Exploring and exploiting plant biomass degradation by Bacteroidetes

Bacteroidetes bacteria have evolved to become excellent biomass degraders. They achieved this by applying carbohydrate-active enzymes (CAZymes) and organizing genes connected to the degradation of specific polysaccharides into discrete gene cassettes, so-called polysaccharide utilization loci (PULs). Consequently, CAZymes and PULs may hold the potential to improve biomass valorization processes in biorefineries and to advance our understanding of human and livestock gut health.CAZymes are extremely diverse in activity and structure, and for some enzyme families only little is known to date. For example, certain carbohydrate esterases (CEs) combine multiple catalytic domains within one protein, resulting in multicatalytic enzyme architectures, and the properties of these have been little explored. In this thesis, I present biochemical data showcasing the existence of intramolecular synergy between the active domains of multicatalytic CEs (BoCE6-CE1). The observed intramolecular synergy facilitated more efficient degradation of xylan-rich biomass compared to non-multicatalytic CEs, giving a possible explanation as to why multicatalytic CEs exist in the genomes of Bacteroidetes species. Well-defined activity profiles of several here characterized CEs support the hypothesis that each catalytic domain fulfills an individual role during concerted plant biomass degradation, explaining why some PULs encode multiple CEs from the same enzyme family. Further, the investigated CEs cleaved xylan decorations and increased the activity of xylanase-mediated biomass degradation up to 20-fold (FjCE6-CE1). During the investigation of the CAZyme repertoire of different species I also identified a remarkably active and promiscuously acting acetyl xylan esterase (DmCE6A), as well as a rare enzyme architecture that may offer new insights into the multitude of interacting enzyme activities necessary to degrade plant biomass (BeCE15A-Rex8A).PULs encode a plethora of CAZymes and have been shown to be vital for the glycan degradation abilities of Bacteroidetes species. However, the investigation of PULs is aggravated by their usually large size, which often limits the scope of genetic studies. In this thesis, I present a new method for the transfer of PULs between Bacteroidetes species, thus expanding the tools available for the identification and characterization of PULs and their components. The PUL transfer was demonstrated for a previously characterized mixed-linkage β-glucan utilization locus and conferred the ability to metabolize mixed-linkage β-glucan to the receptor strain

Cathodes enhance Corynebacterium glutamicum growth with nitrate and promote acetate and formate production

AbstractThe industrially important Corynebacterium glutamicum can only incompletely reduce nitrate into nitrite which then accumulates and inhibits growth. Herein we report that cathodes can resolve this problem and enhance glucose fermentation and growth by promoting nitrite reduction. Cell growth was inhibited at relatively high potentials but was significant when potentials were more reductive (−1.20V with anthraquinone-2-sulfonate as redox mediator or −1.25V vs. Ag/AgCl). Under these conditions, glucose was consumed up to 6 times faster and acetate was produced at up to 11 times higher yields (up to 1.1mol/mol-glucose). Acetate concentrations are the highest reported so far for C. glutamicum under anaerobic conditions, reaching values up to 5.3±0.3g/L. Herein we also demonstrate for the first time formate production (up to 3.4±0.3g/L) by C. glutamicum under strongly reducing conditions, and we attribute this to a possible mechanism of CO2 bioreduction that was electrochemically triggered

Characterization of a novel multidomain CE15-GH8 enzyme encoded by a polysaccharide utilization locus in the human gut bacterium Bacteroides eggerthii

Bacteroidetes are efficient degraders of complex carbohydrates, much thanks to their use of polysaccharide utilization loci (PULs). An integral part of PULs are highly specialized carbohydrate-active enzymes, sometimes composed of multiple linked domains with discrete functions—multicatalytic enzymes. We present the biochemical characterization of a multicatalytic enzyme from a large PUL encoded by the gut bacterium\ua0Bacteroides eggerthii. The enzyme,\ua0BeCE15A-Rex8A, has a rare and novel architecture, with an N-terminal carbohydrate esterase family 15 (CE15) domain and a C-terminal glycoside hydrolase family 8 (GH8) domain. The CE15 domain was identified as a glucuronoyl esterase (GE), though with relatively poor activity on GE model substrates, attributed to key amino acid substitutions in the active site compared to previously studied GEs. The GH8 domain was shown to be a reducing-end\ua0xylose-releasing\ua0exo-oligoxylanase (Rex), based on having activity on xylooligosaccharides but not on longer xylan chains. The full-length\ua0BeCE15A-Rex8A enzyme and the Rex domain were capable of boosting the activity of a commercially available GH11 xylanase on corn cob biomass. Our research adds to the understanding of multicatalytic enzyme architectures and showcases the potential of discovering novel and atypical carbohydrate-active enzymes from mining PULs

Multimodular fused acetyl–feruloyl esterases from soil and gut Bacteroidetes improve xylanase depolymerization of recalcitrant biomass

BackgroundPlant biomass is an abundant and renewable carbon source that is recalcitrant towards both chemical and biochemical degradation. Xylan is the second most abundant polysaccharide in biomass after cellulose, and it possesses a variety of carbohydrate substitutions and non-carbohydrate decorations which can impede enzymatic degradation by glycoside hydrolases. Carbohydrate esterases are able to cleave the ester-linked decorations and thereby improve the accessibility of the xylan backbone to glycoside hydrolases, thus improving the degradation process. Enzymes comprising multiple catalytic glycoside hydrolase domains on the same polypeptide have previously been shown to exhibit intramolecular synergism during degradation of biomass. Similarly, natively fused carbohydrate esterase domains are encoded by certain bacteria, but whether these enzymes can result in similar synergistic boosts in biomass degradation has not previously been evaluated.ResultsTwo carbohydrate esterases with similar architectures, each comprising two distinct physically linked catalytic domains from families 1 (CE1) and 6 (CE6), were selected from xylan-targeting polysaccharide utilization loci (PULs) encoded by the Bacteroidetes species\ua0Bacteroides ovatus\ua0and\ua0Flavobacterium johnsoniae. The full-length enzymes as well as the individual catalytic domains showed activity on a range of synthetic model substrates, corn cob biomass, and Japanese beechwood biomass, with predominant acetyl esterase activity for the N-terminal CE6 domains and feruloyl esterase activity for the C-terminal CE1 domains. Moreover, several of the enzyme constructs were able to substantially boost the performance of a commercially available xylanase on corn cob biomass (close to twofold) and Japanese beechwood biomass (up to 20-fold). Interestingly, a significant improvement in xylanase biomass degradation was observed following addition of the full-length multidomain enzyme from\ua0B. ovatus\ua0versus the addition of its two separated single domains, indicating an intramolecular synergy between the esterase domains. Despite high sequence similarities between the esterase domains from\ua0B. ovatus\ua0and\ua0F. johnsoniae, their addition to the xylanolytic reaction led to different degradation patterns.ConclusionWe demonstrated that multidomain carbohydrate esterases, targeting the non-carbohydrate decorations on different xylan polysaccharides, can considerably facilitate glycoside hydrolase-mediated hydrolysis of xylan and xylan-rich biomass. Moreover, we demonstrated for the first time a synergistic effect between the two fused catalytic domains of a multidomain carbohydrate esterase

A polysaccharide utilization locus from the gut bacterium dysgonomonas mossii encodes functionally distinct carbohydrate esterases

The gut microbiota plays a central role in human health by enzymatically degrading dietary fiber and concomitantly excreting short chain fatty acids that are associated with manifold health benefits. The polysaccharide xylan is abundant in dietary fiber but noncarbohydrate decorations hinder efficient cleavage by glycoside hydrolases (GHs) and need to be addressed by carbohydrate esterases (CEs). Enzymes from carbohydrate esterase families 1 and 6 (CE1 and 6) perform key roles in xylan degradation by removing feruloyl and acetate decorations, yet little is known about these enzyme families especially with regard to their diversity in activity. Bacteroidetes bacteria are dominant members of the microbiota and often encode their carbohydrate-active enzymes in multigene polysaccharide utilization loci (PULs). Here we present the characterization of three CEs found in a PUL encoded by the gut Bacteroidete Dysgonomonas mossii. We demonstrate that the CEs are functionally distinct, with one highly efficient CE6 acetyl esterase and two CE1 enzymes with feruloyl esterase activities. One multidomain CE1 enzyme contains two CE1 domains: an N-terminal domain feruloyl esterase, and a C-terminal domain with minimal activity on model substrates. We present the structure of the C-terminal CE1 domain with the carbohydrate-binding module that bridges the two CE1 domains, as well as a complex of the same protein fragment with methyl ferulate. The investment of D. mossii in producing multiple CEs suggests that improved accessibility of xylan for GHs and cleavage of covalent polysaccharide-polysaccharide and lignin-polysaccharide bonds are important enzyme activities in the gut environment

Enhancement of anaerobic lysine production in Corynebacterium glutamicum electrofermentations

It has been suggested that application of electric potential can affect lysine producing fermentations, although experimental evidence is lacking. To study this hypothesis we used the lysine producer Corynebacterium glutamicum ZW04, and we exposed it to 12 different conditions regarding anaerobic gas environment, applied electrode potential (cathodic, open circuit, anodic), redox mediator and nitrate presence. The gas environment was found to play a major role, with CO2 leading to double the lysine concentrations and yields when compared to N2. Electrode potentials also played a major role, with reductive conditions doubling the titers and increasing the yields of lysine up to 4 times. Addition of the redox mediator anthraquinone-2-sulfonate (AQ2S) under the presence of CO2 and reductive conditions led to additional doubling of the titers, although the yields were not altered considerably. This study demonstrates for the first time that cathodic electrode conditions combined with CO2 and AQ2S as a redox mediator can significantly improve both the yields and the titers of lysine production of a C. glutamicum lysine producing strain, reaching levels that have only been achieved under aerobic conditions