Tailor-Made α-Glucans by Engineering the Processivity of α-Glucanotransferases via Tunnel-Cleft Active Center Interconversions.

The function of polysaccharides is intimately associated with their size, which is largely determined by the processivity of transferases responsible for their synthesis. A tunnel active center architecture has been recognized as a key factor that governs processivity of several glycoside hydrolases (GHs), e.g., cellulases and chitinases. Similar tunnel architecture is also observed in the Limosilactobacillus reuteri 121 GtfB (Lr121 GtfB) α-glucanotransferase from the GH70 family. The molecular element underpinning processivity of these transglucosylases remains underexplored. Here, we report the synthesis of the smallest (α1 → 4)-α-glucan interspersed with linear and branched (α1 → 6) linkages by a novel 4,6-α-glucanotransferase from L. reuteri N1 (LrN1 GtfB) with an open-clefted active center instead of the tunnel structure. Notably, the loop swapping engineering of LrN1 GtfB and Lr121 GtfB based on their crystal structures clarified the impact of the loop-mediated tunnel/cleft structure at the donor subsites -2 to -3 on processivity of these α-glucanotransferases, enabling the tailoring of both product sizes and substrate preferences. This study provides unprecedented insights into the processivity determinants and evolutionary diversification of GH70 α-glucanotransferases and offers a simple route for engineering starch-converting α-glucanotransferases to generate diverse α-glucans for different biotechnological applications.

Polysaccharide utilization loci encoded DUF1735 likely functions as membrane-bound spacer for carbohydrate active enzymes.

Proteins featuring the Domain of Unknown Function 1735 are frequently found in Polysaccharide Utilization Loci, yet their role remains unknown. The domain and vicinity analyzer programs we developed mine the Kyoto Encyclopedia of Genes and Genomes and UniProt to enhance the functional prediction of DUF1735. Our datasets confirmed the exclusive presence of DUF1735 in Bacteroidota genomes, with Bacteroidetes thetaiotaomicron harboring 46 copies. Notably, 97.8% of DUF1735 are encoded in PULs, and 89% are N-termini of multimodular proteins featuring C-termini like Laminin_G_3, F5/8-typeC, and GH18 domains. Predominantly possessing a predicted lipoprotein signal peptide and sharing an immunoglobulin-like ß-sandwich fold with the BACON domain and the N-termini of SusE/F, DUF1735 likely functions as N-terminal, membrane-bound spacer for diverse C-termini involved in PUL-mediated carbohydrate utilization.

Insights into the Structure-Function Relationship of GH70 GtfB α-Glucanotransferases from the Crystal Structure and Molecular Dynamic Simulation of a Newly Characterized Limosilactobacillus reuteri N1 GtfB Enzyme.

α-Glucanotransferases of the CAZy family GH70 convert starch-derived donors to industrially important α-glucans. Here, we describe characteristics of a novel GtfB-type 4,6-α-glucanotransferase of high enzyme activity (60.8 U mg-1) from Limosilactobacillus reuteri N1 (LrN1 GtfB), which produces surprisingly large quantities of soluble protein in heterologous expression (173 mg pure protein per L of culture) and synthesizes the reuteran-like α-glucan with (α1 → 6) linkages in linear chains and branch points. Protein structural analysis of LrN1 GtfB revealed the potential crucial residues at subsites -2∼+2, particularly H265, Y214, and R302, in the active center as well as previously unidentified surface binding sites. Furthermore, molecular dynamic simulations have provided unprecedented insights into linkage specificity hallmarks of the enzyme. Therefore, LrN1 GtfB represents a potent enzymatic tool for starch conversion, and this study promotes our knowledge on the structure-function relationship of GH70 GtfB α-glucanotransferases, which might facilitate the production of tailored α-glucans by enzyme engineering in future.

Novel ß-galactosidase activity and first crystal structure of Glycoside Hydrolase family 154.

Polysaccharide Utilization Loci (PULs) are physically linked gene clusters conserved in the Gram-negative phylum of Bacteroidota and are valuable sources for Carbohydrate Active enZyme (CAZyme) discovery. This study focuses on BD-ß-Gal, an enzyme encoded in a metagenomic PUL and member of the Glycoside Hydrolase family 154 (GH154). BD-ß-Gal showed exo-ß-galactosidase activity with regiopreference for hydrolyzing ß-d-(1,6) glycosidic linkages. Notably, it exhibited a preference for d-glucopyranosyl (d-Glcp) over d-galactopyranosyl (d-Galp) and d-fructofuranosyl (d-Fruf) at the reducing end of the investigated disaccharides. In addition, we determined the high resolution crystal structure of BD-ß-Gal, thus providing the first structural characterization of a GH154 enzyme. Surprisingly, this revealed an (α/α)6 topology, which has not been observed before for ß-galactosidases. BD-ß-Gal displayed low structural homology with characterized CAZymes, but conservation analysis suggested that the active site was located in a central cavity, with conserved E73, R252, and D253 as putative catalytic residues. Interestingly, BD-ß-Gal has a tetrameric structure and a flexible loop from a neighboring protomer may contribute to its reaction specificity. Finally, we showed that the founding member of GH154, BT3677 from Bacteroides thetaiotaomicron, described as ß-glucuronidase, displayed exo-ß-galactosidase activity like BD-ß-Gal but lacked a tetrameric structure.

Structural and time-resolved mechanistic investigations of protein hydrolysis by the acidic proline-specific endoprotease from Aspergillus niger.

Proline-specific endoproteases have been successfully used in, for example, the in-situ degradation of gluten, the hydrolysis of bitter peptides, the reduction of haze during beer production, and the generation of peptides for mass spectroscopy and proteomics applications. Here we present the crystal structure of the extracellular proline-specific endoprotease from Aspergillus niger (AnPEP), a member of the S28 peptidase family with rarely observed true proline-specific endoprotease activity. Family S28 proteases have a conventional Ser-Asp-His catalytic triad, but their oxyanion-stabilizing hole shows a glutamic acid, an amino acid not previously observed in this role. Since these enzymes have an acidic pH optimum, the presence of a glutamic acid in the oxyanion hole may confine their activity to an acidic pH. Yet, considering the presence of the conventional catalytic triad, it is remarkable that the A. niger enzyme remains active down to pH 1.5. The determination of the primary cleavage site of cytochrome c along with molecular dynamics-assisted docking studies indicate that the active site pocket of AnPEP can accommodate a reverse turn of approximately 12 amino acids with proline at the S1 specificity pocket. Comparison with the structures of two S28-proline-specific exopeptidases reveals not only a more spacious active site cavity but also the absence of any putative binding sites for amino- and carboxyl-terminal residues as observed in the exopeptidases, explaining AnPEP's observed endoprotease activity.

Enzymatic glucosylation of polyphenols using glucansucrases and branching sucrases of glycoside hydrolase family 70.

Polyphenols exhibit various beneficial biological activities and represent very promising candidates as active compounds for food industry. However, the low solubility, poor stability and low bioavailability of polyphenols have severely limited their industrial applications. Enzymatic glycosylation is an effective way to improve the physicochemical properties of polyphenols. As efficient transglucosidases, glycoside hydrolase family 70 (GH70) glucansucrases naturally catalyze the synthesis of polysaccharides and oligosaccharides from sucrose. Notably, GH70 glucansucrases show broad acceptor substrate promiscuity and catalyze the glucosylation of a wide range of non-carbohydrate hydroxyl group-containing molecules, including benzenediol, phenolic acids, flavonoids and steviol glycosides. Branching sucrase enzymes, a newly established subfamily of GH70, are shown to possess a broader acceptor substrate binding pocket that acts efficiently for glucosylation of larger size polyphenols such as flavonoids. Here we present a comprehensive review of glucosylation of polyphenols using GH70 glucansucrase and branching sucrases. Their catalytic efficiency, the regioselectivity of glucosylation and the structure of generated products are described for these reactions. Moreover, enzyme engineering is effective for improving their catalytic efficiency and product specificity. The combined information provides novel insights on the glucosylation of polyphenols by GH70 glucansucrases and branching sucrases, and may promote their applications.

Crystal Structure of 4,6-α-Glucanotransferase GtfC-ΔC from Thermophilic Geobacillus 12AMOR1: Starch Transglycosylation in Non-Permuted GH70 Enzymes.

GtfC-type 4,6-α-glucanotransferase (α-GT) enzymes from Glycoside Hydrolase Family 70 (GH70) are of interest for the modification of starch into low-glycemic index food ingredients. Compared to the related GH70 GtfB-type α-GTs, found exclusively in lactic acid bacteria (LAB), GtfCs occur in non-LAB, share low sequence identity, lack circular permutation of the catalytic domain, and feature a single-segment auxiliary domain IV and auxiliary C-terminal domains. Despite these differences, the first crystal structure of a GtfC, GbGtfC-ΔC from Geobacillus 12AMOR1, and the first one representing a non-permuted GH70 enzyme, reveals high structural similarity in the core domains with most GtfBs, featuring a similar tunneled active site. We propose that GtfC (and related GtfD) enzymes evolved from starch-degrading α-amylases from GH13 by acquiring α-1,6 transglycosylation capabilities, before the events that resulted in circular permutation of the catalytic domain observed in other GH70 enzymes (glucansucrases, GtfB-type α-GTs). AlphaFold modeling and sequence alignments suggest that the GbGtfC structure represents the GtfC subfamily, although it has a so far unique alternating α-1,4/α-1,6 product specificity, likely determined by residues near acceptor binding subsites +1/+2.

Characterization of the (Engineered) Branching Sucrase GtfZ-CD2 from Apilactobacillus kunkeei for Efficient Glucosylation of Benzenediol Compounds.

Branching sucrases, a subfamily of Glycoside Hydrolase family (GH70), display transglycosidase activity using sucrose as donor substrate to catalyze glucosylation reaction in the presence of suitable acceptor substrates. In this study, the (α1→3) branching sucrase GtfZ-CD2 from Apilactobacillus kunkeei DSM 12361 was demonstrated to glucosylate benzenediol compounds (i.e., catechol, resorcinol, and hydroquinone) to form monoglucoside and diglucoside products. The production and yield of catechol glucosylated products were significantly higher than that of resorcinol and hydroquinone, revealing a preference for adjacent aromatic hydroxyl groups in glucosylation. Amino residues around acceptor substrate binding subsite +1 were targeted for semirational mutagenesis, yielding GtfZ-CD2 variants with improved resorcinol and hydroquinone glucosylation. Mutant L1560Y with improved hydroquinone mono-glucosylated product synthesis allowed enzymatic conversion of hydroquinone into α-arbutin. This study thus revealed the high potential of GH70 branching sucrases for glucosylating noncarbohydrate molecules. IMPORTANCE Glycosylation represents one of the most important ways to expand the diversity of natural products and improve their physico-chemical properties. Aromatic polyphenol compounds widely found in plants are reported to exhibit various remarkable biological activities; however, they generally suffer from low solubility and stability, which can be improved by glycosylation. Our present study on the glucosylation of benzenediol compounds by GH70 branching sucrase GtfZ-CD2 and its semirational engineering to improve the glucosylation efficiency provides insight into the mechanism of acceptor substrates binding and its glucosylation selectivity. The results demonstrate the potential of using branching sucrase as an effective enzymatic glucosylation tool.

Crystal Structure of Levansucrase from the Gram-Negative Bacterium Brenneria Provides Insights into Its Product Size Specificity.

Microbial levansucrases (LSs, EC 2.4.1.10) have been widely studied for the synthesis of ß-(2,6)-fructans (levan) from sucrose. LSs synthesize levan-type fructo-oligosaccharides, high-molecular-mass levan polymer or combinations of both. Here, we report crystal structures of LS from the G--bacterium Brenneria sp. EniD 312 (Brs-LS) in its apo form, as well as of two mutants (A154S, H327A) targeting positions known to affect LS reaction specificity. In addition, we report a structure of Brs-LS complexed with sucrose, the first crystal structure of a G--LS with a bound substrate. The overall structure of Brs-LS is similar to that of G-- and G+-LSs, with the nucleophile (D68), transition stabilizer (D225), and a general acid/base (E309) in its active site. The H327A mutant lacks an essential interaction with glucosyl moieties of bound substrates in subsite +1, explaining the observed smaller products synthesized by this mutant. The A154S mutation affects the hydrogen-bond network around the transition stabilizing residue (D225) and the nucleophile (D68), and may affect the affinity of the enzyme for sucrose such that it becomes less effective in transfructosylation. Taken together, this study provides novel insights into the roles of structural elements and residues in the product specificity of LSs.

Mutations in Amino Acid Residues of Limosilactobacillus reuteri 121 GtfB 4,6-α-Glucanotransferase that Affect Reaction and Product Specificity.

Limosilactobacillus reuteri 121 4,6-α-glucanotransferase (Lr121 4,6-α-GTase), belonging to the glycosyl hydrolase (GH) 70 GtfB subfamily, converts starch and maltodextrins into linear isomalto/malto polysaccharides (IMMPs) with consecutive (α1 → 6) linkages. The recent elucidation of its crystal structure allowed identification and analysis of further structural features that determine its reaction and product specificity. Herein, sequence alignments between GtfB enzymes with different product linkage specificities (4,6-α-GTase and 4,3-α-GTase) identified amino acid residues in GH70 homology motifs, which may be critical for reaction and product specificity. Based on these alignments, four Lr121 GtfB-ΔN mutants (I1020M, S1057P, H1056G, and Q1126I) were constructed. Compared to wild-type Lr121 GtfB-ΔN, mutants S1057P and Q1126I had considerably improved catalytic efficiencies. Mutants H1056G and Q1126I showed a 9% decrease and an 11% increase, respectively, in the ratio of (α1 → 6) over (α1 → 4) linkages in maltodextrin-derived products. A change in linkage type (e.g., (α1 → 6) linkages to (α1 → 3) linkages) was not observed. The possible functional roles of these Lr121 GtfB-ΔN residues located around the acceptor substrate-binding subsites are discussed. The results provide new insights into structural determinants of the reaction and product specificity of Lr121 GtfB 4,6-α-GTase.

Insights into Broad-Specificity Starch Modification from the Crystal Structure of Limosilactobacillus Reuteri NCC 2613 4,6-α-Glucanotransferase GtfB.

GtfB-type α-glucanotransferase enzymes from glycoside hydrolase family 70 (GH70) convert starch substrates into α-glucans that are of interest as food ingredients with a low glycemic index. Characterization of several GtfBs showed that they differ in product- and substrate specificity, especially with regard to branching, but structural information is limited to a single GtfB, preferring mostly linear starches and featuring a tunneled binding groove. Here, we present the second crystal structure of a 4,6-α-glucanotransferase (Limosilactobacillus reuteri NCC 2613) and an improved homology model of a 4,3-α-glucanotransferase GtfB (L. fermentum NCC 2970) and show that they are able to convert both linear and branched starch substrates. Compared to the previously described GtfB structure, these two enzymes feature a much more open binding groove, reminiscent of and evolutionary closer to starch-converting GH13 α-amylases. Sequence analysis of 287 putative GtfBs suggests that only 20% of them are similarly "open" and thus suitable as broad-specificity starch-converting enzymes.

Crystal structure of an inulosucrase from Halalkalicoccus jeotgali B3T, a halophilic archaeal strain.

Several archaea harbor genes that code for fructosyltransferase (FTF) enzymes. These enzymes have not been characterized yet at structure-function level, but are of extreme interest in view of their potential role in the synthesis of novel compounds for food, nutrition, and pharmaceutical applications. In this study, 3D structure of an inulin-type fructan producing enzyme, inulosucrase (InuHj), from the archaeon Halalkalicoccus jeotgali was resolved in its apo form and with bound substrate (sucrose) molecule and first transglycosylation product (1-kestose). This is the first crystal structure of an FTF from halophilic archaea. Its overall five-bladed ß-propeller fold is conserved with previously reported FTFs, but also shows some unique features. The InuHj structure is closer to those of Gram-negative bacteria, with exceptions such as residue E266, which is conserved in FTFs of Gram-positive bacteria and has possible role in fructan polymer synthesis in these bacteria as compared to fructooligosaccharide (FOS) production by FTFs of Gram-negative bacteria. Highly negative electrostatic surface potential of InuHj, due to a large amount of acidic residues, likely contributes to its halophilicity. The complex of InuHj with 1-kestose indicates that the residues D287 in the 4B-4C loop, Y330 in 4D-5A, and D361 in the unique α2 helix may interact with longer FOSs and facilitate the binding of longer FOS chains during synthesis. The outcome of this work will provide targets for future structure-function studies of FTF enzymes, particularly those from archaea.

Using Mutability Landscapes To Guide Enzyme Thermostabilization.

Thermostabilizing enzymes while retaining their activity and enantioselectivity for applied biocatalysis is an important topic in protein engineering. Rational and computational design strategies as well as directed evolution have been used successfully to thermostabilize enzymes. Herein, we describe an alternative mutability-landscape approach that identified three single mutations (R11Y, R11I and A33D) within the enzyme 4-oxalocrotonate tautomerase (4-OT), which has potential as a biocatalyst for pharmaceutical synthesis, that gave rise to significant increases in apparent melting temperature Tm (up to 20 °C) and in half-life at 80 °C (up to 111-fold). Introduction of these beneficial mutations in an enantioselective but thermolabile 4-OT variant (M45Y/F50A) afforded improved triple-mutant enzyme variants showing an up to 39 °C increase in Tm value, with no reduction in catalytic activity or enantioselectivity. This study illustrates the power of mutability-landscape-guided protein engineering for thermostabilizing enzymes.

Identification of Thermotoga maritima MSB8 GH57 α-amylase AmyC as a glycogen-branching enzyme with high hydrolytic activity.

AmyC, a glycoside hydrolase family 57 (GH57) enzyme of Thermotoga maritima MSB8, has previously been identified as an intracellular α-amylase playing a role in either maltodextrin utilization or storage polysaccharide metabolism. However, the α-amylase specificity of AmyC is questionable as extensive phylogenetic analysis of GH57 and tertiary structural comparison suggest that AmyC could actually be a glycogen-branching enzyme (GBE), a key enzyme in the biosynthesis of glycogen. This communication presents phylogenetic and biochemical evidence that AmyC is a GBE with a relatively high hydrolytic (α-amylase) activity (up to 30% of the total activity), creating a branched α-glucan with 8.5% α-1,6-glycosidic bonds. The high hydrolytic activity is explained by the fact that AmyC has a considerably shorter catalytic loop (residues 213-220) not reaching the acceptor side. Secondly, in AmyC, the tryptophan residue (W 246) near the active site has its side chain buried in the protein interior, while the side chain is at the surface in Tk1436 and Tt1467 GBEs. The putative GBEs from three other Thermotogaceae, with very high sequence similarities to AmyC, were found to have the same structural elements as AmyC, suggesting that GH57 GBEs with relatively high hydrolytic activity may be widespread in nature.

Enantioselective Synthesis of Pharmaceutically Active γ-Aminobutyric Acids Using a Tailor-Made Artificial Michaelase in One-Pot Cascade Reactions.

Chiral γ-aminobutyric acid (GABA) analogues represent abundantly prescribed drugs, which are broadly applied as anticonvulsants, as antidepressants, and for the treatment of neuropathic pain. Here we report a one-pot two-step biocatalytic cascade route for synthesis of the pharmaceutically relevant enantiomers of γ-nitrobutyric acids, starting from simple precursors (acetaldehyde and nitroalkenes), using a tailor-made highly enantioselective artificial "Michaelase" (4-oxalocrotonate tautomerase mutant L8Y/M45Y/F50A), an aldehyde dehydrogenase with a broad non-natural substrate scope, and a cofactor recycling system. We also report a three-step chemoenzymatic cascade route for the efficient chemical reduction of enzymatically prepared γ-nitrobutyric acids into GABA analogues in one pot, achieving high enantiopurity (e.r. up to 99:1) and high overall yields (up to 70%). This chemoenzymatic methodology offers a step-economic alternative route to important pharmaceutically active GABA analogues, and highlights the exciting opportunities available for combining chemocatalysts, natural enzymes, and designed artificial biocatalysts in multistep syntheses.

Mutational Analysis of the Role of the Glucansucrase Gtf180-ΔN Active Site Residues in Product and Linkage Specificity with Lactose as Acceptor Substrate.

Glucansucrase Gtf180-ΔN from Lactobacillus reuteri uses lactose as acceptor substrate to synthesize five glucosylated lactose molecules (F1-F5) with a degree of polymerization (DP) of 3-4 (GL34) and with (α1→2)/(α1→3)/(α1→4) glycosidic linkages. Q1140/W1065/N1029 mutations significantly changed the GL34 product ratios. Q1140 mutations clearly decreased F3 3'-glc-lac with an (α1→3) linkage and increased F4 4',2-glc-lac with (α1→4)/(α1→2) linkages. Formation of F2 2-glc-lac with an (α1→2) linkage and F4 was negatively affected in most W1065 and N1029 mutants, respectively. Mutant N1029G synthesized four new products with additional (α1→3)-linked glucosyl moieties (2xDP4 and 2xDP5). Sucrose/lactose strongly reduced Gtf180-ΔN hydrolytic activity and increased transferase activity of Gtf180-ΔN and mutant N1029G, in comparison to activity with sucrose alone. N1029/W1065/Q1140 thus are key determinants of Gtf180-ΔN linkage and product specificity in the acceptor reaction with lactose. Mutagenesis of key residues in Gtf180-ΔN may allow synthesis of tailor-made mixtures of novel lactose-derived oligosaccharides with potential applications as prebiotic compounds in food/feed and in pharmacy/medicine.

Biochemical and genetic characterization of fungal proline hydroxylase in echinocandin biosynthesis.

An intriguing structural feature of echinocandins is the incorporation of hydroxylated amino acids. Elucidation of the machinery and the mechanism responsible for this modification is critical to generate new echinocandin derivatives with enhanced antifungal activity. In our present study, we biochemically characterized the α-ketoglutarate/Fe2+-dependent proline hydroxylase (HtyE) from two Aspergillus species, Aspergillus pachycristatus and Aspergillus aculeatus, in the respective echinocandin B and aculeacin A biosynthetic gene clusters. Our results showed that both Ap- and Aa-HtyE converted L-proline to trans-4- and trans-3-hydroxyproline, but at different ratios. Both enzymes also effectively hydroxylated C-3 of 4R-methyl-proline, L-pipecolic acid, and D-proline. Our homology modeling and site-directed mutagenesis studies identified Leu182 of Ap-HtyE as a key residue in determining the regioselectivity of Ap-HtyE. Notably, we found that the efficiency in C-3 hydroxylation of 4R-methyl-proline has no direct correlation with the ratio of trans-4-hydroxylproline to trans-3-hydroxylproline catalyzed by HtyE. Deletion of Ap-htyE abolished A. pachycristatus anti-Candida activity and the production of echinocandin B, demonstrating that HtyE is the enzyme responsible for the hydroxylation of L-proline and 4R-methyl-proline in vivo and is essential for the anti-Candida activity of echinocandin B. Our present study thus sheds light on the biochemical basis for the selective hydroxylation of L-proline and 4R-methyl-proline and reveals a new type of biocatalyst with potential for the custom production of hydroxylated proline and pipecolic acid derivatives.

Biochemical characterization of a GH70 protein from Lactobacillus kunkeei DSM 12361 with two catalytic domains involving branching sucrase activity.

The fructophilic bacterium Lactobacillus kunkeei has promising applications as probiotics promoting the health of both honey bees and humans. Here, we report the synthesis of a highly branched dextran by L. kunkeei DSM 12361 and biochemical characterization of a GH70 enzyme (GtfZ). Sequence analysis revealed that GtfZ harbors two separate catalytic cores (CD1 and CD2), predicted to have glucansucrase and branching sucrase specificity, respectively. GtfZ-CD1 was not characterized biochemically due to its unsuccessful expression. With only sucrose as substrate, GtfZ-CD2 was found to mainly catalyze sucrose hydrolysis and leucrose synthesis. When dextran was available as acceptor substrate, GtfZ-CD2 displayed an efficient transglycosidase activity with sucrose as donor substrate. Kinetic analysis showed that the GtfZ-CD2-catalyzed transglycosylation reaction follows a Ping Pong Bi Bi mechanism, indicating the in-turn binding of donor and acceptor substrates in the active site. Structural characterization of the products revealed that GtfZ-CD2 catalyzes the synthesis of single glucosyl (α1 → 3) linked branches onto dextran, resulting in the production of highly branched comb-like α-glucan products. These (α1 → 3) branches can be formed on adjacent positions, as shown when isomaltotriose was used as acceptor substrate. Homology modeling of the GtfZ-CD1 and GtfZ-CD2 protein structure strongly suggests that amino acid differences in conserved motifs II, III, and IV in the catalytic domain contribute to product specificity. Our present study highlights the ability of beneficial lactic acid bacteria to produce structurally complex α-glucans and provides novel insights into the molecular mechanism of an (α1 → 3) branching sucrase.

Biochemical characterization of two GH70 family 4,6-α-glucanotransferases with distinct product specificity from Lactobacillus aviarius subsp. aviarius DSM 20655.

Nine GtfB-like 4,6-α-glucanotransferases (4,6-α-GTs) (represented by GtfX of L. aviarius subsp. aviarius DSM 20655) were identified to show distinct characteristics in conserved motifs I-IV. In particular, the "fingerprint" Tyr in motif III of these nine GtfB-type 4,6-α-GTs was found to be replaced by a Trp. In L. aviarius subsp. aviarius DSM20655, a second GtfB-like protein (GtfY), containing the canonical GtfB Tyr residue in motif III, was located directly upstream of GtfX. Biochemical characterization revealed that both GtfX and GtfY showed GtfB-like 4,6-α-GT activity, cleaving (α1→4) linkages and catalyzing the synthesis of (α1→6) linkages. Nonetheless, they differ in product specificity; GtfY only synthesizes consecutive (α1→6) linkages, yielding linear α-glucan products, but GtfX catalyzes the synthesis of (α1→6) linkages predominantly at branch points (22%) rather than in linear segments (10%). The highly branched α-glucan produced by GtfX from amylose V is resistant to digestion by α-amylase, offering great potential as dietary fibers.

Biotechnological potential of novel glycoside hydrolase family 70 enzymes synthesizing α-glucans from starch and sucrose.

Transglucosidases belonging to the glycoside hydrolase (GH) family 70 are promising enzymatic tools for the synthesis of α-glucans with defined structures from renewable sucrose and starch substrates. Depending on the GH70 enzyme specificity, α-glucans with different structures and physicochemical properties are produced, which have found diverse (potential) commercial applications, e.g. in food, health and as biomaterials. Originally, the GH70 family was established only for glucansucrase enzymes of lactic acid bacteria that catalyze the synthesis of α-glucan polymers from sucrose. In recent years, we have identified 3 novel subfamilies of GH70 enzymes (designated GtfB, GtfC and GtfD), inactive on sucrose but converting starch/maltodextrin substrates into novel α-glucans. These novel starch-acting enzymes considerably enlarge the panel of α-glucans that can be produced. They also represent very interesting evolutionary intermediates between sucrose-acting GH70 glucansucrases and starch-acting GH13 α-amylases. Here we provide an overview of the repertoire of GH70 enzymes currently available with focus on these novel starch-acting GH70 enzymes and their biotechnological potential. Moreover, we discuss key developments in the understanding of structure-function relationships of GH70 enzymes in the light of available three-dimensional structures, and the protein engineering strategies that were recently applied to expand their natural product specificities.