Structure and function of the E. coli dihydroneopterin triphosphate pyrophosphatase: a Nudix enzyme involved in folate biosynthesis

Nudix hydrolases are a superfamily of pyrophosphatases, most of which are involved in clearing the cell of potentially deleterious metabolites and in preventing the accumulation of metabolic intermediates. We determined that the product of the orf17 gene of Escherichia coli, a Nudix NTP hydrolase, catalyzes the hydrolytic release of pyrophosphate from dihydroneopterin triphosphate, the committed step of folate synthesis in bacteria. That this dihydroneopterin hydrolase (DHNTPase) is indeed a key enzyme in the folate pathway was confirmed in vivo: knockout of this gene in E. coli leads to a marked reduction in folate synthesis that is completely restored by a plasmid carrying the gene. We also determined the crystal structure of this enzyme using data to 1.8 A resolution and studied the kinetics of the reaction. These results provide insight into the structural bases for catalysis and substrate specificity in this enzyme and allow the definition of the dihydroneopterin triphosphate pyrophosphatase family of Nudix enzymes.     

A novel glycoside hydrolase family 105: the structure of family 105 unsaturated rhamnogalacturonyl hydrolase complexed with a disaccharide in comparison with family 88 enzyme complexed with the disaccharide

YteR, a hypothetical protein with unknown functions, is derived from Bacillus subtilis strain 168 and has an overall structure similar to that of bacterial unsaturated glucuronyl hydrolase (UGL), although it exhibits little amino acid sequence identity with UGL. UGL releases unsaturated glucuronic acid from glycosaminoglycan treated with glycosaminoglycan lyases. The amino acid sequence of YteR shows a significant homology (26% identity) with the hypothetical protein YesR also from B. subtilis strain 168. To clarify the intrinsic functions of YteR and YesR, both proteins were overexpressed in Escherichia coli, purified, and characterized. Based on their gene arrangements in genome and enzyme properties, YteR and YesR were found to constitute a novel enzyme activity, "unsaturated rhamnogalacturonyl hydrolase," classified as new glycoside hydrolase family 105. This enzyme acts specifically on unsaturated rhamnogalacturonan (RG) obtained from RG type-I treated with RG lyases and releases an unsaturated galacturonic acid. The crystal structure of YteR complexed with unsaturated chondroitin disaccharide (UGL substrate) was obtained and compared to the structure of UGL complexed with the same disaccharide. The UGL substrate is sterically hindered with the active pocket of YteR. The protruding loop of YteR prevents the UGL substrate from being bound effectively. The most likely candidate catalytic residues for general acid/base are Asp143 in YteR and Asp135 in YesR. This is supported by three-dimensional structural and site-directed mutagenesis studies. These findings provide molecular insights into novel enzyme catalysis and sequential reaction mechanisms involved in RG-I depolymerization by bacteria.     

The structure of an enzyme-product complex reveals the critical role of a terminal hydroxide nucleophile in the bacterial phosphotriesterase mechanism

A detailed understanding of the catalytic mechanism of enzymes is an important step toward improving their activity for use in biotechnology. In this paper, crystal soaking experiments and X-ray crystallography were used to analyse the mechanism of the Agrobacterium radiobacter phosphotriesterase, OpdA, an enzyme capable of detoxifying a broad range of organophosphate pesticides. The structures of OpdA complexed with ethylene glycol and the product of dimethoate hydrolysis, dimethyl thiophosphate, provide new details of the catalytic mechanism. These structures suggest that the attacking nucleophile is a terminally bound hydroxide, consistent with the catalytic mechanism of other binuclear metallophosphoesterases. In addition, a crystal structure with the potential substrate trimethyl phosphate bound non-productively demonstrates the importance of the active site cavity in orienting the substrate into an approximation of the transition state.     

Structural analyses of covalent enzyme-substrate analog complexes reveal strengths and limitations of de novo enzyme design

We report the cocrystal structures of a computationally designed and experimentally optimized retro-aldol enzyme with covalently bound substrate analogs. The structure with a covalently bound mechanism-based inhibitor is similar to, but not identical with, the design model, with an RMSD of 1.4 Å over active-site residues and equivalent substrate atoms. As in the design model, the binding pocket orients the substrate through hydrophobic interactions with the naphthyl moiety such that the oxygen atoms analogous to the carbinolamine and β-hydroxyl oxygens are positioned near a network of bound waters. However, there are differences between the design model and the structure: the orientation of the naphthyl group and the conformation of the catalytic lysine are slightly different; the bound water network appears to be more extensive; and the bound substrate analog exhibits more conformational heterogeneity than typical native enzyme–inhibitor complexes. Alanine scanning of the active-site residues shows that both the catalytic lysine and the residues around the binding pocket for the substrate naphthyl group make critical contributions to catalysis. Mutating the set of water-coordinating residues also significantly reduces catalytic activity. The crystal structure of the enzyme with a smaller substrate analog that lacks naphthyl ring shows the catalytic lysine to be more flexible than in the naphthyl–substrate complex; increased preorganization of the active site would likely improve catalysis. The covalently bound complex structures and mutagenesis data highlight the strengths and weaknesses of the de novo enzyme design strategy.     

Human γ-Glutamyl Transpeptidase 1: STRUCTURES OF THE FREE ENZYME,INHIBITOR-BOUND TETRAHEDRAL TRANSITION STATES,AND GLUTAMATE-BOUND ENZYME REVEAL NOVEL MOVEMENT WITHIN THE ACTIVE SITE DURING CATALYSIS*

γ-Glutamyl transpeptidase 1 (GGT1) is a cell surface, N-terminal nucleophile hydrolase that cleaves glutathione and other γ-glutamyl compounds. GGT1 expression is essential in cysteine homeostasis, and its induction has been implicated in the pathology of asthma, reperfusion injury, and cancer. In this study, we report four new crystal structures of human GGT1 (hGGT1) that show conformational changes within the active site as the enzyme progresses from the free enzyme to inhibitor-bound tetrahedral transition states and finally to the glutamate-bound structure prior to the release of this final product of the reaction. The structure of the apoenzyme shows flexibility within the active site. The serine-borate-bound hGGT1 crystal structure demonstrates that serine-borate occupies the active site of the enzyme, resulting in an enzyme-inhibitor complex that replicates the enzyme''s tetrahedral intermediate/transition state. The structure of GGsTop-bound hGGT1 reveals its interactions with the enzyme and why neutral phosphonate diesters are more potent inhibitors than monoanionic phosphonates. These structures are the first structures for any eukaryotic GGT that include a molecule in the active site covalently bound to the catalytic Thr-381. The glutamate-bound structure shows the conformation of the enzyme prior to release of the final product and reveals novel information regarding the displacement of the main chain atoms that form the oxyanion hole and movement of the lid loop region when the active site is occupied. These data provide new insights into the mechanism of hGGT1-catalyzed reactions and will be invaluable in the development of new classes of hGGT1 inhibitors for therapeutic use.     

The structure of an enzyme–product complex reveals the critical role of a terminal hydroxide nucleophile in the bacterial phosphotriesterase mechanism

A detailed understanding of the catalytic mechanism of enzymes is an important step toward improving their activity for use in biotechnology. In this paper, crystal soaking experiments and X-ray crystallography were used to analyse the mechanism of the Agrobacterium radiobacter phosphotriesterase, OpdA, an enzyme capable of detoxifying a broad range of organophosphate pesticides. The structures of OpdA complexed with ethylene glycol and the product of dimethoate hydrolysis, dimethyl thiophosphate, provide new details of the catalytic mechanism. These structures suggest that the attacking nucleophile is a terminally bound hydroxide, consistent with the catalytic mechanism of other binuclear metallophosphoesterases. In addition, a crystal structure with the potential substrate trimethyl phosphate bound non-productively demonstrates the importance of the active site cavity in orienting the substrate into an approximation of the transition state.     

Crystal structures of phosphite dehydrogenase provide insights into nicotinamide cofactor regeneration

The enzyme phosphite dehydrogenase (PTDH) catalyzes the NAD(+)-dependent conversion of phosphite to phosphate and represents the first biological catalyst that has been shown to conduct the enzymatic oxidation of phosphorus. Despite investigation for more than a decade into both the mechanism of its unusual reaction and its utility in cofactor regeneration, there has been a lack of any structural data for PTDH. Here we present the cocrystal structure of an engineered thermostable variant of PTDH bound to NAD(+) (1.7 ? resolution), as well as four other cocrystal structures of thermostable PTDH and its variants with different ligands (all between 1.85 and 2.3 ? resolution). These structures provide a molecular framework for understanding prior mutational analysis and point to additional residues, located in the active site, that may contribute to the enzymatic activity of this highly unusual catalyst.     

Crystal structure of N-succinylarginine dihydrolase AstB, bound to substrate and product, an enzyme from the arginine catabolic pathway of Escherichia coli

The ammonia-producing arginine succinyltransferase pathway is the major pathway in Escherichia coli and related bacteria for arginine catabolism as a sole nitrogen source. This pathway consists of five steps, each catalyzed by a distinct enzyme. Here we report the crystal structure of N-succinylarginine dihydrolase AstB, the second enzyme of the arginine succinyltransferase pathway, providing the first structural insight into enzymes from this pathway. The enzyme exhibits a pseudo 5-fold symmetric alpha/beta propeller fold of circularly arranged betabetaalphabeta modules enclosing the active site. The crystal structure indicates clearly that this enzyme belongs to the amidinotransferase (AT) superfamily and that the active site contains a Cys-His-Glu triad characteristic of the AT superfamily. Structures of the complexes of AstB with the reaction product and a C365S mutant with bound the N-succinylarginine substrate suggest a catalytic mechanism that consists of two cycles of hydrolysis and ammonia release, with each cycle utilizing a mechanism similar to that proposed for arginine deiminases. Like other members of the AT superfamily of enzymes, AstB possesses a flexible loop that is disordered in the absence of substrate and assumes an ordered conformation upon substrate binding, shielding the ligand from the bulk solvent, thereby controlling substrate access and product release.     

The homing endonuclease I-CreI uses three metals, one of which is shared between the two active sites

Homing endonucleases, like restriction enzymes, cleave double-stranded DNA at specific target sites. The cleavage mechanism(s) utilized by LAGLIDADG endonucleases have been difficult to elucidate; their active sites are divergent, and only one low resolution cocrystal structure has been determined. Here we report two high resolution structures of the dimeric I-CreI homing endonuclease bound to DNA: a substrate complex with calcium and a product complex with magnesium. The bound metals in both complexes are verified by manganese anomalous difference maps. The active sites are positioned close together to facilitate cleavage across the DNA minor groove; each contains one metal ion bound between a conserved aspartate (Asp 20) and a single scissile phosphate. A third metal ion bridges the two active sites. This divalent cation is bound between aspartate residues from the active site of each subunit and is in simultaneous contact with the scissile phosphates of both DNA strands. A metal-bound water molecule acts as the nucleophile and is part of an extensive network of ordered water molecules that are positioned by enzyme side chains. These structures illustrate a unique variant of a two-metal endonuclease mechanism is employed by the highly divergent LAGLIDADG enzyme family.     

Structural Elucidation of the Cyclization Mechanism of α-1,6-Glucan by Bacillus circulans T-3040 Cycloisomaltooligosaccharide Glucanotransferase

Bacillus circulans T-3040 cycloisomaltooligosaccharide glucanotransferase belongs to the glycoside hydrolase family 66 and catalyzes an intramolecular transglucosylation reaction that produces cycloisomaltooligosaccharides from dextran. The crystal structure of the core fragment from Ser-39 to Met-738 of B. circulans T-3040 cycloisomaltooligosaccharide glucanotransferase, devoid of its N-terminal signal peptide and C-terminal nonconserved regions, was determined. The structural model contained one catalytic (β/α)₈-barrel domain and three β-domains. Domain N with an immunoglobulin-like β-sandwich fold was attached to the N terminus; domain C with a Greek key β-sandwich fold was located at the C terminus, and a carbohydrate-binding module family 35 (CBM35) β-jellyroll domain B was inserted between the 7th β-strand and the 7th α-helix of the catalytic domain A. The structures of the inactive catalytic nucleophile mutant enzyme complexed with isomaltohexaose, isomaltoheptaose, isomaltooctaose, and cycloisomaltooctaose revealed that the ligands bound in the catalytic cleft and the sugar-binding site of CBM35. Of these, isomaltooctaose bound in the catalytic site extended to the second sugar-binding site of CBM35, which acted as subsite −8, representing the enzyme·substrate complex when the enzyme produces cycloisomaltooctaose. The isomaltoheptaose and cycloisomaltooctaose bound in the catalytic cleft with a circular structure around Met-310, representing the enzyme·product complex. These structures collectively indicated that CBM35 functions in determining the size of the product, causing the predominant production of cycloisomaltooctaose by the enzyme. The canonical sugar-binding site of CBM35 bound the mid-part of isomaltooligosaccharides, indicating that the original function involved substrate binding required for efficient catalysis.     

Outer-membrane phospholipase A: known structure, unknown biological function

Outer-membrane phospholipase A (OMPLA) is one of the few enzymes present in the outer membrane of Gram-negative bacteria. The enzymatic activity of OMPLA is strictly regulated to prevent uncontrolled breakdown of the surrounding phospholipids. The activity of OMPLA can be induced by membrane perturbation and concurs with dimerization of the enzyme. The recently elucidated crystal structures of the inactive, monomeric and an inhibited dimeric form of the enzyme provide detailed structural insight into the functional properties of the enzyme. OMPLA is a serine hydrolase with a unique Asn-156-His-142-Ser-144 catalytic triad. Only in the dimeric state, complete substrate binding pockets and functional oxyanion holes are formed. A model is proposed for the activation of OMPLA in which membrane perturbation causes the formation of non-bilayer structures, resulting in the presentation of phospholipids to the active site of OMPLA and leading to the formation of the active dimeric species. Possible roles for OMPLA in maintaining the cell envelope integrity and in pathogenicity are discussed.     

Dissecting the catalytic mechanism of a plant beta-D-glucan glucohydrolase through structural biology using inhibitors and substrate analogues

Higher plant, family GH3 beta-D-glucan glucohydrolases exhibit exo-hydrolytic and retaining (e-->e) mechanisms of action and catalyze the removal of single glucosyl residues from the non-reducing termini of beta-D-linked glucosidic substrates, with retention of anomeric configuration. The broad specificity beta-D-glucan glucohydrolases are likely to play roles in cell wall re-modelling, turn-over of cell wall components and possibly in plant defence reactions against pathogens. Crystal structures of the barley beta-D-glucan glucohydrolase, obtained from both native enzyme and from the enzyme in complex with a substrate analogues and mechanism-based inhibitors, have enabled the basis of substrate specificity, the mechanism of catalysis, and the role of domain movements during the catalytic cycle to be defined in precise molecular terms. The active site of the enzyme forms a shallow 'pocket' that is located at the interface of two domains of the enzyme and accommodates two glucosyl residues. The propensity of the enzyme to hydrolyze a broad range of substrates with (1-->2)-, (1-->3)-, (1-->4)- and (1-->6)-beta-D-glucosidic linkages is explained from crystal structures of the enzyme in complex with non-hydrolysable S-glycoside substrate analogues, and from molecular modelling. During binding of gluco-oligosaccharides, the glucosyl residue at subsite -1 is locked in a highly constrained position, but the glucosyl residue at the +1 subsite is free to adjust its position between two tryptophan residues positioned at the entry of the active site pocket. The flexibility at subsite +1 and the projection of the remainder of the substrate away from the pocket provide a structural rationale for the capacity of the enzyme to accommodate and hydrolyze glucosides with different linkage positions and hence different overall conformations. While mechanism-based inhibitors with micromolar Ki constants bind in the active site of the enzyme and form esters with the catalytic nucleophile, transition-state mimics bind with their 'glucose' moieties distorted into the 4E conformation, which is critical for the nanomolar binding of these inhibitors to the enzyme. The glucose product of the reaction, which is released from the non-reducing termini of substrates, remains bound to the beta-D-glucan glucohydrolase in the -1 subsite of the active site, until a new substrate molecule approaches the enzyme. If dissociation of the glucose from the enzyme active site could be synchronized throughout the crystal, time-resolved Laue X-ray crystallography could be used to follow the conformational changes that occur as the glucose product diffuses away and the incoming substrate is bound by the enzyme.     

Structural insights into substrate traffic and inhibition in acetylcholinesterase

Acetylcholinesterase (AChE) terminates nerve-impulse transmission at cholinergic synapses by rapid hydrolysis of the neurotransmitter, acetylcholine. Substrate traffic in AChE involves at least two binding sites, the catalytic and peripheral anionic sites, which have been suggested to be allosterically related and involved in substrate inhibition. Here, we present the crystal structures of Torpedo californica AChE complexed with the substrate acetylthiocholine, the product thiocholine and a nonhydrolysable substrate analogue. These structures provide a series of static snapshots of the substrate en route to the active site and identify, for the first time, binding of substrate and product at both the peripheral and active sites. Furthermore, they provide structural insight into substrate inhibition in AChE at two different substrate concentrations. Our structural data indicate that substrate inhibition at moderate substrate concentration is due to choline exit being hindered by a substrate molecule bound at the peripheral site. At the higher concentration, substrate inhibition arises from prevention of exit of acetate due to binding of two substrate molecules within the active-site gorge.     

A Proton Wire and Water Channel Revealed in the Crystal Structure of Isatin Hydrolase

The high resolution crystal structures of isatin hydrolase from Labrenzia aggregata in the apo and the product state are described. These are the first structures of a functionally characterized metal-dependent hydrolase of this fold. Isatin hydrolase converts isatin to isatinate and belongs to a novel family of metalloenzymes that include the bacterial kynurenine formamidase. The product state, mimicked by bound thioisatinate, reveals a water molecule that bridges the thioisatinate to a proton wire in an adjacent water channel and thus allows the proton released by the reaction to escape only when the product is formed. The functional proton wire present in isatin hydrolase isoform b represents a unique catalytic feature common to all hydrolases is here trapped and visualized for the first time. The local molecular environment required to coordinate thioisatinate allows stronger and more confident identification of orthologous genes encoding isatin hydrolases within the prokaryotic kingdom. The isatin hydrolase orthologues found in human gut bacteria raise the question as to whether the indole-3-acetic acid degradation pathway is present in human gut flora.     

Novel Insights into Eukaryotic γ-Glutamyltranspeptidase 1 from the Crystal Structure of the Glutamate-bound Human Enzyme

The enzyme γ-glutamyltranspeptidase 1 (GGT1) is a conserved member of the N-terminal nucleophile hydrolase family that cleaves the γ-glutamyl bond of glutathione and other γ-glutamyl compounds. In animals, GGT1 is expressed on the surface of the cell and has critical roles in maintaining cysteine levels in the body and regulating intracellular redox status. Expression of GGT1 has been implicated as a potentiator of asthma, cardiovascular disease, and cancer. The rational design of effective inhibitors of human GGT1 (hGGT1) has been delayed by the lack of a reliable structural model. The available crystal structures of several bacterial GGTs have been of limited use due to differences in the catalytic behavior of bacterial and mammalian GGTs. We report the high resolution (1.67 Å) crystal structure of glutamate-bound hGGT1, the first of any eukaryotic GGT. Comparisons of the active site architecture of hGGT1 with those of its bacterial orthologs highlight key differences in the residues responsible for substrate binding, including a bimodal switch in the orientation of the catalytic nucleophile (Thr-381) that is unique to the human enzyme. Compared with several bacterial counterparts, the lid loop in the crystal structure of hGGT1 adopts an open conformation that allows greater access to the active site. The hGGT1 structure also revealed tightly bound chlorides near the catalytic residue that may contribute to catalytic activity. These are absent in the bacterial GGTs. These differences between bacterial and mammalian GGTs and the new structural data will accelerate the development of new therapies for GGT1-dependent diseases.     

Structural elucidation of dextran degradation mechanism by streptococcus mutans dextranase belonging to glycoside hydrolase family 66

Dextranase is an enzyme that hydrolyzes dextran α-1,6 linkages. Streptococcus mutans dextranase belongs to glycoside hydrolase family 66, producing isomaltooligosaccharides of various sizes and consisting of at least five amino acid sequence regions. The crystal structure of the conserved fragment from Gln¹⁰⁰ to Ile⁷³² of S. mutans dextranase, devoid of its N- and C-terminal variable regions, was determined at 1.6 Å resolution and found to contain three structural domains. Domain N possessed an immunoglobulin-like β-sandwich fold; domain A contained the enzyme''s catalytic module, comprising a (β/α)₈-barrel; and domain C formed a β-sandwich structure containing two Greek key motifs. Two ligand complex structures were also determined, and, in the enzyme-isomaltotriose complex structure, the bound isomaltooligosaccharide with four glucose moieties was observed in the catalytic glycone cleft and considered to be the transglycosylation product of the enzyme, indicating the presence of four subsites, −4 to −1, in the catalytic cleft. The complexed structure with 4′,5′-epoxypentyl-α-d-glucopyranoside, a suicide substrate of the enzyme, revealed that the epoxide ring reacted to form a covalent bond with the Asp³⁸⁵ side chain. These structures collectively indicated that Asp³⁸⁵ was the catalytic nucleophile and that Glu⁴⁵³ was the acid/base of the double displacement mechanism, in which the enzyme showed a retaining catalytic character. This is the first structural report for the enzyme belonging to glycoside hydrolase family 66, elucidating the enzyme''s catalytic machinery.     

Structure of Putrescine Aminotransferase from Escherichia coli Provides Insights into the Substrate Specificity among Class III Aminotransferases

YgjG is a putrescine aminotransferase enzyme that transfers amino groups from compounds with terminal primary amines to compounds with an aldehyde group using pyridoxal-5′-phosphate (PLP) as a cofactor. Previous biochemical data show that the enzyme prefers primary diamines, such as putrescine, over ornithine as a substrate. To better understand the enzyme''s substrate specificity, crystal structures of YgjG from Escherichia coli were determined at 2.3 and 2.1 Å resolutions for the free and putrescine-bound enzymes, respectively. Sequence and structural analyses revealed that YgjG forms a dimer that adopts a class III PLP-dependent aminotransferase fold. A structural comparison between YgjG and other class III aminotransferases revealed that their structures are similar. However, YgjG has an additional N-terminal helical structure that partially contributes to a dimeric interaction with the other subunit via a helix-helix interaction. Interestingly, the YgjG substrate-binding site entrance size and charge distribution are smaller and more hydrophobic than other class III aminotransferases, which suggest that YgjG has a unique substrate binding site that could accommodate primary aliphatic diamine substrates, including putrescine. The YgjG crystal structures provide structural clues to putrescine aminotransferase substrate specificity and binding.     

Molecular basis for the inhibition of human NMPRTase, a novel target for anticancer agents

Nicotinamide phosphoribosyltransferase (NMPRTase) has a crucial role in the salvage pathway of NAD+ biosynthesis, and a potent inhibitor of NMPRTase, FK866, can reduce cellular NAD+ levels and induce apoptosis in tumors. We have determined the crystal structures at up to 2.1-A resolution of human and murine NMPRTase, alone and in complex with the reaction product nicotinamide mononucleotide or the inhibitor FK866. The structures suggest that Asp219 is a determinant of substrate specificity of NMPRTase, which is confirmed by our mutagenesis studies. FK866 is bound in a tunnel at the interface of the NMPRTase dimer, and mutations in this binding site can abolish the inhibition by FK866. Contrary to current knowledge, the structures show that FK866 should compete directly with the nicotinamide substrate. Our structural and biochemical studies provide a starting point for the development of new anticancer agents.     

Structural characterization of the reaction pathway in phosphoserine phosphatase: crystallographic "snapshots" of intermediate states

Phosphoserine phosphatase (PSP) is a member of a large class of enzymes that catalyze phosphoester hydrolysis using a phosphoaspartate-enzyme intermediate. PSP is a likely regulator of the steady-state d-serine level in the brain, which is a critical co-agonist of the N-methyl-d-aspartate type of glutamate receptors. Here, we present high-resolution (1.5-1.9 A) structures of PSP from Methanococcus jannaschii, which define the open state prior to substrate binding, the complex with phosphoserine substrate bound (with a D to N mutation in the active site), and the complex with AlF3, a transition-state analog for the phospho-transfer steps in the reaction. These structures, together with those described for the BeF3- complex (mimicking the phospho-enzyme) and the enzyme with phosphate product in the active site, provide a detailed structural picture of the full reaction cycle. The structure of the apo state indicates partial unfolding of the enzyme to allow substrate binding, with refolding in the presence of substrate to provide specificity. Interdomain and active-site conformational changes are identified. The structure with the transition state analog bound indicates a "tight" intermediate. A striking structure homology, with significant sequence conservation, among PSP, P-type ATPases and response regulators suggests that the knowledge of the PSP reaction mechanism from the structures determined will provide insights into the reaction mechanisms of the other enzymes in this family.     

Crystal structure of Thermotoga maritima alpha-glucosidase AglA defines a new clan of NAD+-dependent glycosidases

Glycoside hydrolase family 4 represents an unusual group of glucosidases with a requirement for NAD+, divalent metal cations, and reducing conditions. The family is also unique in its inclusion of both alpha- and beta-specific enzymes. The alpha-glucosidase A, AglA, from Thermotoga maritima is a typical glycoside hydrolase family 4 enzyme, requiring NAD+ and Mn2+ as well as strongly reducing conditions for activity. Here we present the crystal structure of the protein complexed with NAD+ and maltose, refined at a resolution of 1.9 A. The NAD+ is bound to a typical Rossman fold NAD+-binding site, and the nicotinamide moiety is localized close to the maltose substrate. Within the active site the conserved Cys-174 and surrounding histidines are positioned to play a role in the hydrolysis reaction. The electron density maps indicate that Cys-174 is oxidized to a sulfinic acid. Most likely, the strongly reducing conditions are necessary to reduce the oxidized cysteine side chain. Notably, the canonical set of catalytic acidic residues common to other glucosidases is not present in the active site. This, combined with a high structural homology to NAD-dependent dehydrogenases, suggests an unusual and possibly unique mechanism of action for a glycoside-hydrolyzing enzyme.