Differential scanning calorimetric and spectroscopic studies on the unfolding of Momordica charantia lectin. Similar modes of thermal and chemical denaturation

Isolation of Momordica charantia seed lectin and glycosidases from the protein bodies: Lectin-glycosidase (β-hexosaminidase) protein body membrane interaction reveals possible physiological function of the lectin

Momordica charantia seeds are known to contain a galactose specific lectin that has been well characterized. Seed extracts also contain glycosidases such as the β-hexosaminidase, α-mannosidase and α-galactosidase. In the present study, lectin was affinity purified from the seed extracts and protein bodies isolated by sucrose density gradient centrifugation. From the protein bodies, lectin was identified and β-hexosaminidase was isolated by lectin affinity chromatography and subsequently separated from other glycosidases by gel filtration. In the native PAGE, the purified β-hexosaminidase migrated as a single band with a molecular weight of ∼235 kDa and by zymogram analysis using 4-methylumbelliferyl N-acetyl-β-D-glucosaminide substrate it was confirmed as β-hexosaminidase. Under reducing conditions in SDS-PAGE, the purified enzyme dissociated into three bands (Mr 33, 20 and 15 kDa). The prominent bands (20 and 15 kDa) showed immunological cross-reactivity with the human Hexosaminidase B antibody in a western blot experiment. In gel digestion of the purified enzyme, followed by proteomic analysis using tandom MS/MS revealed sequence identity as compared to the genomic sequence of the Momordica charantia with a score of 57 (24% sequence coverage). Additionally, by CD analysis the purified β-hexosaminidase showed 39.1% of α-helix. Furthermore, secondary structure variations were observed in presence of substrate, lectin and at different pH values. Protein body membrane prepared from the isolated protein bodies showed a pH dependent interaction with the purified lectin and mixture of glycosidases.

Subunit association, and thermal and chemical unfolding of Cucurbitaceae phloem exudate lectins. A review

Detailed characterization of protein (un)folding intermediates is crucial for understanding the (un)folding pathway, aggregation, stability and their functional properties. In recent years, stress-inducible lectins are being investigated with much interest. In plants phloem proteins PP1 and PP2 are major components of the phloem fluid. While PP1 is a structural protein, PP2 exhibits lectin activity, and was proposed to play key roles in wound sealing, anti-pathogenic activity, and transportation of various molecules including RNA within the plant. Cucurbitaceae fruits contain high concentrations of PP2 lectins, which recognize chitooligosaccharides with high specificity. Although the presence of PP2 lectins in the phloem exudate of Cucurbitaceae species was documented over 40 years ago, so far only a few proteins from this family have been purified and characterized in detail. This review summarizes the results of biophysical studies aimed at investigating the oligomeric status of these lectins, their thermal stability, structural perturbations caused by changes in pH and addition of chaotropic agents and characterization of intermediates observed in the unfolding process. The implications of these results in the functional roles played by PP2 type lectins in their native environment are discussed. Finally, perspectives for future biophysical research on these proteins are given.

A review of thermosensitive antinutritional factors in plant-based foods

Legumes and cereals account for the vast proportion of people's daily intake of plant-based foods. Meanwhile, a large number of antinutritional factors in legumes and cereals hinder the body absorption of nutrients and reduce the nutritional value of food. In this paper, the antinutritional effects, determination, and passivation methods of thermosensitive antinutritional factors such as trypsin inhibitors, urease, lipoxygenase, and lectin were reviewed to provide theoretical help to reduce antinutritional factors in food and improve the utilization rate of plant-based food nutrition. Since trypsin inhibitors and lectin have been more extensively studied and reviewed previously, the review mainly focused on urease and lipoxygenase. This review summarized the information of thermosensitive antinutritional factors, trypsin inhibitors, urease, lipoxygenase, and lectin, in cereals and legumes. The antinutritional effects, and physical and chemical properties of trypsin inhibitors, urease, lipoxygenase, and lectin were introduced. At the same time, the research methods for the detection and inactivation of these four antinutritional factors were also summarized in the order of research conducted time. The rapid determination and inactivation of antinutrients will be the focus of attention for the food industry in the future to improve the nutritional value of food. Exploring what structural changes could passivation technologies bring to antinutritional factors will provide a theoretical basis for further understanding the mechanisms of antinutritional factor inactivation. PRACTICAL APPLICATIONS: Antinutritional factors in plant-based foods hinder the absorption of nutrients and reduce the nutritional value of the food. Among them, thermosensitive antinutritional factors, such as trypsin inhibitors, urease, lipoxygenase, and lectins, have a high proportion among the antinutritional factors. In this paper, we investigate thermosensitive antinutritional factors from three perspectives: the antinutritional effect of thermosensitive antinutritional factors, determination, and passivation methods. The current passivation methods for thermosensitive antinutritional factors revolve around biological, physical, and chemical aspects, and their elimination mechanisms still need further research, especially at the protein structure level. Reducing the level of antinutritional factors in the future food industry while controlling the loss of other nutrients in food is a goal that needs to be balanced. This paper reviews the antinutritional effects of thermosensitive antinutritional factors and passivation methods, expecting to provide new research ideas to improve the nutrient utilization of food.

Combined effects of pH and thermal treatments on IgE-binding capacity and conformational structures of lectin from black kidney bean (Phaseolus vulgaris L.)

Combined effects of pH and thermal treatments on black kidney bean lectin (BKBL) were investigated by response surface methodology (RSM). Low-pH (1.0, 2.0, 3.0) incubation decreased hemagglutination activity (HA) and IgE-binding capacity, but the activities would be restored when the lectin was treated by pH shifting to 7.2. Conformational structure analyses indicated that low-pH induced protein unfolding and pH-shifting treatment resulted in a limited structural rearrangement. Mild heating, such as 60 °C for 3 min, slightly increased the HA and IgE-binding activities of pH shifted BKBL, but no obvious effects in the pH 1.0 incubated BKBL. High-temperature and long-time treatment might induce the protein aggregation, further decreased HA and IgE-binding capacities. RSM results showed both IgE-binding capacity and HA were the lowest under the combination of pH 1.0 incubation with 80 °C heating for 15 min or pH shifting from 1.0 to 7.2 with 100 °C heating for 10 min.

Two-Step Isolation, Purification, and Characterization of Lectin from Zihua Snap Bean (Phaseolus vulgaris) Seeds

A two-step method based on an aqueous two-phase system and Sephadex G-75 was used to separate and purify lectin from the seeds of the Zihua snap bean. The preliminary properties and bioactivity of the Zihua snap bean lectin were characterized by different instrumental methods, such as sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS–PAGE), liquid chromatography-nano electrospray ionization mass spectrometry (Nano LC-ESI-MS/MS), and Fourier transform infrared spectroscopy (FTIR). The hemagglutinating activity of the Zihua snap bean lectin could not be inhibited by glucose, N-acetyl-d-glucosamine, d-galactose, N-acetyl-d-galactosamine, fructose, sucrose, d-maltose, d-trehalose, and lactose. It was found that the hemagglutinating activity of the lectin showed strong dependence on Mn²⁺ and Ca²⁺. The thermal and pH stability of the Zihua snap bean lectin was studied by FTIR and fluorescence spectroscopy. Relatively good stability was observed when the temperature was not higher than 70 °C, as well as in the pH range of 2.0 to 10.0. Digestive stability in vitro was investigated. The untreated lectin was relatively stable to pepsin and trypsin activity, but heat treatment could significantly reduce the digestive stability in vitro. Moreover, the lectin showed an inhibitory effect on the tested bacteria (Staphylococcus aureus (S. aureus), Escherichia coli (E. coli), Bacillus subtilis (B. subtilis)), and it also showed a certain inhibitory effect on the growth of Phytophthora infestans (P. infestans) at higher concentrations.

The Ssl2245-Sll1130 Toxin-Antitoxin System Mediates Heat-induced Programmed Cell Death in Synechocystis sp. PCC6803

Two putative heat-responsive genes, ssl2245 and sll1130, constitute an operon that also has characteristics of a toxin-antitoxin system, thus joining several enigmatic features. Closely related orthologs of Ssl2245 and Sll1130 exist in widely different bacteria, which thrive under environments with large fluctuations in temperature and salinity, among which some are thermo-epilithic biofilm-forming cyanobacteria. Transcriptome analyses revealed that the clustered regularly interspaced short palindromic repeats (CRISPR) genes as well as several hypothetical genes were commonly up-regulated in Δssl2245 and Δsll1130 mutants. Genes coding for heat shock proteins and pilins were also induced in Δsll1130 We observed that the majority of cells in a Δsll1130 mutant strain remained unicellular and viable after prolonged incubation at high temperature (50 °C). In contrast, the wild type formed large cell clumps of dead and live cells, indicating the attempt to form biofilms under harsh conditions. Furthermore, we observed that Sll1130 is a heat-stable ribonuclease whose activity was inhibited by Ssl2245 at optimal temperatures but not at high temperatures. In addition, we demonstrated that Ssl2245 is physically associated with Sll1130 by electrostatic interactions, thereby inhibiting its activity at optimal growth temperature. This association is lost upon exposure to heat, leaving Sll1130 to exhibit its ribonuclease activity. Thus, the activation of Sll1130 leads to the degradation of cellular RNA and thereby heat-induced programmed cell death that in turn supports the formation of a more resistant biofilm for the surviving cells. We suggest to designate Ssl2245 and Sll1130 as MazE and MazF, respectively.

Physico-chemical characteristics and primary structure of an affinity-purified α-D-galactose-specific, jacalin-related lectin from the latex of mulberry (Morus indica)

An α-D-galactose specific lectin belonging to the family of jacalin-related lectins (JRL) has been purified by affinity chromatography on cross-linked guar-gum. Mass spectrometric data revealed that the protein harbors two chains like all the members of galactose-specific jacalin-related lectins (gJRL). De novo sequencing of proteolytic peptides demonstrated that the heavier chain consists of 133 amino acids and the lighter chain comprises of 21 or 24 amino acids. The heavier chain contains one N-glycosylation site (Asn₄₇) occupied with either pauci-mannose type [GlcNAc₂(Fuc)Man₃(Xyl)] or complex type [GlcNAc₂(Fuc)Man₃(Xyl)GlcNAc(Fuc)Gal] N-glycans. Circular dichroism spectroscopy indicated that the secondary structure of the lectin is predominantly made up of β-sheets, and differential scanning calorimetry revealed a thermal denaturation temperature of 77.6 °C. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) cell viability assays on MCF-7 and MDCK cells showed that the lectin is highly cytotoxic towards both cell lines when dosed at micromolar concentrations, suggesting that it may play a role in the defense mechanism of the plant.

Structural studies on a non-toxic homologue of type II RIPs from bitter gourd: Molecular basis of non-toxicity, conformational selection and glycan structure

The structures of nine independent crystals of bitter gourd seed lectin (BGSL), a non-toxic homologue of type II RIPs, and its sugar complexes have been determined. The four-chain, two-fold symmetric, protein is made up of two identical two-chain modules, each consisting of a catalytic chain and a lectin chain, connected by a disulphide bridge. The lectin chain is made up of two domains. Each domain carries a carbohydrate binding site in type II RIPs of known structure. BGSL has a sugar binding site only on one domain, thus impairing its interaction at the cell surface. The adenine binding site in the catalytic chain is defective. Thus, defects in sugar binding as well as adenine binding appear to contribute to the non-toxicity of the lectin. The plasticity of the molecule is mainly caused by the presence of two possible well defined conformations of a surface loop in the lectin chain. One of them is chosen in the sugar complexes, in a case of conformational selection, as the chosen conformation facilitates an additional interaction with the sugar, involving an arginyl residue in the loop. The N-glycosylation of the lectin involves a plant-specific glycan while that in toxic type II RIPs of known structure involves a glycan which is animal as well as plant specific.

Model Analytical Development for Physical, Chemical, and Biological Characterization of Momordica charantia Vegetable Drug

Momordica charantia is a species cultivated throughout the world and widely used in folk medicine, and its medicinal benefits are well documented, especially its pharmacological properties, including antimicrobial activities. Analytical methods have been used to aid in the characterization of compounds derived from plant drug extracts and their products. This paper developed a methodological model to evaluate the integrity of the vegetable drug M. charantia in different particle sizes, using different analytical methods. M. charantia was collected in the semiarid region of Paraíba, Brazil. The herbal medicine raw material derived from the leaves and fruits in different particle sizes was analyzed using thermoanalytical techniques as thermogravimetry (TG) and differential thermal analysis (DTA), pyrolysis coupled to gas chromatography/mass spectrometry (PYR-GC/MS), and nuclear magnetic resonance ((1)H NMR), in addition to the determination of antimicrobial activity. The different particle surface area among the samples was differentiated by the techniques. DTA and TG were used for assessing thermal and kinetic parameters and PYR-GC/MS was used for degradation products chromatographic identification through the pyrograms. The infusions obtained from the fruit and leaves of Momordica charantia presented antimicrobial activity.

Inverse relationship between chitobiase and transglycosylation activities of chitinase-D from Serratia proteamaculans revealed by mutational and biophysical analyses

Serratia proteamaculans chitinase-D (SpChiD) has a unique combination of hydrolytic and transglycosylation (TG) activities. The TG activity of SpChiD can be used for large-scale production of chito-oligosaccharides (CHOS). The multiple activities (hydrolytic and/or chitobiase activities and TG) of SpChiD appear to be strongly influenced by the substrate-binding cleft. Here, we report the unique property of SpChiD substrate-binding cleft, wherein, the residues Tyr28, Val35 and Thr36 control chitobiase activity and the residues Trp160 and Trp290 are crucial for TG activity. Mutants with reduced (V35G and T36G/F) or no (SpChiDΔ30–42 and Y28A) chitobiase activity produced higher amounts of the quantifiable even-chain TG product with degree of polymerization (DP)-6, indicating that the chitobiase and TG activities are inversely related. In addition to its unprecedented catalytic properties, unlike other chitinases, the single modular SpChiD showed dual unfolding transitions. Ligand-induced thermal stability studies with the catalytically inactive mutant of SpChiD (E153A) showed that the transition temperature increased upon binding of CHOS with DP2–6. Isothermal titration calorimetry experiments revealed the exceptionally high binding affinities for E153A to CHOS with DP2–6. These observations strongly support that the architecture of SpChiD substrate-binding cleft adopted to control chitobiase and TG activities, in addition to usual chitinase-mediated hydrolysis.

Structural basis for regulation of stability and activity in glyceraldehyde-3-phosphate dehydrogenases. Differential scanning calorimetry and molecular dynamics

Tissue specific isoforms of human glyceraldehyde-3-phosphate dehydrogenase, somatic (GAPD) and sperm-specific (GAPDS), have been reported to display different levels of both stability and catalytic activity. Here we apply MD simulations to investigate molecular basis of this phenomenon. The protein is a tetramer where each subunit consists of two domains - catalytic and NAD-binding one. We demonstrated key residues responsible for intersubunit and interdomain interactions. Effect of several residues was studied by point mutations. Overall we considered three mutations (Glu96Gln, Glu244Gln and Asp311Asn) disrupting GAPDS-specific salt bridges. Comparison of calculated interaction energies with calorimetric enthalpies confirmed that intersubunit interactions were responsible for enhanced thermostability of GAPDS whereas interdomain interactions had indirect influence on intersubunit contacts. Mutation Asp311Asn was around 10Å far from the active center and corresponded to the closest natural substitution in the isoenzymes. MD simulations revealed that this residue had slight interaction with catalytic residues but influenced the hydrogen bond net and dynamics in active site. These effects can be responsible for a strong influence of this residue on catalytic activity. Overall, our results provide new insight into glyceraldehyde-3-phosphate dehydrogenase structure-function relationships and can be used for the engineering of mutant proteins with modified properties and for development of new inhibitors with indirect influence on the catalytic site.

Guanidine hydrochloride and urea effects upon thermal stability of Glossoscolex paulistus hemoglobin (HbGp)

Glossoscolex paulistus hemoglobin (HbGp) has a molecular mass of 3600kDa. It belongs to the hexagonal bilayer hemoglobin class, which consists of highly cooperative respiratory macromolecules found in mollusks and annelids. The present work focusses on oxy-HbGp thermal stability, in the presence of urea and guanidine hydrochloride (GuHCl), monitored by several techniques. Initially, dynamic light scattering data show that the presence of GuHCl induces the protein oligomeric dissociation, followed by a significant 11-fold increase in the hydrodynamic diameter (DH) values, due to the formation of protein aggregates in solution. In contrast, urea promotes the HbGp oligomeric dissociation, followed by unfolding process at high temperatures, without aggregation. Circular dichroism data show that unfolding critical temperature (Tc) of oxy-HbGp decreases from 57°C, at 0.0 mol/L of the denaturant, to 45°C, in the presence of 3.5 mol/L of urea, suggesting the reduction of HbGp oligomeric stability. Moreover, differential scanning calorimetry results show that at lower GuHCl concentrations, some thermal stabilization of the hemoglobin is observed, whereas at higher concentrations, the reduction of stability takes place. Besides, HbGp is more stable in the presence of urea when compared with the guanidine effect, as deduced from the differences in the concentration range of denaturants.

Comparison of chemical and thermal protein denaturation by combination of computational and experimental approaches. II

Chemical and thermal denaturation methods have been widely used to investigate folding processes of proteins in vitro. However, a molecular understanding of the relationship between these two perturbation methods is lacking. Here, we combined computational and experimental approaches to investigate denaturing effects on three structurally different proteins. We derived a linear relationship between thermal denaturation at temperature T(b) and chemical denaturation at another temperature T(u) using the stability change of a protein (ΔG). For this, we related the dependence of ΔG on temperature, in the Gibbs-Helmholtz equation, to that of ΔG on urea concentration in the linear extrapolation method, assuming that there is a temperature pair from the urea (T(u)) and the aqueous (T(b)) ensembles that produces the same protein structures. We tested this relationship on apoazurin, cytochrome c, and apoflavodoxin using coarse-grained molecular simulations. We found a linear correlation between the temperature for a particular structural ensemble in the absence of urea, T(b), and the temperature of the same structural ensemble at a specific urea concentration, T(u). The in silico results agreed with in vitro far-UV circular dichroism data on apoazurin and cytochrome c. We conclude that chemical and thermal unfolding processes correlate in terms of thermodynamics and structural ensembles at most conditions; however, deviations were found at high concentrations of denaturant.

Crystallization and preliminary X-ray studies of a galactose-specific lectin from the seeds of bitter gourd (Momordica charantia)

A galactose-specific lectin from the seeds of bitter gourd (Momordica charantia) is a four-chain type II ribosome-inactivating protein (RIP) resulting from covalent association through a disulfide bridge between two identical copies of a two-chain unit. The available structural information on such four-chain RIPs is meagre. The bitter gourd lectin was therefore crystallized for structural investigation and the crystals have been characterized. It is anticipated that the structure of the orthorhombic crystals will be analysed using molecular replacement by taking advantage of its sequence, and presumably structural, homology to normal two-chain type II RIPs.