Structural insight into a CE15 esterase from the marine bacterial metagenome

Exploration of three Dyadobacter fermentans enzymes uncovers molecular activity determinants in CE15

Glucuronoyl esterases (GEs) are serine-type hydrolase enzymes belonging to carbohydrate esterase family 15 (CE15), and they play a central role in the reduction of recalcitrance in plant cell walls by cleaving ester linkages between glucuronoxylan and lignin in lignocellulose. Recent studies have suggested that bacterial CE15 enzymes are more heterogeneous in terms of sequence, structure, and substrate preferences than their fungal counterparts. However, the sequence space of bacterial GEs has still not been fully explored, and further studies on diverse enzymes could provide novel insights into new catalysts of biotechnological interest. To expand our knowledge on this family of enzymes, we investigated three unique CE15 members encoded by Dyadobacter fermentans NS114T, a Gram-negative bacterium found endophytically in maize/corn (Zea mays). The enzymes are dissimilar, sharing ≤ 39% sequence identity to each other' and were considerably different in their activities towards synthetic substrates. Combined analysis of their primary sequences and structural predictions aided in establishing hypotheses regarding specificity determinants within CE15, and these were tested using enzyme variants attempting to shift the activity profiles. Together, the results expand our existing knowledge of CE15, shed light into the molecular determinants defining specificity, and support the recent thesis that diverse GEs encoded by a single microorganism may have evolved to fulfil different physiological functions. KEY POINTS: • D. fermentans encodes three CE15 enzymes with diverse sequences and specificities • The Region 2 inserts in bacterial GEs may directly influence enzyme activity • Rational amino acid substitutions improved the poor activity of the DfCE15A enzyme.

Marine Bacteroidetes enzymatically digest xylans from terrestrial plants

Marine Bacteroidetes that degrade polysaccharides contribute to carbon cycling in the ocean. Organic matter, including glycans from terrestrial plants, might enter the oceans through rivers. Whether marine bacteria degrade structurally related glycans from diverse sources including terrestrial plants and marine algae was previously unknown. We show that the marine bacterium Flavimarina sp. Hel_I_48 encodes two polysaccharide utilization loci (PULs) which degrade xylans from terrestrial plants and marine algae. Biochemical experiments revealed activity and specificity of the encoded xylanases and associated enzymes of these PULs. Proteomics indicated that these genomic regions respond to glucuronoxylans and arabinoxylans. Substrate specificities of key enzymes suggest dedicated metabolic pathways for xylan utilization. Some of the xylanases were active on different xylans with the conserved β-1,4-linked xylose main chain. Enzyme activity was consistent with growth curves showing Flavimarina sp. Hel_I_48 uses structurally different xylans. The observed abundance of related xylan-degrading enzyme repertoires in genomes of other marine Bacteroidetes indicates similar activities are common in the ocean. The here presented data show that certain marine bacteria are genetically and biochemically variable enough to access parts of structurally diverse xylans from terrestrial plants as well as from marine algal sources.

Glucuronoyl esterases - enzymes to decouple lignin and carbohydrates and enable better utilization of renewable plant biomass

Glucuronoyl esterases (GEs) are microbial enzymes able to cleave covalent linkages between lignin and carbohydrates in the plant cell wall. GEs are serine hydrolases found in carbohydrate esterase family 15 (CE15), which belongs to the large α/β hydrolase superfamily. GEs have been shown to reduce plant cell wall recalcitrance by hydrolysing the ester bonds found between glucuronic acid moieties on xylan polysaccharides and lignin. In recent years, the exploration of CE15 has broadened significantly and focused more on bacterial enzymes, which are more diverse in terms of sequence and structure to their fungal counterparts. Similar to fungal GEs, the bacterial enzymes are able to improve overall biomass deconstruction but also appear to have less strict substrate preferences for the uronic acid moiety. The structures of bacterial GEs reveal that they often have large inserts close to the active site, with implications for more extensive substrate interactions than the fungal GEs which have more open active sites. In this review, we highlight the recent work on GEs which has predominantly regarded bacterial enzymes, and discuss similarities and differences between bacterial and fungal enzymes in terms of the biochemical properties, diversity in sequence and modularity, and structural variations that have been discovered thus far in CE15.

QM/MM Study of the Reaction Mechanism of Thermophilic Glucuronoyl Esterase for Biomass Treatment

Hydrolysis of lignocellulosic biomass, composed of a lignin-carbohydrate-complex (LCC) matrix, is critical for producing bioethanol from glucose. However, current methods for LCC processing require costly and polluting processes. The fungal Thermothelomyces thermophila glucuronoyl esterase (TtGE) is a promising thermophilic enzyme that hydrolyses LCC ester bonds. This study describes the TtGE catalytic mechanism using QM/MM methods. Two nearly-degenerate rate-determining transition states were found, with barriers of 16 and 17 kcal ⋅ mol^-1 , both with a zwitterionic nature that results from a proton interplay from His346 to either the Ser213-hydroxyl or the lignin leaving group and the rehybridisation of the ester moiety of the substrate to an alkoxide. An oxyanion hole, characteristic of esterases, was provided by the conserved Arg214 through its backbone and sidechain. Our work further suggests that a mutation of Glu267 to a non-negative residue will decrease the energetic barrier in ca. -5 kcal ⋅ mol^-1 , improving the catalytic rate of TtGE.

Mechanism and biomass association of glucuronoyl esterase: an α/β hydrolase with potential in biomass conversion

Glucuronoyl esterases (GEs) are α/β serine hydrolases and a relatively new addition in the toolbox to reduce the recalcitrance of lignocellulose, the biggest obstacle in cost-effective utilization of this important renewable resource. While biochemical and structural characterization of GEs have progressed greatly recently, there have yet been no mechanistic studies shedding light onto the rate-limiting steps relevant for biomass conversion. The bacterial GE OtCE15A possesses a classical yet distinctive catalytic machinery, with easily identifiable catalytic Ser/His completed by two acidic residues (Glu and Asp) rather than one as in the classical triad, and an Arg side chain participating in the oxyanion hole. By QM/MM calculations, we identified deacylation as the decisive step in catalysis, and quantified the role of Asp, Glu and Arg, showing the latter to be particularly important. The results agree well with experimental and structural data. We further calculated the free-energy barrier of post-catalysis dissociation from a complex natural substrate, suggesting that in industrial settings non-catalytic processes may constitute the rate-limiting step, and pointing to future directions for enzyme engineering in biomass utilization.

Zong and coworkers combine computational and experimental methods to decipher in detail the mechanism of action of glucuronoyl esterases, enzymes with significant biotechnological potential for decoupling lignin from polysaccharides in biomass.

Structural and Functional Analysis of a Multimodular Hyperthermostable Xylanase-Glucuronoyl Esterase from Caldicellulosiruptor kristjansonii

The hyperthermophilic bacterium Caldicellulosiruptor kristjansonii encodes an unusual enzyme, CkXyn10C-GE15A, which incorporates two catalytic domains, a xylanase and a glucuronoyl esterase, and five carbohydrate-binding modules (CBMs) from families 9 and 22. The xylanase and glucuronoyl esterase catalytic domains were recently biochemically characterized, as was the ability of the individual CBMs to bind insoluble polysaccharides. Here, we further probed the abilities of the different CBMs from CkXyn10C-GE15A to bind to soluble poly- and oligosaccharides using affinity gel electrophoresis, isothermal titration calorimetry, and differential scanning fluorimetry. The results revealed additional binding properties of the proteins compared to the former studies on insoluble polysaccharides. Collectively, the results show that all five CBMs have their own distinct binding preferences and appear to complement each other and the catalytic domains in targeting complex cell wall polysaccharides. Additionally, through renewed efforts, we have achieved partial structural characterization of this complex multidomain protein. We have determined the structures of the third CBM9 domain (CBM9.3) and the glucuronoyl esterase (GE15A) by X-ray crystallography. CBM9.3 is the second CBM9 structure determined to date and was shown to bind oligosaccharide ligands at the same site but in a different binding mode compared to that of the previously determined CBM9 structure from Thermotoga maritima. GE15A represents a unique intermediate between reported fungal and bacterial glucuronoyl esterase structures as it lacks two inserted loop regions typical of bacterial enzymes and a third loop has an atypical structure. We also report small-angle X-ray scattering measurements of the N-terminal CBM22.1–CBM22.2–Xyn10C construct, indicating a compact arrangement at room temperature.

The coordinated action of glucuronoyl esterase and α-glucuronidase promotes the disassembly of lignin-carbohydrate complexes

Glucuronoxylans represent a significant fraction of woody biomass, and its decomposition is complicated by the presence of lignin–carbohydrate complexes (LCCs). Herein, LCCs from birchwood were used to investigate the potential coordinated action of a glucuronoyl esterase (TtCE15A) and two α‐glucuronidases (SdeAgu115A and AxyAgu115A). When supplementing α‐glucuronidase with equimolar quantities of TtCE15A, total MeGlcpA released after 72 h by SdeAgu115A and AxyAgu115A increased from 52% to 67%, and 61% to 95%, respectively. Based on the combined TtCE15A and AxyAgu115A activities, ~ 34% of MeGlcpA in the extracted birchwood glucuronoxylan was occupied as LCCs. Notably, insoluble LCC fractions reduced soluble α‐glucuronidase concentrations by up to 70%, whereas reduction in soluble TtCE15A was less than 30%, indicating different tendencies to adsorb onto the LCC substrate.

The structural basis of fungal glucuronoyl esterase activity on natural substrates

Structural and functional studies were conducted of the glucuronoyl esterase (GE) from Cerrena unicolor (CuGE), an enzyme catalyzing cleavage of lignin-carbohydrate ester bonds. CuGE is an α/β-hydrolase belonging to carbohydrate esterase family 15 (CE15). The enzyme is modular, comprised of a catalytic and a carbohydrate-binding domain. SAXS data show CuGE as an elongated rigid molecule where the two domains are connected by a rigid linker. Detailed structural information of the catalytic domain in its apo- and inactivated form and complexes with aldouronic acids reveal well-defined binding of the 4-O-methyl-a-D-glucuronoyl moiety, not influenced by the nature of the attached xylo-oligosaccharide. Structural and sequence comparisons within CE15 enzymes reveal two distinct structural subgroups. CuGE belongs to the group of fungal CE15-B enzymes with an open and flat substrate-binding site. The interactions between CuGE and its natural substrates are explained and rationalized by the structural results, microscale thermophoresis and isothermal calorimetry.

Investigation of a thermostable multi-domain xylanase-glucuronoyl esterase enzyme from Caldicellulosiruptor kristjanssonii incorporating multiple carbohydrate-binding modules

BACKGROUND

Efficient degradation of lignocellulosic biomass has become a major bottleneck in industrial processes which attempt to use biomass as a carbon source for the production of biofuels and materials. To make the most effective use of the source material, both the hemicellulosic as well as cellulosic parts of the biomass should be targeted, and as such both hemicellulases and cellulases are important enzymes in biorefinery processes. Using thermostable versions of these enzymes can also prove beneficial in biomass degradation, as they can be expected to act faster than mesophilic enzymes and the process can also be improved by lower viscosities at higher temperatures, as well as prevent the introduction of microbial contamination.

RESULTS

This study presents the investigation of the thermostable, dual-function xylanase-glucuronoyl esterase enzyme CkXyn10C-GE15A from the hyperthermophilic bacterium Caldicellulosiruptor kristjanssonii. Biochemical characterization of the enzyme was performed, including assays for establishing the melting points for the different protein domains, activity assays for the two catalytic domains, as well as binding assays for the multiple carbohydrate-binding domains present in CkXyn10C-GE15A. Although the enzyme domains are naturally linked together, when added separately to biomass, the expected boosting of the xylanase action was not seen. This lack of intramolecular synergy might suggest, together with previous data, that increased xylose release is not the main beneficial trait given by glucuronoyl esterases.

CONCLUSIONS

Due to its thermostability, CkXyn10C-GE15A is a promising candidate for industrial processes, with both catalytic domains exhibiting melting temperatures over 70 °C. Of particular interest is the glucuronoyl esterase domain, as it represents the first studied thermostable enzyme displaying this activity.

Structural and biochemical studies of the glucuronoyl esterase OtCE15A illuminate its interaction with lignocellulosic components

Glucuronoyl esterases (GEs) catalyze the cleavage of ester linkages between lignin and glucuronic acid moieties on glucuronoxylan in plant biomass. As such, GEs represent promising biochemical tools in industrial processing of these recalcitrant resources. However, details on how GEs interact and catalyze degradation of their natural substrates are sparse, calling for thorough enzyme structure-function studies. Presented here is a structural and mechanistic investigation of the bacterial GE OtCE15A. GEs belong to the carbohydrate esterase family 15 (CE15), which is in turn part of the larger α/β-hydrolase superfamily. GEs contain a Ser-His-Asp/Glu catalytic triad, but the location of the catalytic acid in GEs has been shown to be variable, and OtCE15A possesses two putative catalytic acidic residues in the active site. Through site-directed mutagenesis, we demonstrate that these residues are functionally redundant, possibly indicating the evolutionary route toward new functionalities within the family. Structures determined with glucuronate, in both native and covalently bound intermediate states, and galacturonate provide insights into the catalytic mechanism of CE15. A structure of OtCE15A with the glucuronoxylooligosaccharide 2³-(4-O-methyl-α-d-glucuronyl)-xylotriose (commonly referred to as XUX) shows that the enzyme can indeed interact with polysaccharides from the plant cell wall, and an additional structure with the disaccharide xylobiose revealed a surface binding site that could possibly indicate a recognition mechanism for long glucuronoxylan chains. Collectively, the results indicate that OtCE15A, and likely most of the CE15 family, can utilize esters of glucuronoxylooligosaccharides and support the proposal that these enzymes work on lignin-carbohydrate complexes in plant biomass.

Carboxylic Ester Hydrolases in Bacteria: Active Site, Structure, Function and Application

Carboxylic ester hydrolases (CEHs), which catalyze the hydrolysis of carboxylic esters to produce alcohol and acid, are identified in three domains of life. In the Protein Data Bank (PDB), 136 crystal structures of bacterial CEHs (424 PDB codes) from 52 genera and metagenome have been reported. In this review, we categorize these structures based on catalytic machinery, structure and substrate specificity to provide a comprehensive understanding of the bacterial CEHs. CEHs use Ser, Asp or water as a nucleophile to drive diverse catalytic machinery. The α/β/α sandwich architecture is most frequently found in CEHs, but 3-solenoid, β-barrel, up-down bundle, α/β/β/α 4-layer sandwich, 6 or 7 propeller and α/β barrel architectures are also found in these CEHs. Most are substrate-specific to various esters with types of head group and lengths of the acyl chain, but some CEHs exhibit peptidase or lactamase activities. CEHs are widely used in industrial applications, and are the objects of research in structure- or mutation-based protein engineering. Structural studies of CEHs are still necessary for understanding their biological roles, identifying their structure-based functions and structure-based engineering and their potential industrial applications.

Thermophilic enzyme systems for efficient conversion of lignocellulose to valuable products: Structural insights and future perspectives for esterases and oxidative catalysts

Thermophilic enzyme systems are of major importance nowadays in all industrial processes due to their great performance at elevated temperatures. In the present review, an overview of the current knowledge on the properties of thermophilic and thermotolerant carbohydrate esterases and oxidative enzymes with great thermostability is provided, with respect to their potential use in biotechnological applications. A special focus is given to the lytic polysaccharide monooxygenases that are able to oxidatively cleave lignocellulose through the use of oxygen or hydrogen peroxide as co-substrate and a reducing agent as electron donor. Structural characteristics of the enzymes, including active site conformation and surface properties are discussed and correlated with their substrate specificity and thermostability properties.

Enzyme kinetics of fungal glucuronoyl esterases on natural lignin-carbohydrate complexes

Glucuronoyl esterases (CE15 family) enable targeted cleavage of ester linkages in lignin-carbohydrate complexes (LCCs), particularly those linking lignin and glucuronoyl residues in xylan. A substantial challenge in characterization and kinetic analysis of CE15 enzymes has been the lack of proper substrates. Here, we present an assay using an insoluble LCC-rich lignin fraction from birch; lignin-rich pellet (LRP). The assay employs quantification of enzyme reaction products by LC-MS. The kinetics of four fungal CE15 enzymes, PsGE, CuGE, TtGE, and AfuGE originating from lignocellulose-degrading fungi Punctularia strigosozonata, Cerrena unicolor, Thielavia terrestris, and Armillaria fuscipes respectively were characterized and compared using this new assay. All four enzymes had activity on LRP and showed a clear preference for the insoluble substrate compared with smaller soluble LCC mimicking esters. End-product profiles were near identical for the four enzymes but differences in kinetic parameters were observed. TtGE possesses an alternative active site compared with the three other enzymes as it has the position of the catalytic glutamic acid occupied by a serine. TtGE performed poorly compared with the other enzymes. We speculate that glucuronoyl LCCs are not the preferred substrate of TtGE. Removal of an N-terminal CBM on CuGE affected the catalytic efficiently of the enzyme by reducing K_cat by more than 30%. Reaction products were detected from all four CE15s on a similar substrate from spruce indicating a more generic GE activity not limited to the hardwood. The assay with natural substrate represents a novel tool to study the natural function and kinetics of CE15s.

Structure-function analyses reveal that a glucuronoyl esterase from Teredinibacter turnerae interacts with carbohydrates and aromatic compounds

Glucuronoyl esterases (GEs) catalyze the cleavage of ester linkages found between lignin and glucuronic acid moieties on glucuronoxylan in plant biomass. As such, GEs represent promising biochemical tools in industrial processing of these recalcitrant resources. However, details on how GEs interact with their natural substrates are sparse, calling for thorough structure-function studies. Presented here is the structure and biochemical characterization of a GE, TtCE15A, from the bacterium Teredinibacter turnerae, a symbiont of wood-boring shipworms. To gain deeper insight into enzyme-substrate interactions, inhibition studies were performed with both the WT TtCE15A and variants in which we, by using site-directed mutagenesis, substituted residues suggested to have key roles in binding to or interacting with the aromatic and carbohydrate structures of its uronic acid ester substrates. Our results support the hypothesis that two aromatic residues (Phe-174 and Trp-376), conserved in bacterial GEs, interact with aromatic and carbohydrate structures of these substrates in the enzyme active site, respectively. The solved crystal structure of TtCE15A revealed features previously not observed in either fungal or bacterial GEs, with a large inserted N-terminal region neighboring the active site and a differently positioned residue of the catalytic triad. The findings highlight key interactions between GEs and complex lignin-carbohydrate ester substrates and advance our understanding of the substrate specificities of these enzymes in biomass conversion.

Glucuronoyl esterases: diversity, properties and biotechnological potential. A review

Glucuronoyl esterases (GEs) belonging to the carbohydrate esterase family 15 (CE15) are involved in microbial degradation of lignocellulosic plant materials. GEs are capable of degrading complex polymers of lignin and hemicellulose cleaving ester bonds between glucuronic acid residues in xylan and lignin alcohols. GEs promote separation of lignin, hemicellulose and cellulose which is crucial for efficient utilization of biomass as an energy source and feedstock for further processing into products or chemicals. Genes encoding GEs are found in both fungi and bacteria, but, so far, bacterial GEs are essentially unexplored, and despite being discovered >10 years ago, only a limited number of GEs have been characterized. The first laboratory scale example of improved xylose and glucuronic acid release by the synergistic action of GE with cellulolytic enzymes was only reported recently (improved C5 sugar and glucuronic acid yields) and, until now, not much is known about their biotechnology potential. In this review, we discuss the diversity, structure and properties of microbial GEs and consider the status of their action on natural substrates and in biological systems in relation to their future industrial use.

Biochemical and structural features of diverse bacterial glucuronoyl esterases facilitating recalcitrant biomass conversion

BACKGROUND

Lignocellulose is highly recalcitrant to enzymatic deconstruction, where the recalcitrance primarily results from chemical linkages between lignin and carbohydrates. Glucuronoyl esterases (GEs) from carbohydrate esterase family 15 (CE15) have been suggested to play key roles in reducing lignocellulose recalcitrance by cleaving covalent ester bonds found between lignin and glucuronoxylan. However, only a limited number of GEs have been biochemically characterized and structurally determined to date, limiting our understanding of these enzymes and their potential exploration.

RESULTS

Ten CE15 enzymes from three bacterial species, sharing as little as 20% sequence identity, were characterized on a range of model substrates; two protein structures were solved, and insights into their regulation and biological roles were gained through gene expression analysis and enzymatic assays on complex biomass. Several enzymes with higher catalytic efficiencies on a wider range of model substrates than previously characterized fungal GEs were identified. Similarities and differences regarding substrate specificity between the investigated GEs were observed and putatively linked to their positioning in the CE15 phylogenetic tree. The bacterial GEs were able to utilize substrates lacking 4-OH methyl substitutions, known to be important for fungal enzymes. In addition, certain bacterial GEs were able to efficiently cleave esters of galacturonate, a functionality not previously described within the family. The two solved structures revealed similar overall folds to known structures, but also indicated active site regions allowing for more promiscuous substrate specificities. The gene expression analysis demonstrated that bacterial GE-encoding genes were differentially expressed as response to different carbon sources. Further, improved enzymatic saccharification of milled corn cob by a commercial lignocellulolytic enzyme cocktail when supplemented with GEs showcased their synergistic potential with other enzyme types on native biomass.

CONCLUSIONS

Bacterial GEs exhibit much larger diversity than fungal counterparts. In this study, we significantly expanded the existing knowledge on CE15 with the in-depth characterization of ten bacterial GEs broadly spanning the phylogenetic tree, and also presented two novel enzyme structures. Variations in transcriptional responses of CE15-encoding genes under different growth conditions suggest nonredundant functions for enzymes found in species with multiple CE15 genes and further illuminate the importance of GEs in native lignin-carbohydrate disassembly.