Three-dimensional structure of Erwinia chrysanthemi pectin methylesterase reveals a novel esterase active site

Most structures of neutral lipases and esterases have been found to adopt the common alpha/beta hydrolase fold and contain a catalytic Ser-His-Asp triad. Some variation occurs in both the overall protein fold and in the location of the catalytic triad, and in some enzymes the role of the aspartate residue is replaced by a main-chain carbonyl oxygen atom. Here, we report the crystal structure of pectin methylesterase that has neither the common alpha/beta hydrolase fold nor the common catalytic triad. The structure of the Erwinia chrysanthemi enzyme was solved by multiple isomorphous replacement and refined at 2.4 A to a conventional crystallographic R-factor of 17.9 % (R(free) 21.1 %). This is the first structure of a pectin methylesterase and reveals the enzyme to comprise a right-handed parallel beta-helix as seen in the pectinolytic enzymes pectate lyase, pectin lyase, polygalacturonase and rhamnogalacturonase, and unlike the alpha/beta hydrolase fold of rhamnogalacturonan acetylesterase with which it shares esterase activity. Pectin methylesterase has no significant sequence similarity with any protein of known structure. Sequence conservation among the pectin methylesterases has been mapped onto the structure and reveals that the active site comprises two aspartate residues and an arginine residue. These proposed catalytic residues, located on the solvent-accessible surface of the parallel beta-helix and in a cleft formed by external loops, are at a location similar to that of the active site and substrate-binding cleft of pectate lyase. The structure of pectin methylesterase is an example of a new family of esterases.     

Site-directed mutagenesis of potential catalytic residues in 1H-3-hydroxy-4-oxoquinoline 2,4-dioxygenase, and hypothesis on the catalytic mechanism of 2,4-dioxygenolytic ring cleavage

1H-3-Hydroxy-4-oxoquinoline 2,4-dioxygenase (Qdo) is a cofactor-free dioxygenase proposed to belong to the alpha/beta hydrolase fold superfamily of enzymes. Alpha/beta Hydrolases contain a highly conserved catalytic triad (nucleophile-acidic residue-histidine). We previously identified a corresponding catalytically essential histidine residue in Qdo. However, as shown by amino acid replacements through site-directed mutagenesis, nucleophilic and acidic residues of Qdo considered as possible triad residues were not absolutely required for activity. This suggests that Qdo does not contain the canonical catalytic triad of the alpha/beta hydrolase fold enzymes. Some radical trapping agents affected the Qdo-catalyzed reaction. A hypothetical mechanism of Qdo-catalyzed dioxygenation of 1H-3-hydroxy-4-oxoquinoline is compared with the dioxygenation of FMNH2 catalyzed by bacterial luciferase, which also uses a histidine residue as catalytic base.     

Rhamnogalacturonan acetylesterase elucidates the structure and function of a new family of hydrolases

BACKGROUND: The complex polysaccharide rhamnogalacturonan constitutes a major part of the hairy region of pectin. It can have different types of carbohydrate sidechains attached to the rhamnose residues in the backbone of alternating rhamnose and galacturonic acid residues; the galacturonic acid residues can be methylated or acetylated. Aspergillus aculeatus produces enzymes that are able to perform a synergistic degradation of rhamnogalacturonan. The deacetylation of the backbone by rhamnogalacturonan acetylesterase (RGAE) is an essential prerequisite for the subsequent action of the enzymes that cleave the glycosidic bonds. RESULTS: The structure of RGAE has been determined at 1.55 A resolution. RGAE folds into an alpha/beta/alpha structure. The active site of RGAE is an open cleft containing a serine-histidine-aspartic acid catalytic triad. The position of the three residues relative to the central parallel beta sheet and the lack of the nucleophilic elbow motif found in structures possessing the alpha/beta hydrolase fold show that RGAE does not belong to the alpha/beta hydrolase family. CONCLUSIONS: Structural and sequence comparisons have revealed that, despite very low sequence similarities, RGAE is related to seven other proteins. They are all members of a new hydrolase family, the SGNH-hydrolase family, which includes the carbohydrate esterase family 12 as a distinct subfamily. The SGNH-hydrolase family is characterised by having four conserved blocks of residues, each with one completely conserved residue; serine, glycine, asparagine and histidine, respectively. Each of the four residues plays a role in the catalytic function.     

Three-dimensional structure of the catalytic core of acetylxylan esterase from Trichoderma reesei: insights into the deacetylation mechanism

Acetylxylan esterase from Trichoderma reesei removes acetyl side groups from xylan. The crystal structure of the catalytic core of the enzyme was solved at 1.9 A resolution. The core has an alpha/beta/alpha sandwich fold, similar to that of homologous acetylxylan esterase from Penicillium purpurogenum and cutinase from Fusarium solani. All three enzymes belong to family 5 of the carbohydrate esterases and the superfamily of the alpha/beta hydrolase fold. Evidently, the enzymes have diverged from a common ancestor and they share the same catalytic mechanism. The catalytic machinery of acetylxylan esterase from T. reesei was studied by comparison with cutinase, the catalytic site of which is well known. Acetylxylan esterase is a pure serine esterase having a catalytic triad (Ser90, His187, and Asp175) and an oxyanion hole (Thr13 N, and Thr13 O gamma). Although the catalytic triad of acetylxylan esterase has been reported previously, there has been no mention of the oxyanion hole. A model for the binding of substrates is presented on the basis of the docking of xylose. Acetylxylan esterase from T. reesei is able to deacetylate both mono- and double-acetylated residues, but it is not able to remove acetyl groups located close to large side groups such as 4-O-methylglucuronic acid. If the xylopyranoside residue is double-acetylated, both acetyl groups are removed by the catalytic triad: first one acetyl group is removed and then the residue is reorientated so that the nucleophilic oxygen of serine can attack the second acetyl group.     

Molecular markers of serine protease evolution.

The evolutionary history of serine proteases can be accounted for by highly conserved amino acids that form crucial structural and chemical elements of the catalytic apparatus. These residues display non- random dichotomies in either amino acid choice or serine codon usage and serve as discrete markers for tracking changes in the active site environment and supporting structures. These markers categorize serine proteases of the chymotrypsin-like, subtilisin-like and alpha/beta-hydrolase fold clans according to phylogenetic lineages, and indicate the relative ages and order of appearance of those lineages. A common theme among these three unrelated clans of serine proteases is the development or maintenance of a catalytic tetrad, the fourth member of which is a Ser or Cys whose side chain helps stabilize other residues of the standard catalytic triad. A genetic mechanism for mutation of conserved markers, domain duplication followed by gene splitting, is suggested by analysis of evolutionary markers from newly sequenced genes with multiple protease domains.     

Identification of the catalytic residues of alpha-amino acid ester hydrolase from Acetobacter turbidans by labeling and site-directed mutagenesis

The alpha-amino acid ester hydrolase from Acetobacter turbidans ATCC 9325 is capable of hydrolyzing and synthesizing the side chain peptide bond in beta-lactam antibiotics. Data base searches revealed that the enzyme contains an active site serine consensus sequence Gly-X-Ser-Tyr-X-Gly that is also found in X-prolyl dipeptidyl aminopeptidase. The serine hydrolase inhibitor p-nitrophenyl-p'-guanidino-benzoate appeared to be an active site titrant and was used to label the alpha-amino acid ester hydrolase. Electrospray mass spectrometry and tandem mass spectrometry analysis of peptides from a CNBr digest of the labeled protein showed that Ser(205), situated in the consensus sequence, becomes covalently modified by reaction with the inhibitor. Extended sequence analysis showed alignment of this Ser(205) with the catalytic nucleophile of some alpha/beta-hydrolase fold enzymes, which posses a catalytic triad composed of a nucleophile, an acid, and a base. Based on the alignments, 10 amino acids were selected for site-directed mutagenesis (Arg(85), Asp(86), Tyr(143), Ser(156), Ser(205), Tyr(206), Asp(338), His(370), Asp(509), and His(610)). Mutation of Ser(205), Asp(338,) or His(370) to an alanine almost fully inactivated the enzyme, whereas mutation of the other residues did not seriously affect the enzyme activity. Circular dichroism measurements showed that the inactivation was not caused by drastic changes in the tertiary structure. Therefore, we conclude that the catalytic domain of the alpha-amino acid ester hydrolase has an alpha/beta-hydrolase fold structure with a catalytic triad of Ser(205), Asp(338), and His(370). This distinguishes the alpha-amino acid ester hydrolase from the Ntn-hydrolase family of beta-lactam antibiotic acylases.     

The atomic-resolution structure of a novel bacterial esterase

Background: A novel bacterial esterase that cleaves esters on halogenated cyclic compounds has been isolated from an Alcaligenes species. This esterase 713 is encoded by a 1062 base pair gene. The presence of a leader sequence of 27 amino acids suggests that this enzyme is exported from the cytosol. Esterase 713 has been over-expressed in Agrobacterium without this leader sequence. Its amino acid sequence shows no significant homology to any known protein sequence. Results: The crystal structure of esterase 713 has been determined by multiple isomorphous replacement and refined to 1. 1 A resolution. The subunits of this dimeric enzyme comprise a single domain with an alpha/beta hydrolase fold. The catalytic triad has been identified as Ser206-His298-Glu230. The acidic residue of the catalytic triad (Glu230) is located on the beta6 strand of the alpha/beta hydrolase fold, whereas most other alpha/beta hydrolase enzymes have the acidic residue located on the beta7 strand. The oxyanion hole is formed by the mainchain nitrogens of Cys71 and Gln207 as identified by the binding of a substrate analogue, (S)-7-iodo-2,3,4,5-tetrahydro-4-methyl-3-oxo-1H-1, 4-benzodiazepine-2-acetic acid. Cys71 forms a disulphide bond with the neighbouring Cys72. Conclusions: Despite negligible sequence homology, esterase 713 has structural similarities to a number of other esterases and lipases. Residues of the oxyanion hole were confirmed by structural comparison with Rhizomucor miehei lipase. It is proposed that completion of a functional active site requires the formation of the disulphide bond between adjacent residues Cys71 and Cys72 on export of the esterase into the oxidising environment of the periplasmic space.     

His23β and Glu455β of the Pseudomonas sp. 130 Glutaryl-7-Amino Cephalosporanic Acid Acylase Are Crucially Important for Efficient Autoproteolysis and Enzymatic Catalysis

Glutaryl-7-amino cephalosporanic acid acylase is a member of the N-terminal nucleophilic hydrolase family of enzymes. The crystal structure of the acylase reveals there is a Ser-His-Glu motif composed of Ser1beta, His23beta, and Glu455beta near the active site. This mimics the catalytic triad of Ser-His-Asp in serine proteases. Experiments prove that maturation of this enzyme involves autoproteolysis. It has been shown that Ser1beta is the catalytic residue for the autoproteolysis and catalytic reaction. Our works on site-directed mutagenesis followed by the characterization of mutant enzymes demonstrated that His23beta is essential for autoproteolysis whereas Glu455beta is responsible for the efficiency of the process. Neither His23beta nor Glu455beta is essential for the acylase activity, although they affect the catalytic efficiency.     

Multifunctional xylooligosaccharide/cephalosporin C deacetylase revealed by the hexameric structure of the Bacillus subtilis enzyme at 1.9A resolution

Esterases and deacetylases active on carbohydrate ligands have been classified into 14 families based upon amino acid sequence similarities. Enzymes from carbohydrate esterase family seven (CE-7) are unusual in that they display activity towards both acetylated xylooligosaccharides and the antibiotic, cephalosporin C. The 1.9A structure of the multifunctional CE-7 esterase (hereinafter CAH) from Bacillus subtilis 168 reveals a classical alpha/beta hydrolase fold encased within a 32 hexamer. This is the first example of a hexameric alpha/beta hydrolase and is further evidence of the versatility of this particular fold, which is used in a wide variety of biological contexts. A narrow entrance tunnel leads to the centre of the molecule, where the six active-centre catalytic triads point towards the tunnel interior and thus are sequestered away from cytoplasmic contents. By analogy to self-compartmentalising proteases, the tunnel entrance may function to hinder access of large substrates to the poly-specific active centre. This would explain the observation that the enzyme is active on a variety of small, acetylated molecules. The structure of an active site mutant in complex with the reaction product, acetate, reveals details of the putative oxyanion binding site, and suggests that substrates bind predominantly through non-specific contacts with protein hydrophobic residues. Protein residues involved in catalysis are tethered by interactions with protein excursions from the canonical alpha/beta hydrolase fold. These excursions also mediate quaternary structure maintenance, so it would appear that catalytic competence is only achieved on protein multimerisation. We suggest that the acetyl xylan esterase (EC 3.1.1.72) and cephalosporin C deacetylase (EC 3.1.1.41) enzymes of the CE-7 family represent a single class of proteins with a multifunctional deacetylase activity against a range of small substrates.     

Identification of a novel class in the alpha/beta hydrolase fold superfamily: the N-myc differentiation-related proteins

The alpha/beta hydrolases constitute a large protein superfamily that mainly consists of enzymes that catalyze a diverse range of reactions. These proteins exhibit the alpha/beta hydrolase fold, the essential features of which have recently been delineated: the presence of at least five parallel beta-strands, a catalytic triad in a specific order (nucleophile-acid-histidine), and a nucleophilic elbow. Because of the difficulties experimentally in identifying protein structures, we have used a Bayesian computational algorithm (PROBE) to identify the members of this superfamily based on distant sequence relationships. We found that the presence of five sequence motifs, which contain residues important for substrate binding and stabilization of the fold, are required for membership in this superfamily. The superfamily consists of at least 909 members, including the N-myc downstream regulated proteins, which are believed to be involved in cell differentiation. Unlike most of the other superfamily members, the N-myc downstream regulated proteins have never been proposed to possess the alpha/beta hydrolase fold and do not appear to be hydrolases.     

Crystal structure of the catalytic domain of human bile salt activated lipase

Bile-salt activated lipase (BAL) is a pancreatic enzyme that digests a variety of lipids in the small intestine. A distinct property of BAL is its dependency on bile salts in hydrolyzing substrates of long acyl chains or bulky alcoholic motifs. A crystal structure of the catalytic domain of human BAL (residues 1-538) with two surface mutations (N186D and A298D), which were introduced in attempting to facilitate crystallization, has been determined at 2.3 A resolution. The crystal form belongs to space group P2(1)2(1)2(1) with one monomer per asymmetric unit, and the protein shows an alpha/beta hydrolase fold. In the absence of bound bile salt molecules, the protein possesses a preformed catalytic triad and a functional oxyanion hole. Several surface loops around the active site are mobile, including two loops potentially involved in substrate binding (residues 115-125 and 270-285).     

A proposed architecture for lecithin cholesterol acyl transferase (LCAT): identification of the catalytic triad and molecular modeling.

The enzyme cholesterol lecithin acyl transferase (LCAT) shares the Ser/Asp-Glu/His triad with lipases, esterases and proteases, but the low level of sequence homology between LCAT and these enzymes did not allow for the LCAT fold to be identified yet. We, therefore, relied upon structural homology calculations using threading methods based on alignment of the sequence against a library of solved three-dimensional protein structures, for prediction of the LCAT fold. We propose that LCAT, like lipases, belongs to the alpha/beta hydrolase fold family, and that the central domain of LCAT consists of seven conserved parallel beta-strands connected by four alpha-helices and separated by loops. We used the conserved features of this protein fold for the prediction of functional domains in LCAT, and carried out site-directed mutagenesis for the localization of the active site residues. The wild-type enzyme and mutants were expressed in Cos-1 cells. LCAT mass was measured by ELISA, and enzymatic activity was measured on recombinant HDL, on LDL and on a monomeric substrate. We identified D345 and H377 as the catalytic residues of LCAT, together with F103 and L182 as the oxyanion hole residues. In analogy with lipases, we further propose that a potential "lid" domain at residues 50-74 of LCAT might be involved in the enzyme-substrate interaction. Molecular modeling of human LCAT was carried out using human pancreatic and Candida antarctica lipases as templates. The three-dimensional model proposed here is compatible with the position of natural mutants for either LCAT deficiency or Fish-eye disease. It enables moreover prediction of the LCAT domains involved in the interaction with the phospholipid and cholesterol substrates.     

Solution NMR structure of the NlpC/P60 domain of lipoprotein Spr from Escherichia coli: structural evidence for a novel cysteine peptidase catalytic triad

Escherichia coli Spr is a membrane-anchored cell wall hydrolase. The solution NMR structure of the C-terminal NlpC/P60 domain of E. coli Spr described here reveals that the protein adopts a papain-like alpha+beta fold and identifies a substrate-binding cleft featuring several highly conserved residues. The active site features a novel Cys-His-His catalytic triad that appears to be a unique structural signature of this cysteine peptidase family. Moreover, the relative orientation of these catalytic residues is similar to that observed in the analogous Ser-His-His triad, a variant of the classic Ser-His-Asp charge relay system, suggesting the convergent evolution of a catalytic mechanism in quite distinct peptidase families.     

Crystal structure of an acylpeptide hydrolase/esterase from Aeropyrum pernix K1

Acylpeptide hydrolases (APH; also known as acylamino acid releasing enzyme) catalyze the removal of an N-acylated amino acid from blocked peptides. The crystal structure of an APH from the thermophilic archaeon Aeropyrum pernix K1 to 2.1 A resolution confirms it to be a member of the prolyl oligopeptidase family of serine proteases. The structure of apAPH is a symmetric homodimer with each subunit comprised of two domains. The N-terminal domain is a regular seven-bladed beta-propeller, while the C-terminal domain has a canonical alpha/beta hydrolase fold and includes the active site and a conserved Ser445-Asp524-His556 catalytic triad. The complex structure of apAPH with an organophosphorus substrate, p-nitrophenyl phosphate, has also been determined. The complex structure unambiguously maps out the substrate binding pocket and provides a basis for substrate recognition by apAPH. A conserved mechanism for protein degradation from archaea to mammals is suggested by the structural features of apAPH.     

Alpha/beta‐hydrolases: A unique structural motif coordinates catalytic acid residue in 40 protein fold families

The alpha/beta‐hydrolases are a family of acid‐base‐nucleophile catalytic triad enzymes with a common fold, but using a wide variety of substrates, having different pH optima, catalyzing unique catalytic reactions and often showing improved chemical and thermo stability. The ABH enzymes are prime targets for protein engineering. Here, we have classified active sites from 51 representative members of 40 structural ABH fold families into eight distinct conserved geometries. We demonstrate the occurrence of a common structural motif, the catalytic acid zone, at the catalytic triad acid turn. We show that binding of an external ligand does not change the structure of the catalytic acid zone and both the ligand‐free and ligand‐bound forms of the protein belong to the same catalytic acid zone subgroup. We also show that the catalytic acid zone coordinates the position of the catalytic histidine loop directly above its plane, and consequently, fixes the catalytic histidine in a proper position near the catalytic acid. Finally, we demonstrate that the catalytic acid zone plays a key role in multi‐subunit complex formation in ABH enzymes, and is involved in interactions with other proteins. As a result, we speculate that each of the catalytic triad residues has its own supporting structural scaffold, similar to the catalytic acid zone, described above, which together form the extended catalytic triad motif. Each scaffold coordinates the function of its respective catalytic residue, and can even compensate for the loss of protein function, if the catalytic amino acid is mutated.     

Crystal structure of a human peptidyl-tRNA hydrolase reveals a new fold and suggests basis for a bifunctional activity

Peptidyl-tRNA hydrolase (Pth) activity releases tRNA from the premature translation termination product peptidyl-tRNA. Two different enzymes have been reported to encode such activity, Pth present in bacteria and eukaryotes and Pth2 present in archaea and eukaryotes. Here we report the crystallographic structure of the Homo sapiens Pth2 at a 2.0-A resolution as well as its catalytic properties. In contrast to the structure of Escherichia coli Pth, H. sapiens Pth2 has an alpha/beta fold with a four-stranded antiparallel beta-sheet in its core surrounded by two alpha-helices on each side. This arrangement of secondary structure elements generates a fold not previously reported. Its catalytic efficiency is comparable with that reported for the archaeal Sulfolobus solfataricus Pth2 and higher than that of the bacterial E. coli Pth. Several lines of evidence target the active site to two close loops with highly conserved residues. This active site architecture is unrelated to that of E. coli Pth. In addition, intermolecular contacts in the crystal asymmetric unit cell suggest a likely surface for protein-protein interactions related to the Pth2-mediated apoptosis.     

Crystal structure of prolyl aminopeptidase from Serratia marcescens.

Prolyl aminopeptidase from Serratia marcescens specifically catalyzes the removal of N-terminal proline residues from peptides. We have solved its three-dimensional structure at 2.3 A resolution by the multiple isomorphous replacement method. The enzyme consists of two contiguous domains. The larger domain shows the general topology of the alpha/beta hydrolase fold, with a central eight-stranded beta-sheet and six helices. The smaller domain consists of six helices. The catalytic triad (Ser113, His296, and Asp268) is located near the large cavity at the interface between the two domains. Cys271, which is sensitive to SH reagents, is located near the catalytic residues, in spite of the fact that the enzyme is a serine peptidase. The specific residues which make up the hydrophobic pocket line the smaller domain, and the specificity of the exo-type enzyme originates from this smaller domain, which blocks the N-terminal of P1 proline.     

Structure of proline iminopeptidase from Xanthomonas campestris pv. citri: a prototype for the prolyl oligopeptidase family.

The proline iminopeptidase from Xanthomonas campestris pv. citri is a serine peptidase that catalyses the removal of N-terminal proline residues from peptides with high specificity. We have solved its three-dimensional structure by multiple isomorphous replacement and refined it to a crystallographic R-factor of 19.2% using X-ray data to 2.7 A resolution. The protein is folded into two contiguous domains. The larger domain shows the general topology of the alpha/beta hydrolase fold, with a central eight-stranded beta-sheet flanked by two helices and the 11 N-terminal residues on one side, and by four helices on the other side. The smaller domain is placed on top of the larger domain and essentially consists of six helices. The active site, located at the end of a deep pocket at the interface between both domains, includes a catalytic triad of Ser110, Asp266 and His294. Cys269, located at the bottom of the active site very close to the catalytic triad, presumably accounts for the inhibition by thiol-specific reagents. The overall topology of this iminopeptidase is very similar to that of yeast serine carboxypeptidase. The striking secondary structure similarity to human lymphocytic prolyl oligopeptidase and dipeptidyl peptidase IV makes this proline iminopeptidase structure a suitable model for the three-dimensional structure of other peptidases of this family.     

The central cavity from the (alpha/alpha)6 barrel structure of Anabaena sp. CH1 N-acetyl-D-glucosamine 2-epimerase contains two key histidine residues for reversible conversion

N-acetyl-D-glucosamine 2-epimerase (GlcNAc 2-epimerase) catalyzes the reversible epimerization between N-acetyl-D-glucosamine (GlcNAc) and N-acetyl-D-mannosamine (ManNAc). We report here the 2.0 A resolution crystal structure of the GlcNAc 2-epimerase from Anabaena sp. CH1. The structure demonstrates an (alpha/alpha)(6) barrel fold, which shows structural homology with porcine GlcNAc 2-epimerase as well as a number of glycoside hydrolase enzymes and other sugar-metabolizing enzymes. One side of the barrel structure consists of short loops involved in dimer interactions. The other side of the barrel structure is comprised of long loops containing six short beta-sheets, which enclose a putative central active-site pocket. Site-directed mutagenesis of conserved residues near the N-terminal region of the inner alpha helices shows that R57, H239, E308, and H372 are strictly required for activity. E242 and R375 are also essential in catalysis. Based on the structure and kinetic analysis, H239 and H372 may serve as the key active site acid/base catalysts. These results suggest that the (alpha/alpha)(6) barrel represents a steady fold for presenting active-site residues in a cleft at the N-terminal ends of the inner alpha helices, thus forming a fine-tuned catalytic site in GlcNAc 2-epimerase.     

Both helical propensity and side-chain hydrophobicity at a partially exposed site in alpha-helix contribute to the thermodynamic stability of ubiquitin

Pfam family DUF1023 consists entirely of uncharacterized proteins generated by sequencing the genomes of Actinobacteria (Bateman A., et al., Nucleic Acids Res. 2004;32 Database issue:D138-141.) Utilizing sequence similarity detection methods, we infer homology between DUF1023 and alpha/beta hydrolases. DUF1023 proteins conserve the core secondary structures in alpha/beta hydrolase fold, and share similar catalytic machinery as that of alpha/beta hydrolases. We predict DUF1023 spatial structure and deduce that they function as hydrolases utilizing catalytic Ser-His-Asp triad with the serine as a nucleophile.