Single amino acid residue changes in subsite -1 of levansucrase from Zymomonas mobilis 10232 strongly influence the enzyme activities and products

Exploring the role of the residues into catalytic cavity of inulosucrase from Leuconostoc citreum CW28

Inulosucrases are enzymes capable of synthesizing inulin polymers using sucrose as the main substrate. The enzymatic activity relies on the catalytic triad within the active site and residues responsible for substrate recognition and orientation, termed carbohydrate-binding subsites. This study investigates the role of specific residues within the catalytic cavity of a truncated version of IslA4 in enzymatic catalysis. Mutants at residues S425, L499, A602, R618, F619, Y676, Y692, and R696 were constructed and characterized. Characterization results, and in silico structural comparison with other fructansucrases, reveal these residues' functional significance in catalysis. Residue S425 belongs to subsite -1; residues R618 and Y692 are part of subsite +1, and residue R696 belongs to subsites +1 and +2. Residues L499 and A602 are support residues; the former favors the formation of the fructosyl-enzyme intermediate, while the latter stabilizes the acid/base catalyst during catalysis. Residues Y676 and F619 may participate in stabilizing residues at -1/+1 subsites. This study represents the first comprehensive exploration of the structural determinants essential for enzymatic function in the inulosucrase of Leuconostoc citreum, and proposes the identity of residues involved in the -1 to +2 subsites.

Expanding the horizons of levan: from microbial biosynthesis to applications and advanced detection methods

Levan, a β-(2,6)-linked fructose polymer, exhibits diverse properties that impart versatility, rendering it a highly sought-after biopolymer with various industrial applications. Levan can be produced by various microorganisms using sucrose, food industry byproducts and agricultural wastes. Microbial levan represents the most potent cost-effective process for commercial-scale levan production. This study reviews the optimization of levan production by understanding its biosynthesis, physicochemical properties and the fermentation process. In addition, genetic and protein engineering for its increased production and emerging methods for its detection are introduced and discussed. All of these comprehensive studies could serve as powerful tools to optimize levan production and broaden its applications across various industries.

Identification of a Thermostable Levansucrase from Pseudomonas orientalis That Allows Unique Product Specificity at Different Temperatures

The biological production of levan by levansucrase (LS, EC 2.4.1.10) has aroused great interest in the past few years. Previously, we identified a thermostable levansucrase from Celerinatantimonas diazotrophica (Cedi-LS). A novel thermostable LS from Pseudomonas orientalis (Psor-LS) was successfully screened using the Cedi-LS template. The Psor-LS showed maximum activity at 65 °C, much higher than the other LSs. However, these two thermostable LSs showed significantly different product specificity. When the temperature was decreased from 65 to 35 °C, Cedi-LS tended to produce high-molecular-weight (HMW) levan. By contrast, Psor-LS prefers to generate fructooligosaccharides (FOSs, DP ≤ 16) rather than HMW levan under the same conditions. Notably, at 65 °C, Psor-LS would produce HMW levan with an average Mw of 1.4 × 106 Da, indicating that a high temperature might favor the accumulation of HMW levan. In summary, this study allows a thermostable LS suitable for HMW levan and levan-type FOSs production simultaneously.

Molecular insight into regioselectivity of transfructosylation catalyzed by GH68 levansucrase and β-fructofuranosidase

Glycoside hydrolase family 68 (GH68) enzymes catalyze β-fructosyltransfer from sucrose to another sucrose, the so-called transfructosylation. Although regioselectivity of transfructosylation is divergent in GH68 enzymes, there is insufficient information available on the structural factor(s) involved in the selectivity. Here, we found two GH68 enzymes, β-fructofuranosidase (FFZm) and levansucrase (LSZm), encoded tandemly in the genome of Zymomonas mobilis, displayed different selectivity: FFZm catalyzed the β-(2→1)-transfructosylation (1-TF), whereas LSZm did both of 1-TF and β-(2→6)-transfructosylation (6-TF). We identified His79^FFZm and Ala343^FFZm and their corresponding Asn84^LSZm and Ser345^LSZm respectively as the structural factors for those regioselectivities. LSZm with the respective substitution of FFZm-type His and Ala for its Asn84^LSZm and Ser345^LSZm (N84H/S345A-LSZm) lost 6-TF and enhanced 1-TF. Conversely, the LSZm-type replacement of His79^FFZm and Ala343^FFZm in FFZm (H79N/A343S-FFZm) almost lost 1-TF and acquired 6-TF. H79N/A343S-FFZm exhibited the selectivity like LSZm but did not produce the β-(2→6)-fructoside-linked levan and/or long levanooligosaccharides that LSZm did. We assumed Phe189^LSZm to be a responsible residue for the elongation of levan chain in LSZm and mutated the corresponding Leu187^FFZm in FFZm to Phe. An H79N/L187F/A343S-FFZm produced a higher quantity of long levanooligosaccharides than H79N/A343S-FFZm (or H79N-FFZm), although without levan formation, suggesting that LSZm has another structural factor for levan production. We also found that FFZm generated a sucrose analog, β-D-fructofuranosyl α-D-mannopyranoside, by β-fructosyltransfer to d-mannose and regarded His79^FFZm and Ala343^FFZm as key residues for this acceptor specificity. In summary, this study provides insight into the structural factors of regioselectivity and acceptor specificity in transfructosylation of GH68 enzymes.

The molecular basis of the nonprocessive elongation mechanism in levansucrases

Levansucrases (LSs) synthesize levan, a β2-6-linked fructose polymer, by successively transferring the fructosyl moiety from sucrose to a growing acceptor molecule. Elucidation of the levan polymerization mechanism is important for using LSs in the production of size-defined products for application in the food and pharmaceutical industries. For a deeper understanding of the levan synthesis reaction, we determined the crystallographic structure of Bacillus subtilis LS (SacB) in complex with a levan-type fructooligosaccharide and utilized site-directed mutagenesis to identify residues involved in substrate binding. The presence of a levanhexaose molecule in the central catalytic cavity allowed us to identify five substrate-binding subsites (−1, +1, +2, +3, and +4). Mutants affecting residues belonging to the identified acceptor subsites showed similar substrate affinity (Km) values to the wildtype (WT) Km value but had a lower turnover number and transfructosylation/hydrolysis ratio. Of importance, compared with the WT, the variants progressively yielded smaller-sized low-molecular-weight levans, as the affected subsites that were closer to the catalytic site, but without affecting their ability to synthesized high-molecular-weight levans. Furthermore, an additional oligosaccharide-binding site 20 Å away from the catalytic pocket was identified, and its potential participation in the elongation mechanism is discussed. Our results clarify, for the first time, the interaction of the enzyme with an acceptor/product oligosaccharide and elucidate the molecular basis of the nonprocessive levan elongation mechanism of LSs.

Molecular Docking Analysis and Biochemical Evaluation of Levansucrase from Sphingobium chungbukense DJ77

Bacterial exopolymer Levan (β-(2,6) polyfructan) synthesized by levansucrase has attracted interest for various applications due to its low intrinsic viscosity compared with other polysaccharides. We report a novel levansucrase (Lsc) isolated from Sphingobium chunbukense DJ77 and verify its biochemical characteristics by comparative analysis of molecular docking analysis (MOE) and catalytic residue analysis. The complete sequence of the Lsc encoding gene ( lsc) was cloned under the direction of the T7 promoter and purified in an Escherichia coli BL21 (DE3) protein expression system. The enzyme activity analysis and ligand docking MOE study of S. chungbukense DJ77 Lsc revealed that Arg 77, Ser112, Arg 195, Asp196, Glu257, and Gln275 were involved in the sucrose binding and splitting as well as transfructosylation activity. A catalytic comparison of Lsc of S. chungbukense DJ77 with the results of site-directed mutational analysis indicated that Gln275 may coordinate a favorable substrate binding environment, offering broad pH resistance in the range of 5-10. The results suggest that the recombinant E. coli carrying S. chungbukense DJ77 Lsc might produce levan under the regular growth conditions with less need for pH manipulation.

Advances and prospects in metabolic engineering of Zymomonas mobilis

Biorefinery of biomass-based biofuels and biochemicals by microorganisms is a competitive alternative of traditional petroleum refineries. Zymomonas mobilis is a natural ethanologen with many desirable characteristics, which makes it an ideal industrial microbial biocatalyst for commercial production of desirable bioproducts through metabolic engineering. In this review, we summarize the metabolic engineering progress achieved in Z. mobilis to expand its substrate and product ranges as well as to enhance its robustness against stressful conditions such as inhibitory compounds within the lignocellulosic hydrolysates and slurries. We also discuss a few metabolic engineering strategies that can be applied in Z. mobilis to further develop it as a robust workhorse for economic lignocellulosic bioproducts. In addition, we briefly review the progress of metabolic engineering in Z. mobilis related to the classical synthetic biology cycle of "Design-Build-Test-Learn", as well as the progress and potential to develop Z. mobilis as a model chassis for biorefinery practices in the synthetic biology era.

High-throughput assay of levansucrase variants in search of feasible catalysts for the synthesis of fructooligosaccharides and levan

Bacterial levansucrases polymerize fructose residues of sucrose to β-2,6 linked fructans—fructooligosaccharides (FOS) and levan. While β-2,1-linked FOS are widely recognized as prebiotics, the health-related effects of β-2,6 linked FOS are scarcely studied as they are not commercially available. Levansucrase Lsc3 (Lsc-3) of Pseudomonas syringae pv. tomato has very high catalytic activity and stability making it a promising biotechnological catalyst for FOS and levan synthesis. In this study we evaluate feasibility of several high-throughput methods for screening and preliminary characterization of levansucrases using 36 Lsc3 mutants as a test panel. Heterologously expressed and purified His-tagged levansucrase variants were studied for: (1) sucrose-splitting activity; (2) FOS production; (3) ability and kinetics of levan synthesis; (4) thermostability in a Thermofluor assay. Importantly, we show that sucrose-splitting activity as well as the ability to produce FOS can both be evaluated using permeabilized levansucrase-expressing E. coli transformants as catalysts. For the first time we demonstrate the key importance of Trp109, His113, Glu146 and Glu236 for the catalysis of Lsc3. Cost-effective and high-throughput methods presented here are applicable not only in the levansucrase assay, but have a potential to be adapted for high-throughput (automated) study of other enzymes.