A Novel Glutamyl (Aspartyl)-Specific Aminopeptidase A from Lactobacillus delbrueckii with Promising Properties for Application

Legume protein fermented by lactic acid bacteria: Specific enzymatic hydrolysis, protein composition, structure, and functional properties

In recent years, with increasing emphasis on healthy, green, and sustainable consumption concepts, plant-based foods have gained popularity among consumers. As widely sourced plant-based raw materials, legume proteins are considered sustainable and renewable alternatives to animal proteins. However, legume proteins have limited functional properties, which hinder their application in food products. LAB fermentation is a relatively natural processing method that is safer than chemical/physical modification methods and can enrich the functional properties of legume proteins through biodegradation and modification. Therefore, changes in legume protein composition, structure, and functional properties and their related mechanisms during LAB fermentation are described. In addition, the specific enzymatic hydrolysis mechanisms of different LAB proteolytic systems on legume proteins are also focused in this review. The unique proteolytic systems of different LAB induce specific enzymatic hydrolysis of legume proteins, resulting in the production of hydrolysates with diverse functional properties, including solubility, emulsibility, gelability, and foamability, which are determined by the composition (peptide/amino acid) and structure (secondary/tertiary) of legume proteins after LAB fermentation. The correlation between LAB-specific enzymatic hydrolysis, protein composition and structure, and protein functional properties will assist in selecting legume protein raw materials and LAB strains for legume plant-based food products and expand the application of legume proteins in the food industry.

Crystal structure of a newly identified M61 family aminopeptidase with broad substrate specificity that is solely responsible for recycling acidic amino acids

Aminopeptidases with varied substrate specificities are involved in different crucial physiological processes of cellular homeostasis. They also have wide applications in food and pharma industries. Within the bacterial cell, broad specificity aminopeptidases primarily participate in the recycling of amino acids by degrading oligopeptides generated via primary proteolysis mediated by cellular ATP-dependent proteases. However, in bacteria, a truly broad specificity enzyme, which can cleave off acidic, basic, Gly and hydrophobic amino acid residues, is extremely rare. Here, we report structure-function of a putative glycyl aminopeptidase (M61xc) from Xanthomonas campestris pv campestris (Xcc) belonging to the M61 peptidase family. The enzyme exhibits broad specificity and cleaves Ala, Leu, Asp, Glu, Met, Ser, Phe, Tyr, Gly, Arg, and Lys at the N terminus, optimally of peptides with a length of 3-7 amino acids. Further, we report the high-resolution crystal structure of M61xc in the apo form (2.1 Å) and bestatin-bound form (1.95 Å), detailing its catalytic and substrate preference mechanisms. Comparative analysis of enzyme activity in crude cell extracts from both wild-type and m61xc-knockout mutant strains of Xcc has elucidated the unique intracellular role of M61xc. This study suggests that M61xc is the exclusive enzyme in these bacteria that is responsible for liberating Asp/Glu residues from the N-termini of peptides. Also, in view of its broad specificity and peptide degradation ability, it could be considered equivalent to M1 or other oligomeric peptidases from families like M17, M18, M42 or S9, who have an important auxiliary role in post-proteasomal protein degradation in prokaryotes.

Microbial-derived salt-tolerant proteases and their applications in high-salt traditional soybean fermented foods: a review

Different microorganisms can produce different proteases, which can adapt to different industrial requirements such as pH, temperature, and pressure. Salt-tolerant proteases (STPs) from microorganisms exhibit higher salt tolerance, wider adaptability, and more efficient catalytic ability under extreme conditions compared to conventional proteases. These unique enzymes hold great promise for applications in various industries including food, medicine, environmental protection, agriculture, detergents, dyes, and others. Scientific studies on microbial-derived STPs have been widely reported, but there has been little systematic review of microbial-derived STPs and their application in high-salt conventional soybean fermentable foods. This review presents the STP-producing microbial species and their selection methods, and summarizes and analyzes the salt tolerance mechanisms of the microorganisms. It also outlines various techniques for the isolation and purification of STPs from microorganisms and discusses the salt tolerance mechanisms of STPs. Furthermore, this review demonstrates the contribution of modern biotechnology in the screening of novel microbial-derived STPs and their improvement in salt tolerance. It highlights the potential applications and commercial value of salt-tolerant microorganisms and STPs in high-salt traditional soy fermented foods. The review ends with concluding remarks on the challenges and future directions for microbial-derived STPs. This review provides valuable insights into the separation, purification, performance enhancement, and application of microbial-derived STPs in traditional fermented foods.

Flavor differences of soybean and defatted soybean fermented soy sauce and its correlation with the enzyme profiles of the kojis

BACKGROUND

Soybeans and defatted soybeans, commonly used as protein ingredients, have different flavors of their fermented soy sauce. Clarifying the differences between the two soy sauces, as well as the formation mechanism, is an important prerequisite for improving the flavor of defatted soybean soy sauce. To this goal, the aroma characteristics of two soy sauces and their volatile profiles were compared by sensory evaluation and gas chromatography-mass spectrometry, and eight enzyme activities and volatile profiles of matured koji were determined.

RESULTS

Sensory results showed that the acids, fruity and cooked potato-like attributes were higher in whole soybean fermented soy sauce, whereas defatted soybean soy sauce exhibited higher smoky and malty attributes, closely related to the contents of aroma-active compounds in soy sauce, such as isobutyl acetate, 2/3-methylbutanal, acetic acid and 2/3-methylbutanoic acid. The content of most volatiles in the matured kojis showed a consistent trend with that of soy sauce: alcohols, acids, furan(one)s and ketones. Interestingly, acid protease and cellulase activities were 3.3 and 1.6 times higher in the whole soybean koji than in defatted soybean koji, respectively, whereas neutral protease, aminopeptidase, glucoamylase and β-glucosidase were approximately 2.0 times higher in defatted soybean koji.

CONCLUSION

In summary, the flavor differences between soybean and defatted soybean fermented soy sauce were not only caused by the differences in the content of flavor precursors in the materials, but also closely related to the differences in the enzymatic profiles accumulated during the koji-making process. © 2022 Society of Chemical Industry.

Characterization of aspartyl aminopeptidase from Schistosoma japonicum

The tegument of schistosomes is the interface between the worm and the host environment. Some molecules distributed on the tegument participate in host-parasite interactions. Aspartyl aminopeptidase (AAP), identified on the tegument of Schistosoma japonicum (S. japonicum), facilitate protein turnover by acting in concert with other aminopeptidases. In this study, the gene encoding S. japonicum aspartyl aminopeptidase (SjAAP) was cloned, expressed and characterized. Quantitative real-time PCR analysis showed that SjAAP was expressed in all studied developmental stages. The transcript level was higher in 8, 14, 21, and 28 days old worms than the other detected stages. Moreover, the level of expression in 42-day-old male worms was significantly higher than that in females. The recombinant SjAAP (rSjAAP) was expressed as both supernatant and inclusion bodies in Escherichia coli BL21 cells. The enzymatic activity of rSjAAP was 4.45 U/mg. The Km and Vmax values for H-Asp-pNA hydrolysis were discovered to be 5.93 mM and 0.018 mM·min^-1. Immunofluorescence analysis revealed that SjAAP is primarily distributed on the tegument and parenchyma of schistosomes. Western blot showed that rSjAAP possessed good immunogenicity. Although specific antibodies were produced in BALB/c mice vaccinated with rSjAAP emulsified with ISA 206 adjuvant, no significant reduction of worm burden and number of eggs in the liver was observed. Therefore, rSjAAP may not be suitable to act as a potential vaccine candidate against schistosomiasis japonica in mice. However, this study provides some foundation for further exploration of the biological function of this molecule.

Peptidomics as a useful tool in the follow-up of food bioactive peptides

There is an intense research activity on bioactive peptides derived from food proteins in view of their health benefits for consumers. However, their identification is quite challenging as a consequence of their small size and low abundance in complex matrices such as foods or hydrolyzates. Recent advances in peptidomics and bioinformatics are getting improved sensitivity and accuracy and therefore such tools are contributing to the development of sophisticated methodologies for the identification and quantification of peptides. These developments are very useful for the follow-up of peptides released through proteolysis either in the food itself through the action of endogenous peptidases during processing stages like fermentation, drying or ripening, or from food proteins hydrolyzed by commercial peptidases or microorganisms with proteolytic activity. This chapter is presenting the latest advances in peptidomics and its use for the identification and quantification of peptides, and as a useful tool for controlling the proteolysis phenomena in foods and protein hydrolyzates.

Recent Progress in Enzymatic Release of Peptides in Foods of Animal Origin and Assessment of Bioactivity

There is a wide variety of peptides released from food proteins that are able to exert a relevant benefit for human health, such as angiotensin-converting enzyme inhibition, antioxidant, anti-inflammatory, hypoglucemic, or antithrombotic activity, among others. This manuscript is reviewing the recent advances on enzymatic mechanisms for the hydrolysis of proteins from foods of animal origin, including the types of enzymes and mechanisms of action involved, the strategies followed for the isolation and identification of bioactive peptides through advanced proteomic tools, and the assessment of bioactivity and its beneficial effects. Specific applications in fermented and/or ripened foods where a significant number of bioactive peptides have been reported with relevant in vivo physiological effects on laboratory rats and humans as well as the hydrolysis of animal food proteins for the production of bioactive peptides are also reviewed.

Enterococcus faecium NCIMB10415 responds to norepinephrine by altering protein profiles and phenotypic characters

The long-term established symbiosis between gut microbiota and humans is based upon a dynamic equilibrium that, if unbalanced, could lead to the development of diseases. Despite the huge amount of data concerning the microbiota-gut-brain-axis, little information is available on what happens at the molecular level in bacteria, when exposed to human signals. In the present study, the physiological effects exerted by norepinephrine (NE), a human hormone present in significant amounts in the host gut, were analyzed using the commensal/probiotic strain Enterococcus faecium NCIMB10415 as a target. The aim was to compare the protein profiles of treated and untreated bacteria and relating these proteome patterns to some phenotypic modifications important for bacteria-host interaction. Actually, to date, only pathogens have been considered. Combining a gel-free/label-free proteomic analysis with the evaluation of bile salts resistance, biofilm formation and autoaggregation ability (as well as with the bacterial growth kinetics), allowed to detect changes induced by NE treatment on all the tested probiotic properties. Furthermore, exposure to the bioactive molecule increased the abundance of proteins related to stress response and to host-microbe interaction, such as moonlight proteins involved in adhesion and immune stimulation. The results of this investigation demonstrated that, not only pathogens, but also commensal gut bacteria are affected by host-derived hormones, underlining the importance of a correct cross-signalling in the maintenance of gut homeostasis. SIGNIFICANCE: The crucial role played by the human gut microbiota in ensuring host homeostasis and health is definitively ascertained as suggested by the holobiome concept. The present research was intended to shed light on the endocrinological perturbations possibly affecting microbiota. The microbial model used in this study belongs to Enterococcus faecium species, whose controversial role as gut commensal and opportunistic pathogen in the gut ecosystem is well recognized. The results obtained in the present investigation clearly demonstrate that E. faecium NCIMB10415 can sense and respond to norepinephrine, a human hormone abundant at the gut level, by changing protein profiles and physiology, inducing changes that could favor survival and colonization of the host tissues. To our knowledge, this is the first proteomic report concerning the impact of a human hormone on a commensal/probiotic bacterium, since previous research has focused on exploring the effects of neuroendocrine molecules on growth and virulence of pathogenic species.

Therapeutic and biotechnological applications of substrate specific microbial aminopeptidases

Aminopeptidases (EC 3.4.11.) belongs to exoprotease family, which can catalyze the cleavage of peptide bond which connects the N-terminal amino acid to the penultimate residue in a protein. Aminopeptidases catalyze the process of removal of the N-terminal amino acids of target substrates by sequential cleavage of one amino acid residue at a time. Microbial aminopeptidase are of great acceptance as industrial enzymes with varying applications in food and pharma industry since these enzymes possess unique characteristics than aminopeptidases from other sources. This review describes the various applications of microbial aminopeptidases in different industrial sectors. These enzymes are widely used in food industry as a debittering agent as well as in the preparation of protein hydrolysates. In baking, brewing, and cheese making aminopeptidases are extensively used for removing the bitterness of peptides. The inhibitors of these enzymes are found great clinical applications against various diseases such as cancer, diabetes, and viral infections. Aminopeptidases are widely used for the synthesis of biopeptides and amino acids, and found to be efficient than chemical synthesis. These enzymes are capable of hydrolyzing organophosphate compounds, thus having biological as well as environmental significance.

Key Points • Cleaves the amino-terminal amino acid residues from proteins and peptides. • Microbial aminopeptidase are of great acceptance as both therapeutic and industrial enzyme. • Review describes the potential applications of microbial aminopeptidases.

Separation, biochemical characterization and salt-tolerant mechanisms of alkaline protease from Aspergillus oryzae

BACKGROUND

The salt tolerance of proteases secreted by Aspergillus oryzae 3.042 closely relates to the utilization of raw materials and the quality of soy sauce. However, little is known about the salt-tolerant proteases and their salt-tolerant mechanisms.

RESULTS

In this study, we isolated and identified a salt-tolerant alkaline protease (AP, approximately 29 kDa) produced by A. oryzae 3.042. It was considered as a metal-ion-independent serine protease. The optimum and stable pH values were both pH 9.0 and the optimum temperature was 40 °C. Over 20% relative activity of AP remained in the presence of 3.0 mol L^-1 NaCl after 7 days, but its K_m and V_max were only mildly influenced by the presence of 3.0 mol L^-1 NaCl, indicating its outstanding salt tolerance. Furthermore, AP was more stable than non-salt-tolerant protease at high salinity. The salt-tolerant mechanisms of AP could be due to more salt bridges, higher proportion of ordered secondary structures and stronger hydrophobic amino acid residues in the interior.

CONCLUSIONS

The above results are vital for maintaining, activating and/or modulating the activity of AP in high-salt environments. They would also provide theoretical guidance for the modification of AP and the engineering of A. oryzae 3.042 so as to secrete more AP. © 2018 Society of Chemical Industry.

Improving the colloidal and sensory properties of a caseinate hydrolysate using particular exopeptidases

Enzymatic hydrolysis with endopeptidases can be used to modify the colloidal properties of food proteins. In this study, sodium caseinate was hydrolyzed with Sternzym BP 25201, containing a thermolysin-like endopeptidase from Geobacillus stearothermophilus as the only peptidase, to a DH of 2.3 ± 1%. The hydrolysate (pre-hydrolysate) obtained was increased in its foam (+35%) and emulsion stability (+200%) compared to untreated sodium caseinate but showed a bitter taste. This hydrolysate was further treated with the exopeptidases PepN, PepX or PepA, acting on the N-terminus of peptides. Depending on the specificity of the exopeptidase used, changes regarding the hydrolysate properties (hydrophobicity, size), colloidal behavior (emulsions, foams) and taste were observed. No changes regarding the bitterness but further improvements regarding the colloidal stability (foam: +69%, emulsion: +29%) were determined after the application of PepA, which is specific for the hydrophilic amino acids Asp, Glu and Ser. By contrast, treatment with the general aminopeptidase PepN resulted in a non-bitter product, with no significant changes regarding the colloidal properties compared to the pre-hydrolysate (p < 0.05). Similar results to those for PepN (reduced bitterness compared to the pre-hydrolysate, enhanced colloidal stability compared to sodium caseinate) were also obtained using commercial Flavourzyme, which was reduced in its endopeptidase activity (exo-flavourzyme). In conclusion, the modifications obtained with the applied exopeptidases offer a potent tool for researchers and the industry to produce non-bitter protein hydrolysates with increased colloidal properties.

Wheat Gluten Amino Acid Analysis by High-Performance Anion-Exchange Chromatography with Integrated Pulsed Amperometric Detection

The present chapter describes an accurate and user-friendly method for determining amino acid composition of wheat gluten proteins and their gliadin and glutenin fractions. The method consists of hydrolysis of the peptide bonds in 6.0 M hydrochloric acid (HCl) solution at 110 °C for 24 h, followed by evaporation of the acid and separation of the free amino acids by high-performance anion-exchange chromatography with integrated pulsed amperometric detection (HPAEC-IPAD). In contrast to conventional methods, the analysis requires neither pre- or post-column derivatization nor a time-consuming oxidation or derivatization step prior to hydrolysis. Correction factors account for incomplete release of Val and Ile even after hydrolysis for 24 h and for losses of Ser during evaporation. Gradient conditions including an extra eluent allow multiple sequential sample analyses without risk of Glu accumulation on the anion-exchange column which otherwise would result from high Gln levels in gluten proteins.

Generation of bioactive peptides during food processing

Large amounts of peptides are naturally generated in foods through the proteolysis phenomena taking place during processing. Such proteolysis is carried out either by endogenous enzymes in ripened foods or by the combined action of endogenous and microbial enzymes when fermented. Food proteins can also be isolated and hydrolysed by peptidases to produce hydrolysates. endo-peptidases act first followed by the successive action of exo-peptidases (mainly, tri- and di-peptidylpeptidases, aminopeptidases and carboxypeptidases). The generated peptides may be further hydrolysed through the gastrointestinal digestion resulting in a pool of peptides with different sequences and lengths, some of them with relevant bioactivity. However, these peptides should be absorbed intact through the intestinal barrier and reach the blood stream to exert their physiological action. This manuscript is reporting the enzymatic routes and strategies followed for the generation of bioactive peptides.