Unraveling the role of flexible coil near calcium binding site of levansucrase on thermostability and product profile via proline substitution and molecular dynamics simulations

Due to its bioactivity and versatile applications, levan has appeared as a promising biomaterial. Levansucrase is responsible for the conversion of sucrose into levan. With the goal of enhancing levan production, the strategy for enhancing the stability of levansucrase is being intensively studied. To make proteins more stable under high temperatures, proline, the most rigid residue, can be introduced into previously flexible regions. Herein, G249, D250, N251, and H252 on the flexible coil close to the calcium binding site of Bacillus licheniformis levansucrase were replaced with proline. Mutations at G249P greatly enhance both the enzyme's thermodynamic and kinetic stability, while those at H252P improve solely the enzyme's kinetic stability. GPC analysis revealed that G249P synthesize more levan, but H252P generate primarily oligosaccharides. Molecular dynamics simulations (MD) and MM/GBSA analysis revealed that G249P mutation increased not only the stability of levansucrase, but also affinity toward fructan.

Mechanistic Insights into Roseoflavin Synthesis by N,N-8-Demethyl-8-aminoriboflavin Dimethyltransferase (RosA): Molecular Dynamics Simulations and Residue Conservation Analysis

8-Demethyl-8-dimethylaminoriboflavin (Roseoflavin or RoF) is a natural riboflavin analogue found in Streptomyces davaonensis and Streptomyces cinnabarinus. RoF displays potent antibiotic properties because it affects FMN riboswitches and flavoproteins of cellular targets. N,N-8-Demethyl-8-aminoriboflavin dimethyltransferase (RosA) is an enzyme that catalyzes the last step of RoF biosynthesis, a consecutive dimethylation of 8-demethyl-8-aminoriboflavin (AF) to generate RoF. Thus, understanding mechanistic insights into RosA structures and mechanisms could lead to the improvement of the RoF product yield. Herein, mechanistic insights into roseoflavin synthesis by RosA were evaluated using molecular dynamics simulations. The obtained results revealed that RosA possibly catalyzes the reaction by positioning the substrate binding to have proper distance and orientation to the methyl group donor, S-adenosylmethionine. No direct participation of catalytic residues in the reaction was identified. The enzyme's active site structures change drastically to accommodate the ligand binding. On the basis of the MM/GBSA calculations and conservation analysis, the amino acid residues involved in substrate binding were identified. The structural information obtained from this study could be beneficial in designing RosA to efficiently produce roseoflavin.

Cassava pullulanase and its synergistic debranching action with isoamylase 3 in starch catabolism

Pullulanase (EC 3.2.1.41, PUL), a debranching enzyme belonging to glycoside hydrolase family 13 subfamily 13, catalyses the cleavage of α-1,6 linkages of pullulan and β-limit dextrin. The present work studied PUL from cassava Manihot esculenta Crantz (MePUL) tubers, an important economic crop. The Mepul gene was successfully cloned and expressed in E. coli and rMePUL was biochemically characterised. MePUL was present as monomer and homodimer, as judged by apparent mass of ~ 84 - 197 kDa by gel permeation chromatography analysis. Optimal pH and temperature were at pH 6.0 and 50 °C, and enzyme activity was enhanced by the addition of Ca²⁺ ions. Pullulan is the most favourable substrate for rMePUL, followed by β-limit dextrin. Additionally, maltooligosaccharides were potential allosteric modulators of rMePUL. Interestingly, short-chain maltooligosaccharides (DP 2 - 4) were significantly revealed at a higher level when rMePUL was mixed with cassava isoamylase 3 (rMeISA3), compared to that of each single enzyme reaction. This suggests that MePUL and MeISA3 debranch β-limit dextrin in a synergistic manner, which represents a major starch catabolising process in dicots. Additionally, subcellular localisation suggested the involvement of MePUL in starch catabolism, which normally takes place in plastids.

A theoretical study on binding and stabilization of galactose and novel galactose analogues to the human α-galactosidase A variant causing Fabry disease

α-galactosidase A (α-Gal A) catalyzes the hydrolysis of terminal α-galactosyl moieties from globotriaosylceramide, and mutations in this enzyme lead to the lipid metabolism disorder "Fabry disease". Mutation in α-Gal A possibly causes the protein misfolding, which reduces catalytic activity and stability of the enzyme. A recent study demonstrated that the binding of galactose on the α-Gal A catalytic site significantly increases its stability. Herein, the effect of mutation on secondary structure, structural energy, and galactose affinity of α-Gal A (wild type and A143T variant) was investigated using molecular dynamics simulations and free energy calculations based on MM/GBSA method. The results showed that A143T mutation caused the formation of unusual H-bonds that induced the change in secondary structure and binding affinities toward galactose. The amino acid residues involved in galactose binding were identified. The molecular binding mechanism obtained from this study could be helpful for optimizations and designs of new galactose analogs as pharmacological chaperones against Fabry disease.

Allyl ether of mansonone G as a potential anticancer agent for colorectal cancer

Mansonone G (MG), a 1,2-naphthoquinone isolated from the heartwood of Mansonia gagei Drumm, exhibited several pharmacological activities such as anti-bacterial, anti-estrogenic and anti-adipogenic effect. This study evaluated the cytotoxicity of MG and its derivatives as well as determined the mechanism(s) underlying the cytotoxic activity of the most potent MG derivative on two CRC cell lines, HCT-116 cells carrying p53 wild-type and HT-29 cells carrying p53 mutant. We found that MG and its derivatives could inhibit viability of HCT-116 and HT-29 cells in a concentration-dependent manner. Of all semi-synthetic derivatives of MG, allyl ether mansonone G (MG7) was the most potent cytotoxic agent toward cancer cells and less toxic to normal cells. MG7 could induce ROS generation which was associated with cytotoxicity and apoptosis in both HCT-116 and HT-29 cells. Western blot analysis revealed that MG7 downregulated the expression of Bcl-2 and Bcl-xL proteins in both CRC cell lines and upregulated the expression of BAK protein in HT-29 cells. Moreover, MG7 inhibited AKT signaling pathway in both CRC cell lines and modulated ERK1/2 signaling pathway by inhibiting ERK1/2 phosphorylation in HCT-116 cells and activating ERK1/2 phosphorylation in HT-29 cells. Molecular docking revealed that MG7 could bind to the ATP-binding pocket of AKT and ERK1 via hydrophobic interactions.

Emerging roles of GALNT5 on promoting EGFR activation in cholangiocarcinoma: a mechanistic insight

Cholangiocarcinoma (CCA) is a lethal cancer in that the incidence is now increasing worldwide. N-acetylgalactosaminyltransferase 5 (GALNT5), an enzyme that initiates the first step of mucin type-O glycosylation, has been reported to promote aggressiveness of CCA cells via the epithelial to the mesenchymal transition (EMT) process, and Akt/Erk activation. In this study, the clinical and biological relevance of GALNT5 and the molecular mechanisms by which GALNT5 modulated EGFR in promoting CCA progression were examined. Using publicly available datasets, upregulation of GALNT5 in patient CCA tissues and its correlation with EGFR expression was noted. High levels of GALNT5 were significantly associated with the short survival of patients, suggesting a prognostic marker of GALNT5 for CCA. GALNT5 modulated EGFR expression as shown in CCA cell lines. Upregulation of GALNT5 significantly increased EGFR mRNA and protein in GALNT5 overexpressing cells, whereas suppression of GALNT5 expression gave the opposite results. The molecular dynamics simulations and MM/PB(GB)SA-based free energy calculations showed that O-glycosylation on the EGFR extracellular domain enhanced the structural stability, compactness, and H-bond formation of the EGF/GalNAc-EGFR complex compared with those of EGF/EGFR. This stabilized the growth factor binding site and fostered stronger interactions between EGF and EGFR. Using the EGF-induced EGFR activation model, GALNT5 was shown to mediate EGFR stability via a decreased rate of EGFR degradation and enhanced EGFR activity by increasing the binding affinity of EGF/EGFR that consequently increasing the activation of EGFR and its downstream effectors Akt and Erk. In summary, GALNT5 was upregulated in CCA tissues and associated with a worse prognosis. The study identified for the first time the impacts of GALNT5 on EGFR activity by increasing: 1) EGFR expression via a transcriptional-dependent mechanism, 2) EGFR stability by reducing EGFR degradation, and 3) EGFR activation through an increased binding affinity of EGF/EGFR which all together fostered the activation of EGFR. These results expanded the understanding of the molecular mechanism of how GALNT5 impacted CCA progression and suggested GALNT5 as a new target for therapeutic intervention against metastatic CCA.

Computational design of Lactobacillus Acidophilus α-L-rhamnosidase to increase its structural stability

α-L-rhamnosidase catalyzes hydrolysis of the terminal α-L-rhamnose from various natural rhamnoglycosides, including naringin and hesperidin, and has various applications such as debittering of citrus juices in the food industry and flavonoid derhamnosylation in the pharmaceutical industry. However, its activity is lost at high temperatures, limiting its usage. To improve Lactobacillus acidophilus α-L-rhamnosidase stability, we employed molecular dynamics (MD) to identify a highly flexible region, as evaluated by its root mean square fluctuation (RMSF) value, and computational protein design (Rosetta) to increase rigidity and favorable interactions of residues in highly flexible regions. MD results show that five regions have the highest flexibilities and were selected for design by Rosetta. Twenty-one designed mutants with the best ΔΔG at each position and ΔΔG < 0 REU were simulated at high temperature. Eight designed mutants with ΔRMSF of highly flexible regions lower than -10.0% were further simulated at the optimum temperature of the wild type. N88Q, N202V, G207D, Q209M, N211T and Y213K mutants were predicted to be more stable and could maintain their native structures better than the wild type due to increased hydrogen bond interactions of designed residues and their neighboring residues. These designed mutants are promising enzymes with high potential for stability improvement.

Synergistic enzyme cocktail between levansucrase and inulosucrase for superb levan-type fructooligosaccharide synthesis

Inulosucrase (ISC) and levansucrase (LSC) utilise sucrose and produce inulin- and levan-type fructans, respectively. This study aims to propose a new strategy to improve levan-type fructooligosaccharide (L-FOS) production. The effect of ISC/ LSC -mixed reaction was elucidated on L-FOS production. The presence of ISC in the LSC reaction significantly leads to the higher production of L-FOSs as the main products. Furthermore, the different ratios between ISC and LSC affected the distribution of L-FOSs. A greater amount of ISC compared to LSC promoted the synthesis of short-chain L-FOSs. Conversely, when LSC was increased, the synthesis of longer-chain L-FOSs was enhanced. The addition of trisaccharide mixtures obtained from either a single ISC or LSC reaction could enhance L-FOSs synthesis in the LSC reaction. Analysis of these trisaccharides revealed that most species of the oligosaccharides were similar, with 1-kestose being the major one. The supplement of only 1-kestose in the LSC reaction showed similar results to those of the reaction in the presence of trisaccharide mixtures. Moreover, the results were supported by molecular dynamics simulations. This work not only provides an improvement in L-FOS production but also revealed and supported some insights into the mechanism of fructansucrases.

Computational Design of Oligosaccharide-Producing Levansucrase from Bacillus licheniformis RN-01 to Increase Its Stability at High Temperature

Levan-type fructooligosaccharides (LFOs) and levan can potentially be used as ingredients in prebiotics, skincare products, and antitumor agents. The Y246S mutant of Bacillus licheniformis RN-01 levansucrase (oligosaccharide-producing levansucrase, OPL) was reported to productively synthesize LFOs; however, OPL's thermostability is low at high temperatures. To enhance OPL structural stability, this study employed molecular dynamics (AMBER) to identify a highly flexible region, as measured by its average root-mean-square fluctuation (RMSF) value, on the OPL surface and computational protein design (Rosetta) to rigidify and increase favorable interactions to increase its structural stability. AMBER identified region nine (residues 277-317) as a highly flexible region that was selected for design because it has the highest number of residues and the second-highest average RMSF, and it is farthest from the active site. Rosetta designed 14 mutants with the best ΔΔG value in each position, where three mutants have better ΔG than OPL. To determine whether their flexibilities and stabilities are lower than those of OPL, all 14 designed mutants were simulated at high temperature (500 K), and we found that K296E, G309S, and A310W mutants were predicted to be more stable and could retain their native structures better than OPL. Our results suggest that enhanced structural stabilities of these mutants are caused by increased hydrogen bond strengths of the designed residues and their neighboring residues. This study designed K296E, G309S, and A310W mutants of OPL with high potential for stability improvement, and they could potentially be used for the effective production of LFOs.

Fisetin Inhibits Osteogenic Differentiation of Mesenchymal Stem Cells via the Inhibition of YAP

Mesenchymal stem cells (MSCs) are self-renewal and capable of differentiating to various functional cell types, including osteocytes, adipocytes, myoblasts, and chondrocytes. They are, therefore, regarded as a potential source for stem cell therapy. Fisetin is a bioactive flavonoid known as an active antioxidant molecule that has been reported to inhibit cell growth in various cell types. Fisetin was shown to play a role in regulating osteogenic differentiation in animal-derived MSCs; however, its molecular mechanism is not well understood. We, therefore, studied the effect of fisetin on the biological properties of human MSCs derived from chorion tissue and its role in human osteogenesis using MSCs and osteoblast-like cells (SaOs-2) as a model. We found that fisetin inhibited proliferation, migration, and osteogenic differentiation of MSCs as well as human SaOs-2 cells. Fisetin could reduce Yes-associated protein (YAP) activity, which results in downregulation of osteogenic genes and upregulation of fibroblast genes. Further analysis using molecular docking and molecular dynamics simulations suggests that fisetin occupied the hydrophobic TEAD pocket preventing YAP from associating with TEA domain (TEAD). This finding supports the potential application of flavonoids like fisetin as a protein–protein interaction disruptor and also suggesting an implication of fisetin in regulating human osteogenesis.

Conserved Calcium-Binding Residues at the Ca-I Site Involved in Fructooligosaccharide Synthesis by Lactobacillus reuteri 121 Inulosucrase

Inulosucrase is an enzyme that synthesizes inulin-type β-2,1-linked fructooligosaccharides (IFOS) from sucrose. Previous studies have shown that calcium is important for the activity and stability of Lactobacillus reuteri 121 inulosucrase (LrInu). Here, mutational analyses of four conserved calcium-binding site I (Ca-I) residues of LrInu, Asp418, Gln449, Asn488, and Asp520 were performed. Alanine substitution for these residues not only reduced the stability and activity of LrInu, but also modulated the pattern of the IFOS produced. Circular dichroism spectroscopy and molecular dynamics simulation indicated that these mutations had limited impact on the overall conformation of the enzyme. One of Ca-I residues most critical for controlling LrInu-mediated polymerization of IFOS, Asp418, was also subjected to mutagenesis, generating D418E, D418H, D418L, D418N, D418S, and D418W. The activity of these mutants demonstrated that the IFOS chain length could be controlled by a single mutation at the Ca-I site.

Computational design of oligosaccharide producing levansucrase from Bacillus licheniformis RN-01 to improve its thermostability for production of levan-type fructooligosaccharides from sucrose

Levansucrase catalyzes production of levan and levan-type fructooligosaccharides (LFOs) with potential applications in food and pharmaceutical industries such as prebiotics and anti-tumor agents. Previous study found that Y246S mutant of Bacillus licheniformis RN-01 levansucrase (oligosaccharide producing levansucrase, OPL) could effectively produce LFOs but its thermostability is limited at high temperature. In this study, molecular dynamics (MD) and computational protein design were used to create mutants with higher thermostability than OPL by rigidifying highly flexible residues on enzyme surface. MD results show that highly flexible residues suitable for design are K82, N83, D179, and Q308. Two approaches were employed to improve their interactions by allowing them to be amino acids that could potentially form favorable interactions with their neighboring residues or natural amino acids except G, P and C. Flexibilities of designed residues of K82H, N83R, Q308S and K82H/N83R mutants are lower than those of OPL. Experimental results show that characteristics and product patterns of designed mutants are relatively similar to those of OPL. K82H/N83R mutant has higher thermostability than OPL with 1.7-fold increase in t_1/2. Circular dichroism result suggests that designed mutations do not drastically affect secondary structures. This study shows how computational technique can engineer enzyme for thermostability improvement.

Levansucrase from Bacillus amyloliquefaciens KK9 and Its Y237S Variant Producing the High Bioactive Levan-Type Fructooligosaccharides

Levan-typed fructooligosaccharide (LFOS), a β-2,6 linked oligofructose, displays the potential application as a prebiotic and therapeutic dietary supplement. In the present study, LFOS was synthesized using levansucrase from Bacillus amyloliquefaciens KK9 (LsKK9). The wild-type LsKK9 was cloned and expressed in E. coli, and purified by cation exchanger chromatography. Additionally, Y237S variant of LsKK9 was constructed based on sequence alignment and structural analysis to enhance the LFOS production. High-performance anion-exchange chromatography coupled with pulsed amperometric detection (HPAEC-PAD) analysis indicated that Y237S variant efficiently produced a higher amount of short-chain LFOS than wild type. Also, the concentration of enzyme and sucrose in the reactions was optimized. Finally, prebiotic activity assay demonstrated that LFOS produced by Y237S variant had higher prebiotic activity than that of the wild-type enzyme, making the variant enzyme attractive for food biotechnology.

Rational re-design of Lactobacillus reuteri 121 inulosucrase for product chain length control

Fructooligosaccharides (FOSs) are well-known prebiotics that are widely used in the food, beverage and pharmaceutical industries. Inulosucrase (E.C. 2.4.1.9) can potentially be used to synthesise FOSs from sucrose. In this study, inulosucrase from Lactobacillus reuteri 121 was engineered by site-directed mutagenesis to change the FOS chain length. Three variants (R483F, R483Y and R483W) were designed, and their binding free energies with 1,1,1-kestopentaose (GF4) were calculated with the Rosetta software. R483F and R483Y were predicted to bind with GF4 better than the wild type, suggesting that these engineered enzymes should be able to effectively extend GF4 by one residue and produce a greater quantity of GF5 than the wild type. MALDI-TOF MS analysis showed that R483F, R483Y and R483W variants could synthesise shorter chain FOSs with a degree of polymerization (DP) up to 11, 10, and 10, respectively, while wild type produced longer FOSs and in polymeric form. Although the decrease in catalytic activity and the increase of hydrolysis/transglycosylation activity ratio was observed, the variants could effectively synthesise FOSs with the yield up to 73% of substrate. Quantitative analysis demonstrated that these variants produced a larger quantity of GF5 than wild type, which was in good agreement with the predicted binding free energy results. Our findings demonstrate the success of using aromatic amino acid residues, at position D418, to block the oligosaccharide binding track of inulosucrase in controlling product chain length.