PGH(2), a common intermediate of various prostanoids, to form PGF(2α). In this study, we found that AKR1B1 and AKR1B3, but not AKR1B7 and AKR1C3, also catalyzed the isomerization of PGH(2) to PGD(2) in the absence of NADPH or NADP(+). Both PGD(2) and PGF(2α) synthase activities of AKR1B1 and AKR1B3 completely disappeared in the presence of NADP(+) or after heat treatment of these enzymes at 100 °C for 5 min. The K(m), V(max), pK and optimum pH values of the PGD(2) synthase activities of AKR1B1 and AKR1B3 were 23 and 18 μM, 151 and 57 nmol·min(-1)·(mg protein)(-1), 7.9 and 7.6, and pH 8.5 for both AKRs, respectively, and those of PGF(2α) synthase activity were 29 and 33 μM, 169 and 240 nmol·min(-1)·(mg protein)(-1), 6.2 and 5.4, and pH 5.5 and pH 5.0, respectively, in the presence of 0.5 mm NADPH. Site-directed mutagenesis of the catalytic tetrad of AKR1B1, composed of Tyr, Lys, His and Asp, revealed that the triad of Asp43, Lys77 and His110, but not Tyr48, acts as a proton donor in most AKR activities, and is crucial for PGD(2) and PGF(2α) synthase activities. These results, together with molecular docking simulation of PGH(2) to the crystallographic structure of AKR1B1, indicate that His110 acts as a base in concert with Asp43 and Lys77 and as an acid to generate PGD(2) and PGF(2α) in the absence of NADPH or NADP(+) and in the presence of NADPH, respectively.

Identification, cloning and characterization of an aldo-keto reductase from Trypanosoma cruzi with quinone oxido-reductase activity

Drugs currently used for treatment of Trypanosoma cruzi infection, the ethiological agent of Chagas' disease, have shown side effects and variable efficiency. With the aim to describe parasite enzymes involved in the mechanisms of action of trypanocidal drugs and since it has been reported that reductases are crucial in their metabolism, we attempted to identify novel NADPH-dependent oxido-reductases from T. cruzi. The percolation of a soluble fraction of epimastigote lysates through a Cibacron Blue-Sepharose column followed by elution by NADPH yielded a predominant protein with an apparent molecular weight of 32 kDa. This protein was identified by MALDI-TOF as an aldo-keto reductase (AKR) and hence denominated TcAKR. TcAKR was mainly localized in the cytosol and was also present in trypomastigote and amastigote lysates. The recombinant TcAKR (recTcAKR) showed NADPH-dependent reductase activity with the AKR substrates 4-nitrobenzaldehyde and 2-dihydroxyacetone. The saturation curves for both substrates were consistent with the Michaelis-Menten model. We also tested whether recTcAKR may reduce naphthoquinones (NQ), since many of these compounds have displayed important trypanocidal activity. recTcAKR reduced o-NQ (1,2-naphthoquinone, 9,10-phenanthrenquinone and beta-lapachone) with concomitant generation of free radicals but did not exhibit affinity for p-NQ (5-hydroxy-1,4-naphthoquinone, 2-hydroxy-1,4-naphthoquinone, alpha-lapachone and menadione). The substrate saturation curve with o-NQ fitted to a sigmoidal curve, suggesting that recTcAKR presents a cooperative behavior. In addition, three peaks assigned to monomers, dimers and tetramers were obtained when recTcAKR was submitted to a Superose 12 gel chromatography column. TcAKR is the first member of the AKR family described in T. cruzi. Our results indicate that this enzyme may participate in the mechanisms of action of trypanocidal drugs.

Crystal structure and comparative functional analyses of a Mycobacterium aldo-keto reductase

Aldo-keto reductases (AKRs) are a large superfamily of NADPH-dependent enzymes that catalyze the reduction of aldehydes, aldoses, dicarbonyls, steroids, and monosaccharides. While their precise physiological role is generally unknown, AKRs are nevertheless involved in the detoxification of a broad range of toxic metabolites. Mycobacteria contain a number of AKRs, the majority of which are uncharacterised. Here, we report the 1.9 and 1.6 A resolution structures of the apoenzyme and NADPH-bound forms, respectively, of an AKR (MSMEG_2407) from Mycobacterium smegmatis, a close homologue of the M. tuberculosis enzyme Rv2971, whose function is essential to this bacterium. MSMEG_2407 adopted the triosephosphate isomerase (alpha/beta)(8)-barrel fold exhibited by other AKRs. MSMEG_2407 (AKR5H1) bound NADPH via an induced-fit mechanism, in which the NADPH was ligated in an extended fashion. Polar-mediated interactions dominated the interactions with the cofactor, which is atypical of the mode of NADPH binding within the AKR family. Moreover, the nicotinamide ring of NADPH was disordered, and this was attributed to the lack of an "AKR-conserved" bulky residue within the nicotinamide-binding cavity of MSMEG_2407. Enzymatic characterisation of MSMEG_2407 and Rv2971 identified dicarbonyls as a preferred substrate family for hydrolysis, and the frontline antituberculosis drug isoniazid (INH) was shown to inhibit the enzyme activity of both recombinant MSMEG_2407 and Rv2971. However, differences between the affinities of MSMEG_2407 and Rv2971 for dicarbonyls and INH were observed, and this was attributable to amino acid substitutions within the cofactor- and substrate-binding sites. The structures of MSMEG_2407 and the accompanying biochemical characterisation of MSMEG_2407 and Rv2971 provide insight into the structure and function of AKRs from mycobacteria.

Structural and biochemical analyses of YvgN and YtbE from Bacillus subtilis

Bacillus subtilis is one of the most studied gram-positive bacteria. In this work, YvgN and YtbE from B. subtilis, assigned as AKR5G1 and AKR5G2 of aldo-keto reductase (AKR) superfamily. AKR catalyzes the NADPH-dependent reduction of aldehyde or aldose substrates to alcohols. YvgN and YtbE were studied by crystallographic and enzymatic analyses. The apo structures of these proteins were determined by molecular replacement, and the structure of holoenzyme YvgN with NADPH was also solved, revealing the conformational changes upon cofactor binding. Our biochemical data suggest both YvgN and YtbE have preferential specificity for derivatives of benzaldehyde, such as nitryl or halogen group substitution at the 2 or 4 positions. These proteins also showed broad catalytic activity on many standard substrates of AKR, such as glyoxal, dihydroxyacetone, and DL-glyceraldehyde, suggesting a possible role in bacterial detoxification.

The catalytic mechanism of NADH-dependent reduction of 9,10-phenanthrenequinone by Candida tenuis xylose reductase reveals plasticity in an aldo-keto reductase active site

Despite their widely varying physiological functions in carbonyl metabolism, AKR2B5 (Candida tenuis xylose reductase) and many related enzymes of the aldo-keto reductase protein superfamily utilise PQ (9,10-phenanthrenequinone) as a common in vitro substrate for NAD(P)H-dependent reduction. The catalytic roles of the conserved active-site residues (Tyr51, Lys80 and His113) of AKR2B5 in the conversion of the reactive alpha-dicarbonyl moiety of PQ are not well understood. Using wild-type and mutated (Tyr51, Lys80 and His113 individually replaced by alanine) forms of AKR2B5, we have conducted steady-state and transient kinetic studies of the effects of varied pH and deuterium isotopic substitutions in coenzyme and solvent on the enzymatic rates of PQ reduction. Each mutation caused a 10(3)-10(4)-fold decrease in the rate constant for hydride transfer from NADH to PQ, whose value in the wild-type enzyme was determined as approximately 8 x 10(2) s(-1). The data presented support an enzymic mechanism in which a catalytic proton bridge from the protonated side chain of Lys80 (pK=8.6+/-0.1) to the carbonyl group adjacent to the hydride acceptor carbonyl facilitates the chemical reaction step. His113 contributes to positioning of the PQ substrate for catalysis. Contrasting its role as catalytic general acid for conversion of the physiological substrate xylose, Tyr51 controls release of the hydroquinone product. The proposed chemistry of AKR2B5 action involves delivery of both hydrogens required for reduction of the alpha-dicarbonyl substrate to the carbonyl group undergoing (stereoselective) transformation. Hydride transfer from NADH probably precedes the transfer of a proton from Tyr51 whose pK of 7.3+/-0.3 in the NAD+-bound enzyme appears suitable for protonation of a hydroquinone anion (pK=8.8). These results show that the mechanism of AKR2B5 is unusually plastic in the exploitation of the active-site residues, for the catalytic assistance provided to carbonyl group reduction in alpha-dicarbonyls differs from that utilized in the conversion of xylose.

Tyr-51 is the proton donor-acceptor for NAD(H)-dependent interconversion of xylose and xylitol by Candida tenuis xylose reductase (AKR2B5)

Substitution of active-site Tyr-51 by Ala (Y51A) disrupted the activity of Candida tenuis xylose reductase by six orders of magnitude. External bromide brought about unidirectional rate enhancement ( approximately 2x10(3)-fold at 300mM) for NAD(+)-dependent xylitol oxidation by Y51A. Activity of the wild-type reductase was dependent on a single ionizable protein group exhibiting a pK of 9.2+/-0.1 and 7.3+/-0.3 in the holo-enzyme bound with NADH and NAD(+), respectively. This group which had to be protonated for xylose reduction and unprotonated for xylitol oxidation was eliminated in Y51A, consistent with a catalytic acid-base function of Tyr-51. Bromide may complement the xylitol dehydrogenase activity of Y51A by partly restoring the original hydrogen bond between the reactive alcohol and the phenolate of Tyr-51.

A glutathione-specific aldose reductase of Leishmania donovani and its potential implications for methylglyoxal detoxification pathway

Methylglyoxal is mainly catabolized by two major enzymatic pathways. The first is the ubiquitous detoxification pathway, the glyoxalase pathway. In addition to the glyoxalase pathway, aldose reductase pathway also plays a crucial role in lowering the levels of methylglyoxal. The gene encoding aldose reductase (ALR) has been cloned from Leishmania donovani, a protozoan parasite causing visceral leishmaniasis. DNA sequence analysis revealed an open reading frame (ORF) of approximately 855 bp encoding a putative protein of 284 amino acids with a calculated molecular mass of 31.7 kDa and a predicted isoelectric point of 5.85. The sequence identity between L. donovani ALR (LdALR) and mammals and plants is only 36-44%. The ORF is a single copy gene. A protein with a molecular mass that matched the estimated approximately 74 kDa according to the amino acid composition of LdALR with a maltose binding tag present at its N-terminal end was induced by heterologous expression of LdALR in Escherichia coli. In the presence of glutathione, recombinant LdALR reduced methylglyoxal with a K(m) of approximately 112 microM. Comparative structural analysis of the human ALR structure with LdALR model suggests that the active site anchoring the N-terminal end of the glutathione is highly conserved. However, the C-terminal end of the glutathione backbone is expected to be exposed in LdALR, as the residues anchoring the C-terminal end of the glutathione backbone come from the three loop regions in human, which are apparently shortened in the LdALR structure. Thus, the computational analysis provides clues about the expected mode of glutathione binding and its interactions with the protein. This is the first report of the role of an ALR in the metabolic disposal of methylglyoxal in L. donovani and of thiol binding to a kinetoplastid aldose reductase.

The aldo-keto reductase superfamily and its role in drug metabolism and detoxification

The aldo-keto reductase (AKR) superfamily comprises enzymes that catalyze redox transformations involved in biosynthesis, intermediary metabolism, and detoxification. Substrates of AKRs include glucose, steroids, glycosylation end-products, lipid peroxidation products, and environmental pollutants. These proteins adopt a (beta/alpha)(8) barrel structural motif interrupted by a number of extraneous loops and helixes that vary between proteins and bring structural identity to individual families. The human AKR family differs from the rodent families. Due to their broad substrate specificity, AKRs play an important role in the phase II detoxification of a large number of pharmaceuticals, drugs, and xenobiotics.

Molecular basis for prostaglandin production in hosts and parasites

Prostaglandins (PGs) comprise a family of structurally related bioactive lipid mediators that are involved in various symptoms associated with parasitic diseases. The molecular mechanisms of PG biosynthesis in animals have been studied extensively. Currently, several lines of evidence link their production with parasites. In this review we discuss the roles of PGs in parasite pathogenesis and physiology and the recent advances in our understanding of the enzymology of PG production in various parasites.

Catalytic mechanism and substrate selectivity of aldo-keto reductases: insights from structure-function studies of Candida tenuis xylose reductase

Aldo-keto reductases (AKRs) constitute a large protein superfamily of mainly NAD(P)-dependent oxidoreductases involved in carbonyl metabolism. Catalysis is promoted by a conserved tetrad of active site residues (Tyr, Lys, Asp and His). Recent results of structure-function relationship studies for xylose reductase (AKR2B5) require an update of the proposed catalytic mechanism. Electrostatic stabilization by the epsilon-NH3+ group of Lys is a key source of catalytic power of xylose reductase. A molecular-level analysis of the substrate binding pocket of xylose reductase provides a case of how a very broadly specific AKR achieves the requisite selectivity for its physiological substrate and could serve as the basis for the design of novel reductases with improved specificities for biocatalytic applications.

The RACK1 homologue from Trypanosoma brucei is required for the onset and progression of cytokinesis

The receptor for activated C kinase 1 (RACK1) is a conserved scaffold protein that helps regulate a range of cell activities including cell growth, shape, and protein translation. We report that a homologue of RACK1 is required for cytokinesis in pathogenic Trypanosoma brucei. The protein, referred to as TRACK, is comprised of WD repeat elements and can complement cpc2 null mutants of Schizosaccharomyces pombe. TRACK is expressed throughout the trypanosome life cycle and is distributed predominantly in a perinuclear region and the cytoplasm but not along the endoplasmic reticulum, mitochondrion, or cleavage furrow of dividing cells. When tetracycline-inducible RNA interference (RNAi) is used to deplete the cellular content of TRACK, the cells remain metabolically active, but growth is inhibited. In bloodstream forms, growth arrest is due to a delay in the onset of cytokinesis. By contrast, procyclic forms are able to initiate cytokinesis in the absence of TRACK but arrest midway through cell cleavage. The RNAi cells undergo multiple rounds of partial cytokinesis and accumulate nuclei and cytoplasmic extensions with attached flagella. The TRACK RNAi construct is also inducible within infected mice. Under these conditions parasites are eliminated from peripheral blood within 3 days post-infection. Taken as a whole, these data indicate that trypanosomes utilize a RACK1 homologue to regulate the final stages of mitosis. Moreover, disrupting the interaction between TRACK and its partners might be targeted in the design of novel therapies.