The saccharide chain of lupin seed conglutin gamma is not responsible for the protection of the native protein from degradation by trypsin, but facilitates the refolding of the acid-treated protein to the resistant conformation

Antimicrobial and Functional Properties of Duckweed (Wolffia globosa) Protein and Peptide Extracts Prepared by Ultrasound-Assisted Extraction

Wolffia globosa is an interesting alternative plant-based protein source containing up to 40% protein dry weight. Dried duckweed protein extract (PE) was obtained using ultrasound-assisted extraction (UAE) before isoelectric precipitation (pH 3.5) to yield protein concentrate (PC) and protein solution (PS). The PC was hydrolyzed using Alcalase enzyme to obtain protein concentrate hydrolysate (PCH). Among all fractions, PCH exhibited antimicrobial properties by decreasing populations of Vibrio parahaemolyticus and Candida albicans at 0.43 ± 1.31 log reduction (66.21%) and 3.70 ± 0.11 log reduction (99.98%), respectively. The PE and PS also showed high solubilities at pH 8 of 90.49% and 86.84%, respectively. The PE demonstrated the highest emulsifying capacity (EC) (71.29%) at pH 4, while the highest emulsifying stability (ES) (~98%) was obtained from the PE and PS at pH 6 and pH 2, respectively. The major molecular weights (Mw) of the PE, PC, PCH and PS were observed at 25, 45, 63 and 100 kDa, with a decrease in the Mw of the PCH (<5 kDa). The PCH contained the highest total amino acids, with aspartic acid and glutamic acid being the major components. The results revealed the antimicrobial and functional properties of duckweed protein and hydrolysate for the first time and showed their potential for further development as functional food ingredients.

Basic 7S globulin in plants

Soybean seed basic 7S globulin (Bg7S)-like proteins are found in many plant species. Bg7S was originally thought to be a major seed storage protein but was later found to be multifunctional, with stress response, antibacterial activity, hormone receptor-like activity. Moreover, functional differences between Bg7S proteins from legumes and other plants have been revealed. In non-leguminous plants, Bg7S molecules inhibit the invasion of pathogenic microorganisms. However, although leguminous plants have a peptide called leg-insulin that can bind to Bg7S, non-leguminous plants do not have leginsulin. Bg7S in leguminous plants and other plants may have evolved in functionally different directions. Several homologs of Bg7S in plants are reported, but there is no homolog of this protein in peas, suggesting that the pea evolution might have followed a different route when compared to other leguminous plants. Although the functions of Bg7S are well documented in plants, recent studies suggest that this protein is also important in controlling blood glucose level, blood pressure and plasma cholesterol level, and cancer cell antiproliferative actions.

Effects on the Caco-2 Cells of a Hypoglycemic Protein from Lupin Seeds in a Solution and Adsorbed on Polystyrene Nanoparticles to Mimic a Complex Food Matrix

The search for bioactivities influencing the human wellbeing of food proteins and peptides is a topic of broad and current interest. γ-Conglutin (γC) is a lupin seed protein drawing remarkable pharmacological and/or nutraceutical interest, as it is able to reduce hyperglycemia in humans and animal models. The present work deepens our investigations to understand the molecular basis of the in vitro effects of γC by testing the possible metabolic effects on cultivated Caco-2 cells. γC and its derived peptides (obtained via simulated gastrointestinal digestion) did not influence the cell viability at incubation times up to 24 h. The incubation of cells with native or digested γC caused no detectable inflammation processes mediated by Nuclear Factor kappa B (NFκB). We checked if treatment with γC or its derived peptides can elicit the expression of two peptide transporters (Pept-1 and Htp-1) by using an RT-qPCR approach. Native γC caused the halving of Pept-1 expression compared to untreated cells, but this effect disappeared when γC was digested. Either native γC or γC peptides reduced the expression levels of Hpt-1. Finally, this work also sheds light on the possible structural modifications of γC that may occur in the gastrointestinal tract, using an in vitro simulated dispersed system with polystyrene nanoparticles (NPs).

Interaction of γ-conglutin from Lupinus albus with model phospholipid membranes: Investigations on structure, thermal stability and oligomerization status

Interaction with model phospholipid membranes of lupin seed γ-conglutin, a glycaemia-lowering protein from Lupinus albus seeds, has been studied by means of Fourier-Transform infrared spectroscopy at p²H 7.0 and at p²H 4.5. The protein maintains the same secondary structure both at p²H 7.0 and at p²H 4.5, but at p²H 7.0 a higher ¹H/²H exchange was observed, indicating a greater solvent accessibility. The difference in T_m and T_D1/2 of the protein at the abovementioned p²H's has been calculated around 20 °C. Infrared measurements have been then performed in the presence of DMPG and DOPA at p²H 4.5. DMPG showed a little destabilizing effect while DOPA exerted a great stabilizing effect, increasing the T_m of γ-conglutin at p²H 4.5 of more than 20 °C. Since γ-conglutin at p²H 4.5 is in the monomeric form, the interaction with DOPA likely promotes the oligomerization even at p²H 4.5. Interaction between DMPG or DOPA and γ-conglutin has been confirmed by turbidity experiments with DMPC:DMPG or DOPC:DOPA SUVs. Turbidity data also showed high-affinity binding of γ-conglutin to anionic SUVs made up with DOPA. The molecular features outlined in this study are relevant to address the applicative exploitation and to delineate a deeper comprehension of the natural functional role of γ-conglutin.

Advances in the Biology of Seed and Vegetative Storage Proteins Based on Two-Dimensional Electrophoresis Coupled to Mass Spectrometry

Seed storage proteins play a fundamental role in plant reproduction and human nutrition. They accumulate during seed development as reserve material for germination and seedling growth and are a major source of dietary protein for human consumption. Storage proteins encompass multiple isoforms encoded by multi-gene families that undergo abundant glycosylations and phosphorylations. Two-dimensional electrophoresis (2-DE) is a proteomic tool especially suitable for the characterization of storage proteins because of their peculiar characteristics. In particular, storage proteins are soluble multimeric proteins highly represented in the seed proteome that contain polypeptides of molecular mass between 10 and 130 kDa. In addition, high-resolution profiles can be achieved by applying targeted 2-DE protocols. 2-DE coupled with mass spectrometry (MS) has traditionally been the methodology of choice in numerous studies on the biology of storage proteins in a wide diversity of plants. 2-DE-based reference maps have decisively contributed to the current state of our knowledge about storage proteins in multiple key aspects, including identification of isoforms and quantification of their relative abundance, identification of phosphorylated isoforms and assessment of their phosphorylation status, and dynamic changes of isoforms during seed development and germination both qualitatively and quantitatively. These advances have translated into relevant information about meaningful traits in seed breeding such as protein quality, longevity, gluten and allergen content, stress response and antifungal, antibacterial, and insect susceptibility. This review addresses progress on the biology of storage proteins and application areas in seed breeding using 2-DE-based maps.

Reverse phase HPLC method for detection and quantification of lupin seed γ-conglutin

A simple, selective and accurate reverse phase HPLC method was developed for detection and quantitation of γ-conglutin from lupin seed extract. A linear gradient of water and acetonitrile containing trifluoroacetic acid (TFA) on a reverse phase column (Agilent Zorbax 300SB C-18), with a flow rate of 0.8ml/min was able to produce a sharp and symmetric peak of γ-conglutin with a retention time at 29.16min. The identity of γ-conglutin in the peak was confirmed by mass spectrometry (MS/MS identification) and sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) analysis. The data obtained from MS/MS analysis was matched against the specified database to obtain the exact match for the protein of interest. The proposed method was validated in terms of specificity, linearity, sensitivity, precision, recovery and accuracy. The analytical parameters revealed that the validated method was capable of selectively performing a good chromatographic separation of γ-conglutin from the lupin seed extract with no interference of the matrix. The detection and quantitation limit of γ-conglutin were found to be 2.68μg/ml and 8.12μg/ml respectively. The accuracy (precision and recovery) analysis of the method was conducted under repeatable conditions on different days. Intra-day and inter-day precision values less than 0.5% and recovery greater than 97% indicated high precision and accuracy of the method for analysis of γ-conglutin. The method validation findings were reproducible and can be successfully applied for routine analysis of γ-conglutin from lupin seed extract.

Emulsifying and Foaming Properties of Different Protein Fractions Obtained from a Novel Lupin Variety AluProt-CGNA(®) (Lupinus luteus)

The use of vegetable proteins as food ingredient is becoming increasingly important due to their high versatility and environmental acceptability. This work describes a chemical characterization and techno-functional properties (emulsifying and foaming properties) of 3 protein fractions obtained from a protein-rich novel lupin variety, AluProt-CGNA(®) . This nongenetically modified variety have a great protein content in dehulled seeds (60.6 g protein/100 g, dry matter), which is higher than soybean and other lupin varieties. A simple procedure was utilized to obtain 3 different fractions by using alkali solubilization and isoelectric precipitation. Fractions 1 and 3 were mainly composed of protein and polysaccharides (NNE), whereas fraction 2 was mainly composed by protein (97%, w/w). Fraction 3 presented interesting and potential foaming properties in comparison to the other fractions evaluated in the study. Besides, its solubility, foaming and emulsifying capacity were practically not affected by pH variations. The 3 fractions also presented good emulsion stability, reaching values above a 95%. SDS-PAGE showed that fractions 1 and 2 contained mainly conglutin α, β, and δ, but in different ratios, whereas fraction 3 contained mainly conglutin γ and albumins. The results of this work will provide better understanding for the utilization of each protein fractions as potential ingredients in food industry.

Proteomics for exploiting diversity of lupin seed storage proteins and their use as nutraceuticals for health and welfare

Lupins have a variety of both traditional and modern uses. In the last decade, reports assessing the benefits of lupin seed proteins have proliferated and, nowadays, the pharmaceutical industry is interested in lupin proteins for human health. Modern genomics and proteomics have hugely contributed to describing the diversity of lupin storage genes and, above all, proteins. Most of these studies have been centered on few edible lupin species. However, Lupinus genus comprises hundreds of species spread throughout the Old and New Worlds, and these resources have been scarcely explored and exploited. We present here a detailed review of the literature on the potential of lupin seed proteins as nutraceuticals, and the use of -omic tools to analyze seed storage polypeptides in main edible lupins and their diversity at the Lupinus inter- and intra-species level. In this sense, proteomics, more than any other, has been a key approach. Proteomics has shown that lupin seed protein diversity, where post-translational modifications yield a large number of peptide variants with a potential concern in bioactivity, goes far beyond gene diversity. The future extended use of second and third generation proteomics should definitely help to go deeper into coverage and characterization of lupin seed proteome.

BIOLOGICAL SIGNIFICANCE

Some important topics concerning storage proteins from lupin seeds are presented and analyzed in an integrated way in this review. Proteomic approaches have been essential in characterizing lupin seed protein diversity, which goes far beyond gene diversity since the protein level adds to the latter differential proteolytic cleavage of conglutin pro-proteins and a diverse array of glycosylation forms and sites. Proteomics has also proved helpful for screening and studying Lupinus germplasm with the future aim of exploiting and improving food production, quality, and nutritional values.

In-depth glycoproteomic characterization of γ-conglutin by high-resolution accurate mass spectrometry

The molecular characterization of bioactive food components is necessary for understanding the mechanisms of their beneficial or detrimental effects on human health. This study focused on γ-conglutin, a well-known lupin seed N-glycoprotein with health-promoting properties and controversial allergenic potential. Given the importance of N-glycosylation for the functional and structural characteristics of proteins, we studied the purified protein by a mass spectrometry-based glycoproteomic approach able to identify the structure, micro-heterogeneity and attachment site of the bound N-glycan(s), and to provide extensive coverage of the protein sequence. The peptide/N-glycopeptide mixtures generated by enzymatic digestion (with or without N-deglycosylation) were analyzed by high-resolution accurate mass liquid chromatography-multi-stage mass spectrometry. The four main micro-heterogeneous variants of the single N-glycan bound to γ-conglutin were identified as Man2(Xyl) (Fuc) GlcNAc2, Man3(Xyl) (Fuc) GlcNAc2, GlcNAcMan3(Xyl) (Fuc) GlcNAc2 and GlcNAc 2Man3(Xyl) (Fuc) GlcNAc2. These carry both core β1,2-xylose and core α1-3-fucose (well known Cross-Reactive Carbohydrate Determinants), but corresponding fucose-free variants were also identified as minor components. The N-glycan was proven to reside on Asn131, one of the two potential N-glycosylation sites. The extensive coverage of the γ-conglutin amino acid sequence suggested three alternative N-termini of the small subunit, that were later confirmed by direct-infusion Orbitrap mass spectrometry analysis of the intact subunit.

Spectroscopic studies on the pH-dependent structural dynamics of γ-conglutin, the blood glucose-lowering protein of lupin seeds

γ-Conglutin is a blood glucose-lowering protein purified from lupin (Lupinus albus, L.) seed. Despite various features of this protein have already been studied, no function in the seed nor any mechanism of action as a hypoglycemic nutraceutical compound have been identified so far. The lupin protein was shown to exist both in monomeric and multimeric forms as a function of pH. However, a detailed description of the pH-dependent structural dynamics of this protein, as the basis to investigate the reason/s of its functional behaviour, is not available yet. In this study, multiple and independent spectroscopic approaches, including light scattering associated to size exclusion chromatography of both untreated and covalently cross-linked protein, near and far UV circular dichroism, intrinsic and extrinsic fluorescence measurements, have been used to monitor oligomeric and conformational modifications caused by pH changes. Altogether, the results revealed a tetramer-dimer-monomer transition between neutral to slightly acidic pH and a dramatic and abrupt conformational change below pH 3.5. According to these findings, a model depicting γ-conglutin structural dynamics was drawn. This model highlights the primary role of amino acid side group electrostatic interactions in the oligomer association/dissociation equilibria and in the pH-driven collapse of the native conformation.

Roles of glycosylation on the antifungal activity and apoplast accumulation of StAPs (Solanum tuberosum aspartic proteases)

Specific roles of glycosylation appear to be protein-dependent. Plant aspartic proteases (APs) contain two or more consensus N-glycosylation sites; however, the importance of them is not well understood. StAPs (Solanum tuberosum aspartic proteases) are bifunctional proteins with both proteolytic and antimicrobial activities. These proteins are accumulated into the intercellular washing fluid of potato tubers and leaves after wounding or infection. In this paper we investigated the importance of glycosylation on the StAPs apoplast accumulation, biochemical parameters, and fungicidal activity. Assays to evaluate the importance of StAPs glycosylation groups by using glycosylation inhibitors demonstrate that carbohydrate portions are essential to StAPs accumulation into the apoplast of tubers and leaves after wounding or detachment, respectively. Bifunctional activity of StAPs is differentially affected by this post-translational modification. Results obtained show that not significant changes were produced in the physicochemical properties after StAPs deglycosylation (pH and thermal-optimum activity and index of protein surface hydrophobicity). Otherwise, StAPs antifungal activity is affected by deglycosylation. Deglycosylated StAPs (dgStAPs) fungicidal activity is lower than native StAPs at all concentrations and times assayed. In summary, glycosylation has not a significant role on the StAPs conformational structure. However, it is involved in the StAPs subcellular accumulation and antifungal activity suggesting that it could be necessary for StAPs membrane and/or protein interactions and subsequently its biological function(s).

Metal ions restore the proteolytic resistance of denatured conglutin gamma, a lupin seed glycoprotein, by promoting its refolding

The susceptibility to trypsin of conglutin gamma, a lupin seed glycoprotein affected by this enzyme only when in a non-native conformation, was used to study the effect of Zn(2+) and other metal ions on the structural dynamics of the protein. When acid-treated trypsin-susceptible conglutin gamma was incubated at neutral pH in the presence of Zn(2+), it became resistant to tryptic attack, contrary to the protein treated in the absence of Zn(2+). The time course of this refolding event has been quantitatively evaluated by SDS-PAGE. Amino acid sequencing of the major polypeptide fragments, produced by trypsin before completion of the refolding process, indicated that only a few cleavable bonds were accessible to the enzyme. This suggested that the presence of metal ions affected the pathway of degradation of the protein, by inducing its folding. Among the other metal ions tested, Ni(2+) also promoted the adoption of a trypsin-resistant conformation of conglutin gamma, whereas Mn(2+) and Ca(2+) had only much lower effects. The relevance of these findings for a deeper understanding of the in vivo degradation of plant food proteins and how it is affected by metal ions are discussed.

Cloning, sequencing and expression in the seeds and radicles of two Lupinus albus conglutin gamma genes

Two genes encoding conglutin gamma have been isolated from a Lupinus albus genomic library and sequenced. The expression of conglutin gamma was studied by partial amino acid sequencing of the mature seed protein and by nucleotide sequencing of reverse transcriptase-polymerase chain reaction products from various tissues during the plant life cycle.

Thermal stabilities of lupin seed conglutin gamma protomers and tetramers

Various experimental approaches have been used in this work to assess the thermal stabilities of lupin seed conglutin gamma at two pH values, 4.5 and 7.5, at which the protein exists as a protomer and a tetramer, respectively. The patterns of thermal unfolding at the two pH values differed significantly; the tetramer aggregated and became insoluble, whereas the protomer was still soluble after thermal treatment. Also, the midpoint transition temperatures were dramatically different, being 60.3 and 75.1 degrees C for the protomer and tetramer, respectively. The behavior of conglutin gamma at neutral pH was also affected by disulfide formation/interchange, in that some unfolded protein molecules became covalently stabilized. More detailed analyses by differential scanning calorimetry and indirect fluorescence measurements, using 8-anilino-1-naphthalenesulfonic acid as a probe, confirmed the remarkable differences observed in the thermal stabilities of the two protein forms and allowed models for their unfolding patterns to be drawn.

A molecular basis for glycosylation-induced conformational switching

BACKGROUND

Asparagine-linked glycosylation has the capacity to greatly influence the structure and function of glycoproteins. In most cases, however, it is unclear specifically how the carbohydrate moiety interacts with the protein to influence its conformation.

RESULTS

A series of glycosylation based on the critical A285 glycosylation site of the hemagglutinin glycoprotein from influenza from influenza virus was used as a model system to study the effects of asparagine-linked glycosylation. Derivatization of this peptide with a family of short carbohydrates reveals that subtle changes in the structure of the carbohydrate have a dramatic impact on peptide conformation. Modification of the hemagglutinin glycopeptide with a truncated version of the native carbohydrate induces a beta-turn structure similar to the structure found in the native protein. Replacement of the C2 and C2' N-acetyl groups of the carbohydrates with hydroxyl moieties results in a less well-ordered peptide conformation.

CONCLUSIONS

It is likely that the N-acetyl groups of the carbohydrates have a critical role in promoting the more compact beta-turn conformation through steric interactions with the peptide. This study has demonstrated that relatively small changes in carbohydrate composition can have dramatic ramifications on glycopeptide conformation.

Glycans of higher plant peroxidases: recent observations and future speculations

Plant peroxidases are composed of a peptide and associated heme, calcium and glycans. The 3D structure of the major cationic peanut peroxidase has revealed the sites of the heme and calcium. But the diffraction of the glycans was not sufficient to show their structure. This review presents research that has been executed to obtain putative glycans and their binding sites, and to gain an indirect insight into these glycans. It also offers approaches that will be used to determine the function of the glycans on the peanut peroxidase. Some comparisons are made with other plant glycoproteins including peroxidases from plants other than peanut.

Analysis of the conserved glycosylation site in the nicotinic acetylcholine receptor: potential roles in complex assembly

BACKGROUND

Assembly of the functional nicotinic acetylcholine receptor (nAChR) is dependent on a series of exquisitely coordinated events including polypeptide synthesis and processing, side-chain elaboration through post-translational modifications, and subunit oligomerization. A 17-residue sequence that includes a cystine disulfide and an N-linked glycosylation site is conserved in the extracellular domain of each of the nAChR subunits, and is involved in intersubunit interactions that are critical for assembly of intact, pentameric complexes. A polypeptide representing the relevant sequence from the alpha-subunit of the nAChR (Ac-Tyr-Cys-Glu-Ile-Ile-Val-Thr-His-Phe-Pro-Phe-Asp-Gln-Gln Asn-Cys-Thr-NH2) is small enough to allow detailed structural analysis, which may provide insight into the role of glycosylation in the maturation process that leads to ion-channel assembly. We therefore investigated the effect of N-linked glycosylation on the structure of this heptadecapeptide.

RESULTS

Thermodynamic analysis shows that glycosylation alters disulfide formation in the loop peptide, shifting the equilibrium in favor of the disulfide. Spectroscopic studies reveal that the cis/trans amide isomer ratio of the proline is also affected by the modification, with a resultant shift in the equilibrium in favor of the trans isomer, even though the proline is several residues removed from the glycosylation site. Two-dimensional NMR analysis of the glycopeptide does not indicate the presence of any specific interactions between the carbohydrate and the peptide.

CONCLUSIONS

These studies demonstrate that glycosylation can have a significant influence on disulfide formation and proline isomerization in a local peptide sequence. As both these processes are considered slow steps in protein folding, it is evident that N-linked glycosylation has important indirect roles that influence the folding of the receptor subunit and assembly of the pentameric complex.