Cloning and expression of a novel prolyl endopeptidase from Aspergillus oryzae and its application in beer stabilization

One-step high efficiency separation of prolyl endopeptidase from Aspergillus niger and its application

Prolyl endopeptidase from Aspergillus niger (An-PEP) is an enzyme that recognizes C-terminal peptide bonds of amino acid chains and cleaves them by hydrolysis. An aqueous two-phase system (ATPS) was used to separate An-PEP from fermentation broth. Through single factor experiments, the ATPS containing 16 % (w/w) PEG2000 and 15 % (w/w) (NH4)2SO4 at pH 6.0 obtained the recovery of 79.74 ± 0.16 % and the purification coefficient of 7.64 ± 0.08. It was then used to produce soy protein isolate peptide (SPIP) by hydrolysis of soy protein isolate (SPI), and SPIP-Ferrous chelate (SPIP-Fe) was prepared with SPIP and Fe2+. The chelation conditions were optimized by RSM, as the chelation time was 30 min, chelation temperature was 25 °C, SPIP mass to VC mass was two to one and pH was 6.0. The obtained chelation rate was 82.56 ± 2.30 %. The change in the structures and functional features of SPIP before and after chelation were investigated. The FTIR and UV-Vis results indicated that the chelation of Fe2+ and SPIP depended mainly on the formation of amide bonds. The fluorescence, SEM and amino acid composition analysis results indicated that Fe2+ could induce and stabilize the surface conformation and change the amino acid distribution on the surfaces of SPIP. The chelation of SPIP and Fe2+ resulted in the enhancement of radical scavenging activities and ACE inhibitory activities. This work provided a new perspective for the further development of peptide-Fe chelates for iron supplement.

Application of Strain Selection Technology in Alcoholic Beverages: A Review

The diversity of alcohol beverage microorganisms is of great significance for improving the brewing process and the quality of alcohol beverage products. During the process of making alcoholic beverages, a group of microorganisms, represented by yeast and lactic acid bacteria, conducts fermentation. These microorganisms have complex synergistic or competitive relationships, and the participation of different microorganisms has a major impact on the fermentation process and the flavor and aroma of the product. Strain selection is one of the key steps. Utilizing scientific breeding technology, the relationship between strains can be managed, the composition of the alcoholic beverage microbial community can be improved, and the quality and flavor of the alcoholic beverage products can be increased. Currently, research on the microbial diversity of alcohol beverages has received extensive attention. However, the selection technology for dominant bacteria in alcohol beverages has not yet been systematically summarized. To breed better-quality alcohol beverage strains and improve the quality and characteristics of wine, this paper introduces the microbial diversity characteristics of the world's three major brewing alcohols: beer, wine, and yellow wine, as well as the breeding technologies of related strains. The application of culture selection technology in the study of microbial diversity of brewed wine was reviewed and analyzed. The strain selection technology and alcohol beverage process should be combined to explore the potential application of a diverse array of alcohol beverage strains, thereby boosting the quality and flavor of the alcohol beverage and driving the sustainable development of the alcoholic beverage industry.

Characterization, Semirational Design for pH Robustness, and the Application in Bioactive Peptide Production of a X-Prolyl Dipeptidyl Aminopeptidase from Lactococcus lactis MY-3

PepXLcMY-3, an X-prolyl dipeptidyl aminopeptidase derived from Lactobacillus lactis MY-3, was screened and recombinantly expressed in Escherichia coli. The enzyme could exhibit about 40% activity within the pH range of 6.0-10. To further improve the pH robustness, site E396 located in the active pocket was discovered through alanine scanning. The mutant E396I displayed both developed activity and kcat/Km. The optimal pH of E396I shifted from 6.0 to 10 compared to WT, with the relative activity within the pH range of 6.0-10 significantly increased. The site K648 was then proposed by semirational design. The activity of mutant E396I/K648D reached 4.03 U/mg. The optimal pH was restored to 6.0, and the pH stability was further improved. E396I/K648D could totally hydrolyze β-casomorphin 7 within 30 min. The hydrolysate showed 64.5% inhibition on angiotensin I converting enzyme, which was more efficient than those produced by E396I and WT, 23.2 and 44.7%, respectively.

Microbial proteases and their applications

Proteases (proteinases or peptidases) are a class of hydrolases that cleave peptide chains in proteins. Endopeptidases are a type of protease that hydrolyze the internal peptide bonds of proteins, forming shorter peptides; exopeptidases hydrolyze the terminal peptide bonds from the C-terminal or N-terminal, forming free amino acids. Microbial proteases are a popular instrument in many industrial applications. In this review, the classification, detection, identification, and sources of microbial proteases are systematically introduced, as well as their applications in food, detergents, waste treatment, and biotechnology processes in the industry fields. In addition, recent studies on techniques used to express heterologous microbial proteases are summarized to describe the process of studying proteases. Finally, future developmental trends for microbial proteases are discussed.

Degradation Mechanism of Soybean Protein B3 Subunit Catalyzed by Prolyl Endopeptidase from Aspergillus niger during Soy Sauce Fermentation

Soy sauce secondary precipitate formed due to the B₃ subunit seriously affects soy sauce's appearance quality. In this study, a prolyl endopeptidase (APE) from Aspergillus niger, which could degrade approximately 50% of the B₃ subunit and increase proline content by 24% in soy sauce, was isolated and identified. The results showed that APE was an acidic salt-tolerant serine protease (62 kDa), which was optimally active at 40 °C and pH 4.0, and retained more than 69% activity in 3 M NaCl solution over 10 days. As a potential substrate of APE, the B₃ subunit contains 10 proline residues. High salinity could not damage the hydrogen bonds, salt bridges, and interior hydrophobicity of APE; thus, the spatial structures and activity of APE in 3 M NaCl solution were stable within 3 days and decreased thereafter. High salinity made the B₃ subunit more rigid and lowered the catalytic activity of APE on the B₃ subunit, hindering complete hydrolysis of the B₃ subunit. This was the first report about the APE capable of degrading the B₃ subunit and reducing the secondary precipitate of soy sauce, providing a new possibility to solve the secondary precipitate of soy sauce.

Post-Proline Cleaving Enzymes (PPCEs): Classification, Structure, Molecular Properties, and Applications

Proteases or peptidases are hydrolases that catalyze the breakdown of polypeptide chains into smaller peptide subunits. Proteases exist in all life forms, including archaea, bacteria, protozoa, insects, animals, and plants due to their vital functions in cellular processing and regulation. There are several classes of proteases in the MEROPS database based on their catalytic mechanisms. This review focuses on post-proline cleaving enzymes (PPCEs) from different peptidase families, as well as prolyl endoprotease/oligopeptidase (PEP/POP) from the serine peptidase family. To date, most PPCEs studied are of microbial and animal origins. Recently, there have been reports of plant PPCEs. The most common PEP/POP are members of the S9 family that comprise two conserved domains. The substrate-limiting β-propeller domain prevents unwanted digestion, while the α/β hydrolase catalyzes the reaction at the carboxyl-terminal of proline residues. PPCEs display preferences towards the Pro-X bonds for hydrolysis. This level of selectivity is substantial and has benefited the brewing industry, therapeutics for celiac disease by targeting proline-rich substrates, drug targets for human diseases, and proteomics analysis. Protein engineering via mutagenesis has been performed to improve heat resistance, pepsin-resistant capability, specificity, and protein turnover of PPCEs for pharmacological applications. This review aims to synthesize recent structure-function studies of PPCEs from different families of peptidases to provide insights into the molecular mechanism of prolyl cleaving activity. Despite the non-exhaustive list of PPCEs, this is the first comprehensive review to cover the biochemical properties, biological functions, and biotechnological applications of PPCEs from the diverse taxa.

Effective separation of prolyl endopeptidase from Aspergillus Niger by aqueous two phase system and its characterization and application

Aspergillus niger prolyl endopeptidase (An-PEP) has become a research focus because of its advantages in specifically cleaving the C-terminal peptide bond of proline residues, especially it was an industrial food-grade acidic PEP. Aqueous two-phase system (ATPS) was first applied for separating An-PEP from fermentation broth. Via response surface method (RSM) experiment, an effectively separation of An-PEP was achieved by ATPS containing27% (w/w) ethanol and 14.5% (w/w) (NH₄)₂SO₄ at pH 6.0 with the recovery of 90.29 ± 0.23% and purification coefficient of 15.35 ± 0.30. The purified An-PEP was characterized by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), fourier transform infrared (FTIR) and fluorescence spectrometry. The optimum temperature and pH of An-PEP were 40 °C and 4.5-5.0, respectively. An-PEP was activated and stabilized by Ca²⁺ but inhibited by Fe³⁺. The enzymatic application of purified An-PEP was evaluated by hydrolyzing egg white protein (EWP) to prepare bioactive peptides. The obtained hydrolysates had good scavenging ability of OH and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) free radicals, angiotensin converting enzyme (ACE) inhibitory activity and anti-gout activity. This research realized a low-cost, high-efficiency and simple separation technology of An-PEP and provided a broader idea for the preparation of bioactive peptides and the application of An-PEP.

Gluten-Free Brewing: Issues and Perspectives

Celiac disease (CD) is an immune-mediated gluten-sensitive enteropathy. Currently, it affects around 1% of world population, but it is constantly growing. Celiac patients have to follow a strict gluten-free (GF) diet. Beer is one of the most consumed beverages worldwide, but it is not safe for people with CD. It has a gluten content usually above the safe threshold (20 ppm), determined by the official method for hydrolyzed foods (R5-competitive-ELISA). The demand on the market for GF beers is increasingly growing. This review aims to provide a comprehensive overview of different strategies to produce GF beer, highlighting strengths and weaknesses of each approach and taking into account technological and sensory issues. GF cereals or pseudocereals have poor brewing attitudes (if used as main raw material) and give the beer unusual flavour. Instead, enzymatic treatments allow traditional brewing process followed by gluten content reduction. A survey on 185 GF-producing breweries (both industrial and craft) from all over the world have been considered to assess which approach is most used. Beers brewed with GF cereals and pseudocereals (used in well-balanced proportions) are more common than gluten-removed (GR) beers, obtained by enzymatic treatment.

An Energy Optimization Strategy Based on the Perfect Conformation of Prolyl Endopeptidase for Improving Catalytic Efficiency

Prolyl endopeptidases (PEPs) hydrolyze proteins to yield bioactive peptides and are effective in the treatment of celiac disease. However, the catalytic efficiency of PEPs still has the potential to be improved, which could further strengthen their industrial and therapeutic applications. Herein, a novel rational design strategy based on a "near-attack conformation" of the catalytic state of PEP was adopted. Constrained dynamic simulations were applied, followed by the virtual screening of potentially favorable mutants according to their binding free energy. We redesigned Sphaerobacter thermophiles PEP with high-temperature activity/stability, a wide range of pH stabilities, and high proline specificity. As a result, the k_cat value of two PEP mutants (I462W and Q560Y) increased by 208.2 and 150.1%, respectively, and the k_cat/K_M increased by 32.7 and 6.3%, respectively. These data revealed that the PEP mutants had improved catalytic efficiency and that our strategy can be applied for enzyme engineering.

Effects of UPR and ERAD pathway on the prolyl endopeptidase production in Pichia pastoris by controlling of nitrogen source

Prolyl endopeptidase (PEP) is very useful in various industries, while the high cost of enzyme production remains a major obstacle for its industrial applications. Pichia pastoris has been used for the PEP production; however, the fermentation process has not be investigated and little is known about the impact of excessive PEP production on the host cell physiology. Here, we optimized the nitrogen source to improve the PEP expression level and further evaluated the cellular response including UPR and ERAD. During methanol induction phase the PEP activity (1583 U/L) was increased by 1.48-fold under the optimized nitrogen concentration of NH4+ (300 mmol/L) and casamino acids [1.0% (w/v)] in a 3-L bioreactor. Evaluated by RT-PCR the UPR and ERAD pathways were confirmed to be activated. Furthermore, a strong decrease of ERAD-related gene transcription was observed with the addition of nitrogen source, which contributed to a higher PEP expression level.

Diversity, Application, and Synthetic Biology of Industrially Important Aspergillus Fungi

The filamentous fungal genus Aspergillus consists of over 340 officially recognized species. A handful of these Aspergillus fungi are predominantly used for food fermentation and large-scale production of enzymes, organic acids, and bioactive compounds. These industrially important Aspergilli primarily belong to the two major Aspergillus sections, Nigri and Flavi. Aspergillus oryzae (section Flavi) is the most commonly used mold for the fermentation of soybeans, rice, grains, and potatoes. Aspergillus niger (section Nigri) is used in the industrial production of various enzymes and organic acids, including 99% (1.4 million tons per year) of citric acid produced worldwide. Better understanding of the genomes and the signaling mechanisms of key Aspergillus species can help identify novel approaches to enhance these commercially significant strains. This review summarizes the diversity, current applications, key products, and synthetic biology of Aspergillus fungi commonly used in industry.

AoS28D, a proline-Xaa carboxypeptidase secreted by Aspergillus oryzae

Prolyl peptidases of the MEROPS S28 family are of particular interest because they are key enzymes in the digestion of proline-rich peptides. A BLAST analysis of the Aspergillus oryzae genome revealed sequences coding for four proteases of the S28 family. Three of these proteases, AoS28A, AoS28B, and AoS28C, were previously characterized as acidic prolyl endopeptidases. The fourth protease, AoS28D, showed high sequence divergence with other S28 proteases and belongs to a phylogenetically distinct cluster together with orthologous proteases from other Aspergillus species. The objective of the present paper was to characterize AoS28D protease in terms of substrate specificity and activity. AoS28D produced by gene overexpression in A. oryzae and in Pichia pastoris was a 70-kDa glycoprotein with a 10-kDa sugar moiety. In contrast with other S28 proteases, AoS28D did not hydrolyze internal Pro-Xaa bonds of several tested peptides. Similarly, to human lysosomal Pro-Xaa carboxypeptidase, AoS28D demonstrated selectivity for cleaving C-terminal Pro-Xaa bonds which are resistant to carboxypeptidases of the S10 family concomitantly secreted by A. oryzae. Therefore, AoS28D could act in synergy with these enzymes during sequential degradation of a peptide from its C-terminus.