Transient oxidation as a mechanistic strategy in enzymatic catalysis

Trihydroxybenzaldoximes are Redox Cycling Inhibitors of ThDP-Dependent DXP Synthase

Pathogenic bacteria must swiftly adapt to dynamic infection environments in order to survive and colonize in the host. 1-Deoxy-d-xylulose-5-phosphate synthase (DXPS) is thought to play a critical role in bacterial adaptation during infection and is a promising drug target. DXPS utilizes a thiamine diphosphate (ThDP) cofactor to catalyze the decarboxylative condensation of pyruvate and d-glyceraldehyde-3-phosphate (d-GAP) to form DXP, a precursor to isoprenoids and B vitamins. DXPS follows a ligand-gated mechanism in which pyruvate reacts with ThDP to form a long-lived lactyl-ThDP (LThDP) adduct which is coordinated by an active-site network of residues. d-GAP binding ostensibly disrupts this network to activate LThDP for decarboxylation. Our lab previously reported trihydroxybenzaldoxime inhibitors which are competitive with respect to d-GAP, and uncompetitive with respect to pyruvate, suggesting they bind after E-LThDP complex formation. Here, we conducted mechanistic studies to determine if these compounds inhibit DXPS by preventing LThDP activation or if they act as inducers of LThDP activation. We discovered that the catechol moiety of the trihydroxybenzaldoxime scaffold undergoes oxidation under alkaline aerobic conditions, and inhibitory potency is reduced under oxygen restriction. Leveraging long-range 1H-15N HSQC NMR and electrochemical measurements, we demonstrated that the oxidized form of the trihydroxybenzaldoxime induces LThDP decarboxylation and accepts electrons from the resulting carbanion, resulting in reduction to the catechol and formation of acetyl-ThDP which hydrolyzes to form acetate. Under aerobic conditions the catechol is reoxidized. Thus, these compounds act as redox cycling, substrate-wasting inhibitors of DXP formation. These findings uncover a novel activity and mechanism of DXPS inhibition which may have implications for DXPS-mediated redox activity in bacteria. Further exploration of redox active DXPS probes may provide new insights for inhibition strategies and selective probe development.

Trihydroxybenzaldoximes are redox cycling inhibitors of ThDP-dependent DXP synthase

Pathogenic bacteria must swiftly adapt to dynamic infection environments in order to survive and colonize in the host. 1-Deoxy-d-xylulose-5-phosphate synthase (DXPS) is thought to play a critical role in bacterial adaptation during infection and is a promising drug target. DXPS utilizes a thiamine diphosphate (ThDP) cofactor to catalyze the decarboxylative condensation of pyruvate and D-glyceraldehyde-3-phosphate (d-GAP) to form DXP, a precursor to isoprenoids and B vitamins. DXPS follows a ligand-gated mechanism in which pyruvate reacts with ThDP to form a long-lived lactyl-ThDP (LThDP) adduct which is coordinated by an active-site network of residues. d-GAP binding ostensibly disrupts this network to activate LThDP for decarboxylation. Our lab previously reported trihydroxybenzaldoximes inhibitors which are competitive with respect to D-GAP, and uncompetitive with respect to pyruvate, suggesting they bind after E-LThDP complex formation. Here, we conducted mechanistic studies to determine if these compounds inhibit DXPS by preventing LThDP activation or if they act as inducers of LThDP activation. We discovered that the catechol moiety of the trihydroxybenzaldoxime scaffold undergoes oxidation under alkaline aerobic conditions, and inhibitory potency is reduced under oxygen restriction. Leveraging long range 1H-15N HSQC NMR and electrochemical measurements, we demonstrated that the oxidized form of the trihydroxybenzaldoxime induces LThDP decarboxylation. The oxime moiety accepts electrons from the resulting carbanion, resulting in formation of acetyl-ThDP which hydrolyzes to form acetate. SAR studies revealed that the catechol attenuates the redox activity of the oxime moiety, and under aerobic conditions these compounds are oxidized and thus act as redox cycling inhibitors of DXPS. Further exploration of redox active DXPS probes may provide new insights for inhibition strategies and selective probe development.

Biosynthesis of the α-D-Mannosidase Inhibitor (-)-Swainsonine

(-)-Swainsonine is a polyhydroxylated indolizidine alkaloid with potent inhibitory activity against α-D-mannosidases. In this work, we successfully reconstituted swainsonine biosynthetic pathway both in vivo and in vitro. Our study unveiled an unexpected epimerization mechanism involving two α-ketoglutarate-dependent non-heme iron dioxygenases (SwnH2 and SwnH1), and an unusual imine reductase (SwnN), which displays substrate-dependent stereospecificity. The stereochemical outcome of SwnN-catalyzed iminium reduction is ultimately dictated by SwnH1-catalyzed C8-hydroxylation. We also serendipitously discovered that an O -acetyl group can serve as a detachable protecting/directing group, altering the site-selectivity of SwnH2-catalyzed hydroxylation while maintaining the stereoselectivity. Insights gained from the biochemical characterization of these tailoring enzymes enabled biocatalytic synthesis of a new polyhydroxylated indolizidine alkaloid, opening doors to the biosynthesis of diverse natural product-based glycosidase inhibitors.

Structural basis of catalysis and substrate recognition by the NAD(H)-dependent α-d-glucuronidase from the glycoside hydrolase family 4

Members of the glycoside hydrolase family 4 (GH4) employ an unusual glycosidic bond cleavage mechanism utilizing NAD(H) and a divalent metal ion, under reducing conditions. These enzymes act upon a diverse range of glycosides, and unlike most other GH families, homologs here are known to accommodate both α- and β-anomeric specificities within the same active site. Here, we report the catalytic properties and the crystal structures of TmAgu4B, an α-d-glucuronidase from the hyperthermophile Thermotoga maritima. The structures in three different states include the apo form, the NADH bound holo form, and the ternary complex with NADH and the reaction product d-glucuronic acid, at 2.15, 1.97 and 1.85 Å resolutions, respectively. These structures reveal the step-wise route of conformational changes required in the active site to achieve the catalytically competent state, and illustrate the direct role of residues that determine the reaction mechanism. Furthermore, a structural transition of a helical region in the active site to a turn geometry resulting in the rearrangement of a unique arginine residue governs the exclusive glucopyranosiduronic acid recognition in TmAgu4B. Mutational studies show that modifications of the glycone binding site geometry lead to catalytic failure and indicate overlapping roles of specific residues in catalysis and substrate recognition. The data highlight hitherto unreported molecular features and associated active site dynamics that determine the structure-function relationships within the unique GH4 family.

Isotope Probing of the UDP-Apiose/UDP-Xylose Synthase Reaction: Evidence of a Mechanism via a Coupled Oxidation and Aldol Cleavage

The C-branched sugar d-apiose (Api) is essential for plant cell-wall development. An enzyme-catalyzed decarboxylation/pyranoside ring-contraction reaction leads from UDP-α-d-glucuronic acid (UDP-GlcA) to the Api precursor UDP-α-d-apiose (UDP-Api). We examined the mechanism of UDP-Api/UDP-α-d-xylose synthase (UAXS) with site-selectively 2 H-labeled and deoxygenated substrates. The analogue UDP-2-deoxy-GlcA, which prevents C-2/C-3 aldol cleavage as the plausible initiating step of pyranoside-to-furanoside conversion, did not give the corresponding Api product. Kinetic isotope effects (KIEs) support an UAXS mechanism in which substrate oxidation by enzyme-NAD+ and retro-aldol sugar ring-opening occur coupled in a single rate-limiting step leading to decarboxylation. Rearrangement and ring-contracting aldol addition in an open-chain intermediate then give the UDP-Api aldehyde, which is intercepted via reduction by enzyme-NADH.

Structural and functional characterization of the Clostridium perfringens N-acetylmannosamine-6-phosphate 2-epimerase essential for the sialic acid salvage pathway

Pathogenic bacteria are endowed with an arsenal of specialized enzymes to convert nutrient compounds from their cell hosts. The essential N-acetylmannosamine-6-phosphate 2-epimerase (NanE) belongs to a convergent glycolytic pathway for utilization of the three amino sugars, GlcNAc, ManNAc, and sialic acid. The crystal structure of ligand-free NanE from Clostridium perfringens reveals a modified triose-phosphate isomerase (β/α)8 barrel in which a stable dimer is formed by exchanging the C-terminal helix. By retaining catalytic activity in the crystalline state, the structure of the enzyme bound to the GlcNAc-6P product identifies the topology of the active site pocket and points to invariant residues Lys(66) as a putative single catalyst, supported by the structure of the catalytically inactive K66A mutant in complex with substrate ManNAc-6P. (1)H NMR-based time course assays of native NanE and mutated variants demonstrate the essential role of Lys(66) for the epimerization reaction with participation of neighboring Arg(43), Asp(126), and Glu(180) residues. These findings unveil a one-base catalytic mechanism of C2 deprotonation/reprotonation via an enolate intermediate and provide the structural basis for the development of new antimicrobial agents against this family of bacterial 2-epimerases.

The evolution of enzyme function in the isomerases

The advent of computational approaches to measure functional similarity between enzymes adds a new dimension to existing evolutionary studies based on sequence and structure. This paper reviews research efforts aiming to understand the evolution of enzyme function in superfamilies, presenting a novel strategy to provide an overview of the evolution of enzymes belonging to an individual EC class, using the isomerases as an exemplar.

Structure and mechanism of human UDP-xylose synthase: evidence for a promoting role of sugar ring distortion in a three-step catalytic conversion of UDP-glucuronic acid

UDP-xylose synthase (UXS) catalyzes decarboxylation of UDP-d-glucuronic acid to UDP-xylose. In mammals, UDP-xylose serves to initiate glycosaminoglycan synthesis on the protein core of extracellular matrix proteoglycans. Lack of UXS activity leads to a defective extracellular matrix, resulting in strong interference with cell signaling pathways. We present comprehensive structural and mechanistic characterization of the human form of UXS. The 1.26-Å crystal structure of the enzyme bound with NAD⁺ and UDP reveals a homodimeric short-chain dehydrogenase/reductase (SDR), belonging to the NDP-sugar epimerases/dehydratases subclass. We show that enzymatic reaction proceeds in three chemical steps via UDP-4-keto-d-glucuronic acid and UDP-4-keto-pentose intermediates. Molecular dynamics simulations reveal that the d-glucuronyl ring accommodated by UXS features a marked ⁴C₁chair to B_O,3boat distortion that facilitates catalysis in two different ways. It promotes oxidation at C₄ (step 1) by aligning the enzymatic base Tyr¹⁴⁷ with the reactive substrate hydroxyl and it brings the carboxylate group at C₅ into an almost fully axial position, ideal for decarboxylation of UDP-4-keto-d-glucuronic acid in the second chemical step. The protonated side chain of Tyr¹⁴⁷ stabilizes the enolate of decarboxylated C₄ keto species (²H₁half-chair) that is then protonated from the Si face at C₅, involving water coordinated by Glu¹²⁰. Arg²⁷⁷, which is positioned by a salt-link interaction with Glu¹²⁰, closes up the catalytic site and prevents release of the UDP-4-keto-pentose and NADH intermediates. Hydrogenation of the C₄ keto group by NADH, assisted by Tyr¹⁴⁷ as catalytic proton donor, yields UDP-xylose adopting the relaxed ⁴C₁chair conformation (step 3).

Redox mechanism of glycosidic bond hydrolysis catalyzed by 6-phospho-alpha-glucosidase: a DFT study

Glycosidic bonds are remarkably resistant to cleavage by chemical hydrolysis. Glycoside hydrolases catalyze their selective hydrolysis in oligosaccharides, polysaccharides, and glycoconjugates by following nonredox catalytic pathways or a net redox-neutral catalytic pathway using NAD(+) and divalent metal ions as cofactors. GlvA (6-phospho-alpha-glucosidase) is a glycosidase belonging to family GH4 and follows a regioselective redox-neutral mechanism of glycosidic-bond hydrolysis that favors alpha- over beta-glycosides. Its proposed catalytic mechanism can be divided into two half-reactions: the first one activates the glucopyranose ring by successively forming intermediates that are oxidized at the 3-, 2-, and 1-positions of the ring, which ultimately facilitate the heterolytic deglycosylation. The second half-reaction is essentially the reverse of the first half-reaction, beginning with the pyranose ring hydroxylation at the anomeric carbon, and it is followed by 3-reduction and regeneration of the active forms of the catalytic site and its cofactors. We investigated the NAD(+)-dependent redox mechanism of glycosidic bond hydrolysis as catalyzed by GlvA through the combined application of density functional theory and a self-consistent reaction field to a large active-site model obtained from the crystallographic structure of the enzyme, then we applied natural bond orbital and second-order perturbation analyses to monitor the electron flow and change in oxidation state on each atomic center along the reaction coordinate to rationalize the energetics and regioselectivity of this catalytic mechanism. We find that in GlvA, the redox catalytic mechanism of hydrolysis is driven by the gradual strengthening of the axial endo-anomeric component within the hexose ring along the reaction coordinate to facilitate the heterolytic dissociation of the axial C1-O bond. In addition, the combined influence of specific components of the generalized anomeric effect fully explains the regioselectivity observed in the catalytic activity of GlvA.