Aldose reductase inhibitors: 2013-present

INTRODUCTION

Aldose reductase (ALR2) is both the key enzyme of the polyol pathway, whose activation under hyperglycemic conditions leads to the development of chronic diabetic complications, and the crucial promoter of inflammatory and cytotoxic conditions, even under a normoglycemic status. Accordingly, it represents an excellent drug target and a huge effort is being done to disclose novel compounds able to inhibit it.

AREAS COVERED

This literature survey summarizes patents and patent applications published over the last 5 years and filed for natural, semi-synthetic and synthetic ALR2 inhibitors. Compounds described have been discussed and analyzed from both chemical and functional angles.

EXPERT OPINION

Several ALR2 inhibitors with a promising pre-clinical ability to address diabetic complications and inflammatory diseases are being developed during the observed timeframe. Natural compounds and plant extracts are the prevalent ones, thus confirming the use of phytopharmaceuticals as an increasingly pursued therapeutic trend also in the ALR2 inhibitors field. Intriguing hints may be taken from synthetic derivatives, the most significant ones being represented by the differential inhibitors ARDIs. Differently from classical ARIs, these compounds should fire up the therapeutic efficacy of the class while minimizing its side effects, thus overcoming the existing limits of this kind of inhibitors.

Crystal structure of yeast xylose reductase in complex with a novel NADP-DTT adduct provides insights into substrate recognition and catalysis

Aldose reductases (ARs) belonging to the aldo-keto reductase (AKR) superfamily catalyze the conversion of carbonyl substrates into their respective alcohols. Here we report the crystal structures of the yeast Debaryomyces nepalensis xylose reductase (DnXR, AKR2B10) in the apo form and as a ternary complex with a novel NADP-DTT adduct. Xylose reductase, a key enzyme in the conversion of xylose to xylitol, has several industrial applications. The enzyme displayed the highest catalytic efficiency for l-threose (138 ± 7 mm^-1 ·s^-1 ) followed by d-erythrose (30 ± 3 mm^-1 ·s^-1 ). The crystal structure of the complex reveals a covalent linkage between the C4N atom of the nicotinamide ring of the cosubstrate and the S1 sulfur atom of DTT and provides the first structural evidence for a protein mediated NADP-low-molecular-mass thiol adduct. We hypothesize that the formation of the adduct is facilitated by an in-crystallo Michael addition of the DTT thiolate to the specific conformation of bound NADPH in the active site of DnXR. The interactions between DTT, a four-carbon sugar alcohol analog, and the enzyme are representative of a near-cognate product ternary complex and provide significant insights into the structural basis of aldose binding and specificity and the catalytic mechanism of ARs. DATABASE: Structural data are available in the PDB under the accession numbers 5ZCI and 5ZCM.

Engineering the cofactor specificity of an alcohol dehydrogenase via single mutations or insertions distal to the 2'-phosphate group of NADP(H)

There have been many reports exploring the engineering of the cofactor specificity of aldo-keto reductases (AKRs), as this class of proteins is ubiquitous and exhibits many useful activities. A common approach is the mutagenesis of amino acids involved in interactions with the 2'-phosphate group of NADP(H) in the cofactor binding pocket. We recently performed a 'loop-grafting' approach to engineer the substrate specificity of the thermostable alcohol dehydrogenase D (AdhD) from Pyrococcus furiosus and we found that a loop insertion after residue 211, which is on the back side of the cofactor binding pocket, could also alter cofactor specificity. Here, we further explore this approach by introducing single point mutations and single amino acid insertions at the loop insertion site. Six different mutants of AdhD were created by either converting glycine 211 to cysteine or serine or by inserting alanine, serine, glycine or cysteine between the 211 and 212 residues. Several mutants gained activity with NADP+ above the wild-type enzyme. And remarkably, it was found that all of the mutants investigated resulted in some degree of reversal of cofactor specificity in the oxidative direction. These changes were generally a result of changes in conformations of the ternary enzyme/cofactor/substrate complexes as opposed to changes in affinities or binding energies of the cofactors. This study highlights the role that amino acids which are distal to the cofactor binding pocket but are involved in substrate interactions can influence cofactor specificity in AdhD, and this strategy should translate to other AKR family members.

Characterization of AKR1B16, a novel mouse aldo-keto reductase

Aldo-keto reductases (AKRs) are distributed in three families and multiple subfamilies in mammals. The mouse Akr1b3 gene is clearly orthologous to human AKR1B1, both coding for aldose reductase, and their gene products show similar tissue distribution, regulation by osmotic stress and kinetic properties. In contrast, no unambiguous orthologs of human AKR1B10 and AKR1B15.1 have been identified in rodents. Although two more AKRs, AKR1B7 and AKR1B8, have been identified and characterized in mouse, none of them seems to exhibit properties similar to the human AKRs. Recently, a novel mouse AKR gene, Akr1b16, was annotated and the respective gene product, AKR1B16 (sharing 83% and 80% amino acid sequence identity with AKR1B10 and AKR1B15.1, respectively), was expressed as insoluble and inactive protein in a bacterial expression system. Here we describe the expression and purification of a soluble and enzymatically active AKR1B16 from E. coli using three chaperone systems. A structural model of AKR1B16 allowed the estimation of its active-site pocket volume, which was much wider (402 Å³) than those of AKR1B10 (279 Å³) and AKR1B15.1 (60 Å³). AKR1B16 reduced aliphatic and aromatic carbonyl compounds, using NADPH as a cofactor, with moderate or low activity (highest k_cat values around 5 min^-1). The best substrate for the enzyme was pyridine-3-aldehyde. AKR1B16 showed poor inhibition with classical AKR inhibitors, tolrestat being the most potent. Kinetics and inhibition properties resemble those of rat AKR1B17 but differ from those of the human enzymes. In addition, AKR1B16 catalyzed the oxidation of 17β-hydroxysteroids in a NADP⁺-dependent manner. These results, together with a phylogenetic analysis, suggest that mouse AKR1B16 is an ortholog of rat AKR1B17, but not of human AKR1B10 or AKR1B15.1. These human enzymes have no counterpart in the murine species, which is evidenced by forming a separate cluster in the phylogenetic tree and by their unique activity with retinaldehyde.

A pathogenesis related-10 protein CaARP functions as aldo/keto reductase to scavenge cytotoxic aldehydes

Pathogenesis related-10 (PR-10) proteins are present as multigene family in most of the higher plants. The role of PR-10 proteins in plant is poorly understood. A sequence analysis revealed that a large number of PR-10 proteins possess conserved motifs found in aldo/keto reductases (AKRs) of yeast and fungi. We took three PR-10 proteins, CaARP from chickpea, ABR17 from pea and the major pollen allergen Bet v1 from silver birch as examples and showed that these purified recombinant proteins possessed AKR activity using various cytotoxic aldehydes including methylglyoxal and malondialdehyde as substrates and the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) as co-factor. Essential amino acids for this catalytic activity were identified by substitution with other amino acids. CaARP was able to discriminate between the reduced and oxidized forms of NADP independently of its catalytic activity and underwent structural change upon binding with NADPH. CaARP protein was preferentially localized in cytosol. When expressed in bacteria, yeast or plant, catalytically active variants of CaARP conferred tolerance to salinity, oxidative stress or cytotoxic aldehydes. CaARP-expressing plants showed lower lipid peroxidation product content in presence or absence of stress suggesting that the protein functions as a scavenger of cytotoxic aldehydes produced by metabolism and lipid peroxidation. Our result proposes a new biochemical property of a PR-10 protein.

Substrate Specificity, Inhibitor Selectivity and Structure-Function Relationships of Aldo-Keto Reductase 1B15: A Novel Human Retinaldehyde Reductase

Human aldo-keto reductase 1B15 (AKR1B15) is a newly discovered enzyme which shares 92% amino acid sequence identity with AKR1B10. While AKR1B10 is a well characterized enzyme with high retinaldehyde reductase activity, involved in the development of several cancer types, the enzymatic activity and physiological role of AKR1B15 are still poorly known. Here, the purified recombinant enzyme has been subjected to substrate specificity characterization, kinetic analysis and inhibitor screening, combined with structural modeling. AKR1B15 is active towards a variety of carbonyl substrates, including retinoids, with lower kcat and Km values than AKR1B10. In contrast to AKR1B10, which strongly prefers all-trans-retinaldehyde, AKR1B15 exhibits superior catalytic efficiency with 9-cis-retinaldehyde, the best substrate found for this enzyme. With ketone and dicarbonyl substrates, AKR1B15 also shows higher catalytic activity than AKR1B10. Several typical AKR inhibitors do not significantly affect AKR1B15 activity. Amino acid substitutions clustered in loops A and C result in a smaller, more hydrophobic and more rigid active site in AKR1B15 compared with the AKR1B10 pocket, consistent with distinct substrate specificity and narrower inhibitor selectivity for AKR1B15.

Identification of a determinant for strict NADP(H)-specificity and high sensitivity to mixed-type steroid inhibitor of rabbit aldo-keto reductase 1C33 by site-directed mutagenesis

In rabbit tissues, hydroxysteroid dehydrogenase belonging to the aldo-keto reductase (AKR) superfamily exists in six isoforms (AKRs: 1C5 and 1C29-1C33), sharing >73% amino acid sequence identity. AKR1C33 is strictly NADPH-specific, in contrast to dual NADPH/NADH specificity of the other isoforms. All coenzyme-binding residues of the structurally elucidated AKR1C5 are conserved in other isoforms, except that S217 (interacting with the pyrophosphate moiety) and T273 (interacting with the 2'-phosphate moiety) are replaced with F217 and N272, respectively, in AKR1C33. To explore the determinants for the NADPH specificity of AKR1C33, we prepared its F217S and N272T mutant enzymes. The mutation of F217S, but not N272T, converted AKR1C33 into a dually coenzyme-specific form that showed similar kcat values for NAD(P)H to those of AKR1C32. The reverse mutation (S217F) in dually coenzyme-specific AKR1C32 produced a strictly NADPH-specific form. The F217S mutation also abolished the activity towards 3-keto-5β-cholestanes that are substrates specific to AKR1C33, and markedly decreased the sensitivity to 4-pregnenes (such as deoxycorticosterone and medroxyprogesterone acetate) that were found to be potent mixed-type inhibitors of the wild-type enzyme. The results indicate the important role of F217 in the strict NADPH-dependency, as well as its involvement in the unique catalytic properties of AKR1C33.

Modular exchange of substrate-binding loops alters both substrate and cofactor specificity in a member of the aldo-keto reductase superfamily

Substrate specificity in the aldo-keto reductase (AKR) superfamily is determined by three mobile loops positioned at the top of the canonical (α/β)(8)-barrel structure. These loops have previously been demonstrated to be modular in a well-studied class of AKRs, in that exchanging loops between two similar hydroxysteroid dehydrogenases resulted in a complete alteration of substrate specificity (Ma,H. and Penning,T.M. (1999) Proc. Natl Acad. Sci. USA, 96, 11161-11166). Here, we further examine the modularity of these loops by grafting those from human aldose reductase (hAR) into the hyperthermostable AKR, alcohol dehydrogenase D (AdhD), from Pyrococcus furiosus. Replacement of Loops A and B was sufficient to impart hAR activity into AdhD, and the resulting chimera retained the thermostability of the parent enzyme. However, no active chimeras were observed when the hAR loops were grafted into a previously engineered cofactor specificity mutant of AdhD, which displayed similar kinetics to hAR with the model substrate dl-glyceraldehyde. The non-additivity of these mutations suggests that efficient turnover is more dependent on the relative positioning of the cofactor and substrate in the active site than on binding of the individual species. The ability to impart the substrate specificities of mesostable AKRs into a thermostable scaffold will be useful in a variety of applications including immobilized enzyme systems for bioelectrocatalysis and fine chemical synthesis.

Structure of the His269Arg mutant of the rat aldose reductase-like protein AKR1B14 complexed with NADPH

Rat aldose reductase-like protein (AKR1B14) is an orthologue of mouse vas deferens protein (AKR1B7) and plays roles in the detoxification of reactive aldehydes and synthesis of prostaglandin F(2α). Here, the 1.87 Å resolution crystal structure of the His269Arg mutant of AKR1B14 complexed with NADPH is described and shows that the negatively charged 2'-phosphate group of the coenzyme forms an ionic interaction with the positively charged guanidinium group of Arg269 that is also observed in the human aldose reductase (AKR1B1) structure. Previous experiments on the site-directed mutagenesis of His269 to Arg, Phe and Met revealed fourfold, sevenfold and 127-fold increases in the K(m) for NADPH, respectively, which are in agreement with the present molecular-modelling and X-ray crystallographic studies. This is the first tertiary structure of a mutant form of this AKR1B7 orthologue to be reported in order to investigate the structure-function relationship of the nonconserved His269 and its role in coenzyme binding.

Broadening the cofactor specificity of a thermostable alcohol dehydrogenase using rational protein design introduces novel kinetic transient behavior

Cofactor specificity in the aldo-keto reductase (AKR) superfamily has been well studied, and several groups have reported the rational alteration of cofactor specificity in these enzymes. Although most efforts have focused on mesostable AKRs, several putative AKRs have recently been identified from hyperthermophiles. The few that have been characterized exhibit a strong preference for NAD(H) as a cofactor, in contrast to the NADP(H) preference of the mesophilic AKRs. Using the design rules elucidated from mesostable AKRs, we introduced two site-directed mutations in the cofactor binding pocket to investigate cofactor specificity in a thermostable AKR, AdhD, which is an alcohol dehydrogenase from Pyrococcus furiosus. The resulting double mutant exhibited significantly improved activity and broadened cofactor specificity as compared to the wild-type. Results of previous pre-steady-state kinetic experiments suggest that the high affinity of the mesostable AKRs for NADP(H) stems from a conformational change upon cofactor binding which is mediated by interactions between a canonical arginine and the 2'-phosphate of the cofactor. Pre-steady-state kinetics with AdhD and the new mutants show a rich conformational behavior that is independent of the canonical arginine or the 2'-phosphate. Additionally, experiments with the highly active double mutant using NADPH as a cofactor demonstrate an unprecedented transient behavior where the binding mechanism appears to be dependent on cofactor concentration. These results suggest that the structural features involved in cofactor specificity in the AKRs are conserved within the superfamily, but the dynamic interactions of the enzyme with cofactors are unexpectedly complex.

Molecular cloning and biochemical characterization of a novel erythrose reductase from Candida magnoliae JH110

Background

Erythrose reductase (ER) catalyzes the final step of erythritol production, which is reducing erythrose to erythritol using NAD(P)H as a cofactor. ER has gained interest because of its importance in the production of erythritol, which has extremely low digestibility and approved safety for diabetics. Although ERs were purified and characterized from microbial sources, the entire primary structure and the corresponding DNA for ER still remain unknown in most of erythritol-producing yeasts. Candida magnoliae JH110 isolated from honeycombs produces a significant amount of erythritol, suggesting the presence of erythrose metabolizing enzymes. Here we provide the genetic sequence and functional characteristics of a novel NADPH-dependent ER from C. magnoliae JH110.

Results

The gene encoding a novel ER was isolated from an osmophilic yeast C. magnoliae JH110. The ER gene composed of 849 nucleotides encodes a polypeptide with a calculated molecular mass of 31.4 kDa. The deduced amino acid sequence of ER showed a high degree of similarity to other members of the aldo-keto reductase superfamily including three ER isozymes from Trichosporonoides megachiliensis SNG-42. The intact coding region of ER from C. magnoliae JH110 was cloned, functionally expressed in Escherichia coli using a combined approach of gene fusion and molecular chaperone co-expression, and subsequently purified to homogeneity. The enzyme displayed a temperature and pH optimum at 42°C and 5.5, respectively. Among various aldoses, the C. magnoliae JH110 ER showed high specific activity for reduction of erythrose to the corresponding alcohol, erythritol. To explore the molecular basis of the catalysis of erythrose reduction with NADPH, homology structural modeling was performed. The result suggested that NADPH binding partners are completely conserved in the C. magnoliae JH110 ER. Furthermore, NADPH interacts with the side chains Lys252, Thr255, and Arg258, which could account for the enzyme's absolute requirement of NADPH over NADH.

Conclusions

A novel ER enzyme and its corresponding gene were isolated from C. magnoliae JH110. The C. magnoliae JH110 ER with high activity and catalytic efficiency would be very useful for in vitro erythritol production and could be applied for the production of erythritol in other microorganisms, which do not produce erythritol.

Investigation of the role of a conserved glycine motif in the Saccharomyces cerevisiae xylose reductase

All yeast xylose reductases, with the exception of that from Schizosaccharomyces pombe, possess the catalytic and coenzyme-binding elements from both the aldo-keto reductase and short-chain dehydrogenase-reductase (SDR) enzyme families in their primary sequences. In the Saccharomyces cerevisiae xylose reductase (XR), the SDR-like coenzyme-binding GXXXGXG motif (Gly motif) is located between residues 128 and 134, with the third Gly residue being replaced by an Asp. We used site-directed mutagenesis to study the role of this SDR-like Gly motif in the S. cerevisiae xylose reductase. Site-directed mutagenesis of the individual conserved Gly residue positions (G128A, G132A, D134G, and D134A) did not significantly affect the specific activity, kinetic constants (K(m), K(cat), and K(cat)/K(m)), or dissociation constants (K(d)) in any of the variants compared with the wild type. Deletion of the entire Gly motif produced an unstable protein that could not be purified. These results indicate that the SDR-like Gly motif likely provides support to the overall structure of the enzyme, but it does not contribute directly to coenzyme binding in this XR.

The coenzyme specificity of Candida tenuis xylose reductase (AKR2B5) explored by site-directed mutagenesis and X-ray crystallography

CtXR (xylose reductase from the yeast Candida tenuis; AKR2B5) can utilize NADPH or NADH as co-substrate for the reduction of D-xylose into xylitol, NADPH being preferred approx. 33-fold. X-ray structures of CtXR bound to NADP+ and NAD+ have revealed two different protein conformations capable of accommodating the presence or absence of the coenzyme 2'-phosphate group. Here we have used site-directed mutagenesis to replace interactions specific to the enzyme-NADP+ complex with the aim of engineering the co-substrate-dependent conformational switch towards improved NADH selectivity. Purified single-site mutants K274R (Lys274-->Arg), K274M, K274G, S275A, N276D, R280H and the double mutant K274R-N276D were characterized by steady-state kinetic analysis of enzymic D-xylose reductions with NADH and NADPH at 25 degrees C (pH 7.0). The results reveal between 2- and 193-fold increases in NADH versus NADPH selectivity in the mutants, compared with the wild-type, with only modest alterations of the original NADH-linked xylose specificity and catalytic-centre activity. Catalytic reaction profile analysis demonstrated that all mutations produced parallel effects of similar magnitude on ground-state binding of coenzyme and transition state stabilization. The crystal structure of the double mutant showing the best improvement of coenzyme selectivity versus wild-type and exhibiting a 5-fold preference for NADH over NADPH was determined in a binary complex with NAD+ at 2.2 A resolution.

Dissection of the physiological interconversion of 5alpha-DHT and 3alpha-diol by rat 3alpha-HSD via transient kinetics shows that the chemical step is rate-determining: effect of mutating cofactor and substrate-binding pocket residues on catalysis

3Alpha-hydroxysteroid dehydrogenases (3alpha-HSDs) catalyze the interconversion between 5alpha-dihydrotestosterone (5alpha-DHT), the most potent androgen, and 3alpha-androstanediol (3alpha-diol), a weak androgen metabolite. To identify the rate-determining step in this physiologically important reaction, rat liver 3alpha-HSD (AKR1C9) was used as the protein model for the human homologues in fluorescence stopped-flow transient kinetic and kinetic isotope effect studies. Using single and multiple turnover experiments to monitor the NADPH-dependent reduction of 5alpha-DHT, it was found that k(lim) and k(max) values were identical to k(cat), indicating that chemistry is rate-limiting overall. Kinetic isotope effect measurements, which gave (D)k(cat) = 2.4 and (D)2(O)k(cat) = 3.0 at pL 6.0, suggest that the slow chemical transformation is significantly rate-limiting. When the NADP(+)-dependent oxidation of 3alpha-diol was monitored, single and multiple turnover experiments showed a k(lim) and burst kinetics consistent with product release as being rate-limiting overall. When NAD(+) was substituted for NADP(+), burst phase kinetics was eliminated, and k(max) was identical to k(cat). Thus with the physiologically relevant substrates 5alpha-DHT plus NADPH and 3alpha-diol plus NAD(+), the slowest event is chemistry. R276 forms a salt-linkage with the phosphate of 2'-AMP, and when it is mutated, tight binding of NAD(P)H is no longer observed [Ratnam, K., et al. (1999) Biochemistry 38, 7856-7864]. The R276M mutant also eliminated the burst phase kinetics observed for the NADP(+)-dependent oxidation of 3alpha-diol. The data with the R276M mutant confirms that the release of the NADPH product is the slow event; and in its absence, chemistry becomes rate-limiting. W227 is a critical hydrophobic residue at the steroid binding site, and when it is mutated to alanine, k(cat)/K(m) for oxidation is significantly depressed. Burst phase kinetics for the NADP(+)-dependent turnover of 3alpha-diol by W227A was also abolished. In the W227A mutant, the slow release of NADPH is no longer observed since the chemical transformation is now even slower. Thus, residues in the cofactor and steroid-binding site can alter the rate-determining step in the NADP(+)-dependent oxidation of 3alpha-diol to make chemistry rate-limiting overall.

Optimizing an artificial metabolic pathway: engineering the cofactor specificity of Corynebacterium 2,5-diketo-D-gluconic acid reductase for use in vitamin C biosynthesis

The strict cofactor specificity of many enzymes can potentially become a liability when these enzymes are to be employed in an artificial metabolic pathway. The preference for NADPH over NADH exhibited by the Corynebacterium 2,5-diketo-D-gluconic acid (2,5-DKG) reductase may not be ideal for use in industrial scale vitamin C biosynthesis. We have previously reported making a number of site-directed mutations at five sites located in the cofactor-binding pocket that interact with the 2'-phosphate group of NADPH. These mutations conferred greater activity with NADH upon the Corynebacterium 2,5-DKG reductase [Banta, S., Swanson, B. A., Wu, S., Jarnagin, A., and Anderson, S. (2002) Protein Eng. 15, 131-140; (1)]. The best of these mutations have now been combined to see if further improvements can be obtained. In addition, several chimeric mutants have been produced that contain the same residues as are found in other members of the aldo-keto reductase superfamily that are naturally able to use NADH as a cofactor. The most active mutants obtained in this work were also combined with a previously reported substrate-binding pocket double mutant, F22Y/A272G. Mutant activity was assayed using activity-stained native polyacrylamide gels. Superior mutants were purified and subjected to a simplified kinetic analysis. The simplified kinetic analysis was extended for the most active mutants in order to obtain the kinetic parameters in the full-ordered bi bi rate equation in the absence of products, with both NADH and NADPH as cofactors. The best mutant 2,5-DKG reductase produced in this work was the F22Y/K232G/R238H/A272G quadruple mutant, which exhibits activity with NADH that is more than 2 orders of magnitude higher than that of the wild-type enzyme, and it retains a high level of activity with NADPH. This new 2,5-DKG reductase may be a valuable new catalyst for use in vitamin C biosynthesis.

Identification of lysine-78 as an essential residue in the Saccharomyces cerevisiae xylose reductase

Yeast xylose reductases are hypothesized as hybrid enzymes as their primary sequences contain elements of both the aldo-keto reductases (AKR) and short chain dehydrogenase/reductase (SDR) enzyme families. During catalysis by members of both enzyme families, an essential Lys residue H-bonds to a Tyr residue that donates proton to the aldehyde substrate. In the Saccharomyces cerevisiae xylose reductase, Tyr49 has been identified as the proton donor. However, the primary sequence of the enzyme contains two Lys residues, Lys53 and Lys78, corresponding to the conserved motifs for SDR and AKR enzyme families, respectively, that may H-bond to Tyr49. We used site-directed mutagenesis to substitute each of these Lys residues with Met. The activity of the K53M variant was slightly decreased as compared to the wild-type, while that of the K78M variant was negligible. The results suggest that Lys78 is the essential residue that H-bonds to Tyr49 during catalysis and indicate that the active site residues of yeast xylose reductases match those of the AKR, rather than SDR, enzymes. Intrinsic enzyme fluorescence spectroscopic analysis suggests that Lys78 may also contribute to the efficient binding of NADPH to the enzyme.

Alteration of the specificity of the cofactor-binding pocket of Corynebacterium 2,5-diketo-D-gluconic acid reductase A

The NADPH-dependent 2,5-diketo-D-gluconic acid (2,5-DKG) reductase enzyme is a required component in some novel biosynthetic vitamin C production processes. This enzyme catalyzes the conversion of 2,5-DKG to 2-keto-L-gulonic acid, which is an immediate precursor to L-ascorbic acid. Forty unique site-directed mutations were made at five residues in the cofactor-binding pocket of 2,5-DKG reductase A in an attempt to improve its ability to use NADH as a cofactor. NADH is more stable, less expensive and more prevalent in the cell than is NADPH. To the best of our knowledge, this is the first focused attempt to alter the cofactor specificity of a member of the aldo-keto reductase superfamily by engineering improved activity with NADH into the enzyme. Activity of the mutants with NADH or NADPH was assayed using activity-stained native polyacrylamide gels. Eight of the mutants at three different sites were identified as having improved activity with NADH. These mutants were purified and subjected to a kinetic characterization with NADH as a cofactor. The best mutant obtained, R238H, produced an almost 7-fold improvement in catalysis with NADH compared with the wild-type enzyme. Surprisingly, most of this catalytic improvement appeared to be due to an improvement in the apparent kcat for the reaction rather than a large improvement in the affinity of the enzyme for NADH.

Critical residues for the specificity of cofactors and substrates in human estrogenic 17beta-hydroxysteroid dehydrogenase 1: variants designed from the three-dimensional structure of the enzyme

Human estrogenic 17beta-hydroxysteroid dehydrogenase is an NADP(H)-preferring enzyme. It possesses 11- and 4-fold higher specificity toward NADP(H) over NAD(H) for oxidation and reduction, respectively, as demonstrated by kinetic studies. To elucidate the roles of the amino acids involved in cofactor specificity, we generated variants by site-directed mutagenesis. The results showed that introducing a positively charged residue, lysine, at the Ser12 position increased the enzyme's preference for NADP(H) more than 20-fold. Substitution of the negatively charged residue, aspartic acid, into the Leu36 position switched the enzyme's cofactor preference from NADPH to NAD with a 220-fold change in the ratio of the specificity toward the two cofactors in the case of oxidation. This variant dramatically abolished the enzyme's reductase function and stimulated its dehydrogenase activity, as shown by enzyme activity in intact cells. The substrate-binding pocket was also studied with four variants: Ser142Gly, Ser142Cys, His221Ala, and Glu282Ala. The Ser142Gly variant abolished most of the enzyme's oxidation and reduction activities. The residual reductase activity in vitro is less than 2% that of the wild-type enzyme. However, the Ser142Cys variant was fully inactive, both as a partially purified protein and in intact cells. This suggests that the bulky sulfhydryl group of cysteine entirely disrupted the catalytic triad and that the Ser142 side chain is important for maintaining the integrity of this triad. His221 variation weakened the apparent affinity for estrone, as demonstrated by a 30-fold increase in Michaelis-Menten constant, supporting its important role in substrate binding. This residue may play an important role in substrate inhibition via the formation of a dead-end complex. The formerly suggested importance of Glu282 could not be confirmed.

Stereospecific interaction of a novel spirosuccinimide type aldose reductase inhibitor, AS-3201, with aldose reductase

Aldose reductase (AR) is an NADPH-dependent enzyme implicated in diabetic complications. AS-3201 [(R)-(-)-2-(4-bromo-2-fluorobenzyl)-1,2,3,4-tetrahydropyrrolo[1,2-a]pyrazine-4-spiro-3'-pyrrolidine-1,2',3,5'-tetrone] is a structurally novel and potent ARI with an inhibitor constant (K(i) = 10(-)(10) M) 2000-fold lower than that of its optical antipode (S-isomer). To elucidate the inhibition modes and the stereochemical differences in their inhibitory potencies, we examined the interaction of these R- and S-isomers with AR under physiological conditions. Enzyme kinetic analysis, which was performed by using physiological substrates at 37 degrees C, showed that both isomers selectively act on the E-NADP(+) complex in both the forward and reverse reactions of AR. However, fluorometric titration analysis demonstrated that the affinities of the isomers for the E-NADP(+) complex are about the same as those for the E-NADPH complex and the apoenzyme. These results suggested that the selective binding to the E-NADP(+) complex arises from the predominance of this enzyme form during steady-state turnover rather than from binding specificity. Both the competition with a known active site-directed ARI and the protective effect on AR inactivation by N-bromosuccinimide showed that the isomers bind to the active site of the enzyme, but the thermodynamic parameters for the binding to AR indicated that additional hydrogen bonds and/or van der Waals interactions contribute to the energetic stabilization in the E-R-isomer complex. Molecular modeling, together with the deductions from spectroscopic studies, suggested that the succinimide ring and the 4-bromo-2-fluorobenzyl group of the R-isomer are optimally located for formation of a hydrogen-bonding network with AR, and that the latter benzyl group is also effective for the differentiation between AR and aldehyde reductase (a closely related enzyme).

Relevance of aldo-keto reductase family members to the pathobiology of diabetic nephropathy and renal development

Aldo-keto reductases (AKRs) are a family of monomeric oxido-reductases with molecular weight ranging from 35-40 kDa and currently includes upwards of 60 members. They are expressed in a wide variety of tissues, where they catalyze the NADPH-dependent reduction of various aliphatic and aromatic aldehydes and ketones. The functions of most of the family members are not well defined. But two members, aldehyde reductase (AKRIA) and aldose reductase (AKRIB), have been extensively studied. The latter has received the most attention since being relevant to the complications of diabetes mellitus. It is up-regulated during hyperglycemia, and at the same time there is an increased activity of the sorbitol pathway and non-enzymatic glycation of proteins with ensuing damage in various tissues. It is developmentally regulated in the ocular lens, and is believed to modulate lens fiber morphogenesis during fetal life. Unlike the other AKR family members that are ubiquitously expressed, recently a renal-specific oxio-reductase has been described that is expressed exclusively in the proximal tubules. Although, it has no homology with other AKR members, it binds to NADPH with high affinity and is up-regulated in streptozotocin-induced diabetes in mice. It is also developmentally regulated and seems to selectively modulate renal tubulogenesis during embryonic life.

Channeling efficiency in the bifunctional methylenetetrahydrofolate dehydrogenase/cyclohydrolase domain: the effects of site-directed mutagenesis of NADP binding residues

The three-dimensional structure of the dehydrogenase-cyclohydrolase bifunctional domain of the human trifunctional enzyme indicates that Arg-173 and Ser-197 are within 3 A of the 2'-phosphate of bound NADP. Site-directed mutagenesis confirms that Arg-173 is essential for efficient binding and cannot be substituted by lysine. R173A and R173K have detectable dehydrogenase activity, but the K(m) values for NADP are increased by at least 500-fold. The S197A mutant has a K(m) for NADP that is only 20-fold higher than wild-type, indicating that it plays a supporting role. Forward and reverse cyclohydrolase activities of all the mutants were unchanged, except that the reverse cyclohydrolase activity of mutants that bind NADP poorly, or lack Ser-197, cannot be stimulated by 2',5'-ADP. The 50% channeling efficiency in the forward direction is not improved by the addition of exogenous NADPH and cannot be explained by premature dissociation of the dinucleotide from the ternary complex. As well, channeling is unaffected in mutants that exhibit a wide range of dinucleotide binding. Given that dinucleotide binding is unrelated to substrate channeling efficiency in the D/C domain, we propose that the difference in forward and reverse channeling efficiencies can be explained solely by the movement of the methenylH(4)folate between two overlapping subsites to which it has different binding affinities.

Mutation of nicotinamide pocket residues in rat liver 3 alpha-hydroxysteroid dehydrogenase reveals different modes of cofactor binding

Rat liver 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD), an aldo-keto reductase, binds NADP(+) in an extended anti-conformation across an (alpha/beta)(8)-barrel. The orientation of the nicotinamide ring, which permits stereospecific transfer of the 4-pro-R hydride from NAD(P)H to substrate, is achieved by hydrogen bonds formed between the C3-carboxamide of the nicotinamide ring and Ser 166, Asn 167, and Gln 190 and by pi-stacking between this ring and Tyr 216. These residues were mutated to yield S166A, N167A, Q190A, and Y216S. In these mutants, K(d)(NADP(H)) increased by 2-11-fold but without a significant change in K(d)(NAD(H)). Steady-state kinetic parameters showed that K(m)(NADP)()+ increased 13-151-fold, and this was accompanied by comparable decreases in k(cat)/K(m)(NADP)()+. By contrast, K(m)(NAD)()+ increased 4-8-fold, but changes in k(cat)/K(m)(NAD)()+ were more dramatic and ranged from 23- to 930-fold. Corresponding changes in binding energies indicated that each residue contributed equally to the binding of NADP(H) in the ground and transition states. However, the same residues stabilized the binding of NAD(H) only in the transition state. These observations suggest that different modes of binding exist for NADP(H) and NAD(H). Importantly, these modes were revealed by mutating residues in the nicotinamide pocket indicating that direct interactions with the 2'-phosphate in the adenine mononucleotide is not the sole determinant of cofactor preference. The single mutations were unable to invert or racemize the stereochemistry of hydride transfer even though the nicotinamide pocket can accommodate both anti- and syn-conformers once the necessary hydrogen bonds are eliminated. When 4-pro-R-[(3)H]NADH was used to monitor incorporation into [(14)C]-5alpha-dihydrotestosterone, a decrease in the (3)H:(14)C ratio was observed in the mutants relative to wild-type enzyme reflecting a pronounced primary kinetic isotope effect. This observation coupled with the change in the binding energy for NAD(P)(H) in the transition state suggests that these mutants have altered the reaction trajectory for hydride transfer.

The arginine 276 anchor for NADP(H) dictates fluorescence kinetic transients in 3 alpha-hydroxysteroid dehydrogenase, a representative aldo-keto reductase

Fluorescence stopped-flow studies were conducted with recombinant rat liver 3 alpha-HSD, an aldo-keto reductase (AKR) that plays critical roles in steroid hormone inactivation, to characterize the binding of nicotinamide cofactor, the first step in the kinetic mechanism. Binding of NADP(H) involved two events: the fast formation of a loose complex (E.NADP(H)), followed by a conformational change in enzyme structure leading to a tightly bound complex (E.NADP(H)), which was observed as a fluorescence kinetic transient. Binding of NAD(H) was not characterized by a similar kinetic transient, implying a difference in the mode of binding of the two cofactors. Unlike previously characterized AKRs, the rates associated with the formation and decay of E.NADP(H) and E.NADP(H) were much faster than kcat for the oxidoreduction of various substrates, indicating that binding and release of cofactor is not rate-limiting overall in 3 alpha-HSD. Mutation of Arg 276, a highly conserved residue in AKRs that forms a salt bridge with the adenosine 2'-phosphate of NADP(H), resulted in large changes in Km and Kd for NADP(H) that were not observed with NAD(H). The loss in free energy associated with the increase in Kd for NADP(H) is consistent with the elimination of an electrostatic link. Importantly, this mutation abolished the kinetic transient associated with NADPH binding. Thus, anchoring of the adenosine 2'-phosphate of NADPH by Arg 276 appears to be obligatory for the fluorescence kinetic transients to be observed. The removal of Trp 86, a residue involved in fluorescence energy transfer with NAD(P)H, also abolished the kinetic transient, but mutation of Trp 227, a residue on a mobile loop associated with cofactor binding, did not. It is concluded that in 3 alpha-HSD, the time dependence of the change in Trp 86 fluorescence is due to cofactor anchoring, and thus, Trp 86 is a distal reporter of this event. Further, the loop movement that accompanies cofactor binding is spectrally silent.

Site-directed mutagenesis studies of the NADPH-binding domain of rat steroid 5alpha-reductase (isozyme-1) I: analysis of aromatic and hydroxylated amino acid residues

Previous studies have shown that the reduced nicotinamide adenine dinucleotide phosphate (NADPH)- binding domain of rat liver microsomal steroid 5alpha-reductase isozyme-1 (r5alphaR-1) is in a highly conserved region of the polypeptide sequence (residues 160-190). In this study, we investigated, by site-directed mutagenesis, the role of hydroxylated and aromatic amino acids within the NADPH-binding domain. The r5alphaR-1 cDNA was cloned into a pCMV vector, and the double strand site-directed mutagenesis method was used to create mutants Y179F, Y179S, Y189F, Y189S, S164A, S164T, and Y187F, which were subsequently expressed in COS-1 cells. Kinetic studies of the expressed enzymes showed that the mutation Y179F resulted in an approximately 40-fold increase in the Km for NADPH versus wild-type, with only a 2-fold increase in the Km for testosterone. The mutants Y189F and S164A showed smaller increases (4 and 6-fold) in Kms for NADPH and no significant change in the Km for testosterone, whereas Y189S had kinetic properties similar to the wild-type r5alphaR-1. Mutants Y179S and S164T both resulted in inactive enzymes, whereas F187Y showed an approximately 5-fold decrease in Km for NADPH and a significant increase (approximately 18-fold) in the Km for testosterone. The results suggest that the -OH functionality of Y179 is involved in cofactor binding, but is not essential for the activity of the enzyme, whereas the -OH functionalities of Y189 and S164 play lesser roles in cofactor binding to r5alphaR-1 and may not be required for enzyme activity. On the other hand, the residue F187 may be important for the binding of both NADPH and testosterone.

The structure and function of yeast xylose (aldose) reductases

Yeast xylose (aldose) reductases are members of the aldo-keto reductase family of enzymes which are widely distributed in a variety of other organisms. In yeasts, these enzymes catalyse the first step of xylose metabolism where xylose is converted to xylitol. In the past 16 years, xylose reductases from yeasts able to ferment or utilize xylose have been isolated and studied mainly because of their importance in xylose bioconversions. In recent years, genes encoding xylose reductases from several yeasts have been cloned and sequenced. A comparison of the primary sequences of yeast xylose reductases with the much better characterized human aldose reductase and human aldehyde reductase reveals that the yeast enzymes are hybrids between aldo-keto reductases and the short chain dehydrogenases/reductases families of enzymes. Why this is so and its evolutionary significance is presently not known. This short review will critically examine the structure and function information that can be gleaned from the sequence comparison. Several interesting questions arise from the sequence comparison and these can provide fruitful areas for further investigations.

Aldose reductase: a window to the treatment of diabetic complications?

Kinetic studies on the aldose reductase protein (AR2) have shown that it does not behave as a classical enzyme in relation to ring aldose sugars. These results have been confirmed by X-ray crystallography studies, which have pinpointed binding sites for pharmacological "aklose reductase inhibitors" (ARIs). As with non-enzymic glycation reactions, there is probably a free-radical element involved derived from monosaccharide autoxidation. In the case of AR2, there is free radical oxidation of NADPH by autoxidising monosaccharides, enhanced in the presence of the NADPH-binding protein. Whatever the behaviour of AR2, many studies have showed that sorbitol production is not an initiating aetiological factor in the development of diabetic complications in humans. Vitamin E (alpha-tocopherol), other antioxidants and high fat diets can delay or prevent cataract in diabetic animals even though sorbitol and fructose levels are not modified; vitamin C acts as an AR1 in humans. Protein post-translational modification by glyc-oxidation or other events is probably the key factor in the aetiology of diabetic complications. There is now no need to invoke AR2 in xylitol biosynthesis. Xylitol can be produced in the lens from glucose, via a pathway involving the enzymes myo-inositol-oxygen oxidoreductase, D-glucuronate reductase. L-gulonate NAD(+)-3-oxidoreductase and L-iditol-NAD(+)-5-oxidoreductase, all of which have recently been found in bovine and rat lens. This chapter investigates the molecular events underlying AR2 and its binding and kinetics. Induction of the protein by osmotic response elements is discussed, with detailed analysis of recent in vitro and in vivo experiments on numerous ARIs. These have a number of actions in the cell which are not specific, and which do not involve them binding to AR2. These include peroxy-radical scavenging and recently discovered effects of metal ion chelation. In controlled experiments, it has been found that incubation of rat lens homogenate with glucose and the copper chelator o-phenanthroline abolishes production of sorbitol. Taken together, these results suggest AR2 is a vestigial NADPH-binding protein, perhaps similar in function to a number of non-mammalian crystallins which have been recruited into the lens. There is mounting evidence for the binding of reactive aldehyde moieties to the protein, and the involvement of AR2 either as a 'housekeeping' protein, or in a free-radial-mediated 'catalytic' role. Interfering with the NADPH binding and flux levels--possibly involving free radicals and metal ions--has a deleterious effect. We have yet to determine whether aldose reductase is the black sheep of the aldehyde reductase family, or whether it is a skeleton in the cupboard, waiting to be clothed in the flesh of new revelations in the interactions between proteins, metal ions and redox metabolites.

Switch of coenzyme specificity of mouse lung carbonyl reductase by substitution of threonine 38 with aspartic acid

Mouse lung carbonyl reductase, a member of the short-chain dehydrogenase/reductase (SDR) family, exhibits coenzyme specificity for NADP(H) over NAD(H). Crystal structure of the enzyme-NADPH complex shows that Thr-38 interacts with the 2'-phosphate of NADPH and occupies the position spatially similar to an Asp residue of the NAD(H)-dependent SDRs that hydrogen-bonds to the hydroxyl groups of the adenine ribose of the coenzymes. Using site-directed mutagenesis, we constructed a mutant mouse lung carbonyl reductase in which Thr-38 was replaced by Asp (T38D), and we compared kinetic properties of the mutant and wild-type enzymes in both forward and reverse reactions. The mutation resulted in increases of more than 200-fold in the Km values for NADP(H) and decreases of more than 7-fold in those for NAD(H), but few changes in the Km values for substrates or in the kcat values of the reactions. NAD(H) provided maximal protection against thermal and urea denaturation of T38D, in contrast to the effective protection by NADP(H) for the wild-type enzyme. Thus, the single mutation converted the coenzyme specificity from NADP(H) to NAD(H). Calculation of free energy changes showed that the 2'-phosphate of NADP(H) contributes to its interaction with the wild-type enzyme. Changing Thr-38 to Asp destabilized the binding energies of NADP(H) by 3.9-4.5 kcal/mol and stabilized those of NAD(H) by 1.2-1.4 kcal/mol. These results indicate a significant role of Thr-38 in NADP(H) binding for the mouse lung enzyme and provide further evidence for the key role of Asp at this position in NAD(H) specificity of the SDR family proteins.

Probing the coenzyme and substrate binding events of CDP-D-glucose 4,6-dehydratase: mechanistic implications

NAD+-dependent nucleotidyl diphosphohexose 4,6-dehydratases which transform nucleotidyl diphosphohexoses into corresponding 4-keto-6-deoxy sugar derivatives are essential to the formation of all 6-deoxyhexoses. Studies of the CDP-D-glucose 4,6-dehydratase (Eod) from Yersinia had shown that this dimeric protein binds only 1 equiv of NAD+/mol of enzyme and, unlike other enzymes of the same class, displays a unique NAD+ requirement for full catalytic activity. Analysis of the primary sequence revealed an extended ADP-binding fold (GHTGFKG) which deviates from the common Rossman consensus (GXGXXG) and thus may have contributed to Eod's limited NAD+ affinity. In particular, the presence of His17 in the beta-turn region and that of Lys21 in a position typically occupied by a small hydrophobic residue may impose electronic or steric perturbations to this essential binding motif. To better understand the correlation between the binding properties and primary sequence, mutants (H17G and K21I) were constructed to provide enzymes containing an ADP binding region which more closely resembles the Rossman-type fold. Analysis of the cofactor and substrate binding characteristics of the wild-type and mutant enzymes helped define the presence of two binding sites for both CDP-d_glucose and NAD+ per enzyme molecule. While both mutants displayed enhanced NAD+ affinity, the H17G mutation resulted in an enzyme with slightly higher kcat and a 3-fold increase in catalytic efficiency (kcat/Km). The large anticooperativity found for NAD+ binding (K1=40.3 + or - 0.4 nM, K2=539.8 + or - 4.8 nM) may explain why the cofactor binding sites of wild-type Eod are only half-occupied. Further examination also revealed the purified Eod to contain sequestered NADH and that the affinity of Eod for NADH(K1=0.21 + or - 0.01 nM, K2= 7.46 + or -0.25 nM) is much higher than that for NAD+. Thus, it is possible that Eod's half-site saturation of NAD+ per enzyme dimer may also be attributed to a significant portion of the cofactor binding sites being occupied by NADH. Interestingly, the sequestered NADH is released upon binding with CDP-D-glucose. These results implicate a new kinetic mechanism for Eod catalysis.

Structure of porcine aldehyde reductase holoenzyme

Aldehyde reductase, a member of the aldo-keto reductase superfamily, catalyzes the NADPH-dependent reduction of a variety of aldehydes to their corresponding alcohols. The structure of porcine aldehyde reductase-NADPH binary complex has been determined by x-ray diffraction methods and refined to a crystallographic R-factor of 0.20 at 2.4 A resolution. The tertiary structure of aldehyde reductase is similar to that of aldose reductase and consists of an alpha/beta-barrel with the active site located at the carboxy terminus of the strands of the barrel. Unlike aldose reductase, the N epsilon 2 of the imidazole ring of His 113 in aldehyde reductase interacts, through a hydrogen bond, with the amide group of the nicotinamide ring of NADPH.