High-level expression of a novel α-galactosidase gene from Rhizomucor miehei in Pichia pastoris and characterization of the recombinant enyzme

Biochemical characterization and insights into the potency of the acidic Aspergillus niger NRC114 purified α-galactosidase in removing raffinose family oligosaccharides from soymilk yogurt

BACKGROUND

Because humans lack α-galactosidase, foods containing certain oligosaccharides from the raffinose family, such as soybeans and other legumes, may disrupt digestion and cause flatulence.

RESULTS

Aspergillus niger NRC114 α-galactosidase was purified using protein precipitation, gel filtration, and ion exchange chromatography steps, which resulted in a 123-fold purification. The purified enzyme was found to be 64 kDa using the SDS-PAGE approach. The optimum pH and temperature of the purified α-galactosidase were detected at pH 3.5 and 60 ºC, respectively. The pure enzyme exhibited potent acidic pH stability at pH 3.0 and pH 4.0 for 2 h, and it retained its full activity at 50 ºC and 60 ºC for 120 min and 90 min, respectively. The enzyme was activated using 2.5 mM of K⁺, Mg²⁺, Co²⁺, or Zn²⁺ by 14%, 23%, 28%, and 11%, respectively. The K_m and V_max values of the purified enzyme were calculated to be 0.401 µM and 14.65 μmol min^-1, respectively. The soymilk yogurt showed an increase in its total phenolic content and total flavonoids after enzyme treatment, as well as several volatile compounds that were detected and identified using GC-MS analysis. HPLC analysis clarified the enzymatic action in the hydrolysis of raffinose family oligosaccharides.

CONCLUSION

The findings of this study indicate the importance of A. niger NRC114 α-galactosidase enzyme for future studies, especially its applications in a variety of biological fields.

Microbial succession and its correlation with the dynamics of volatile compounds involved in fermented minced peppers

Fermented minced peppers are a traditional fermented food that has a unique flavor due to various microbial communities involved in fermentation. Understanding the changes in microbial communities and volatile components of fermented minced peppers is particularly important to unveil the formation of unique flavor of fermented peppers. In this study, the microbial communities and volatile compounds in fermented minced pepper was analyzed by high-throughput sequencing and GC-MS, as well as their underlying correlations were also established. Results indicated that 17 genera were identified as dominant microorganisms in the fermentation of minced pepper, accompanied by the detection of 64 volatile compounds. Further hierarchical clustering analysis (HCA) displayed that dynamic change of volatile metabolites were involved in the fermentation process, where alkane volatile components were mainly generated in the early stage (3–5 days), and alcohols volatile components were in the middle stage (7–17 days), while ester volatile components were mainly produced in both the early stage (3–5 days) and last stage (17–20 days). Bidirectional orthogonal partial least squares (O2PLS) analysis revealed that 11 genera were core functional microorganisms of fermented minced pepper. Cladosporium and Hansenpora were significantly correlated with the formation of 9 and 6 volatiles, respectively. These findings provide new insights into aroma profile variation of fermented minced peppers and underlying mechanism of characteristic aroma formation during fermentation.

Isolation and Identification of an α-Galactosidase-Producing Lactosphaera pasteurii Strain and Its Enzymatic Expression Analysis

α-Galactosidase (EC 3.2.1.22) refers to a group of enzymes that hydrolyze oligosaccharides containing α-galactoside-banded glycosides, such as stachyose, raffinose, and verbascose. These enzymes also possess great potential for application in sugar production, and in the feed and pharmaceutical industries. In this study, a strain of Lactosphaera pasteurii (WHPC005) that produces α-galactosidase was identified from the soil of Western Hunan, China. It was determined that the optimal temperature and pH for this α-galactosidase were 45 °C and 5.5, respectively. The activity of α-galactosidase was inhibited by K⁺, Al³⁺, Fe³⁺, fructose, sucrose, lactose, galactose, SDS, EDTA, NaCl, and (NH₄)₂SO₄, and enhanced by Ca²⁺, Fe²⁺, Mn², Zn²⁺, glucose, and raffinose. The optimal inducer was raffinose, and the optimal induction concentration was 30 μmol/L. The α-galactosidase gene was cloned using random fragment cloning methods. Sequence analysis demonstrated that the open reading frame of the α-galactosidase gene was 1230 bp, which encodes a putative protein of 409 amino acids in length. Bioinformatics analysis showed that the isoelectric point and molecular weight of this α-galactosidase were 4.84 and 47.40 kD, respectively. Random coils, alpha helixes, and beta turns were observed in its secondary structure, and conserved regions were found in the tertiary structure of this α-galactosidase. Therefore, this α-galactosidase-producing bacterial strain has the potential for application in the feed industry.

Molecular advances in microbial α-galactosidases: challenges and prospects

α-Galactosidase (α-D-galactosidase galactohydrolase; EC 3.2.1.22), is an industrially important enzyme that hydrolyzes the galactose residues in galactooligosaccharides and polysaccharides. The industrial production of α-galactosidase is currently insufficient owing to the high production cost, low production efficiency and low enzyme activity. Recent years have witnessed an increase in the worldwide research on molecular techniques to improve the production efficiency of microbial α-galactosidases. Cloning and overexpression of the gene sequences coding for α-galactosidases can not only increase the enzyme yield but can confer industrially beneficial characteristics to the enzyme protein. This review focuses on the molecular advances in the overexpression of α-galactosidases in bacterial and yeast/fungal expression systems. Recombinant α-galactosidases have improved biochemical and hydrolytic properties compared to their native counterparts. Metabolic engineering of microorganisms to produce high yields of α-galactosidase can also assist in the production of value-added products. Developing new variants of α-galactosidases through directed evolution can yield enzymes with increased catalytic activity and altered regioselectivity. The bottlenecks in the recombinant production of α-galactosidases are also discussed. The knowledge about the hurdles in the overexpression of recombinant proteins illuminates the emerging possibilities of developing a successful microbial cell factory and widens the opportunities for the production of industrially beneficial α-galactosidases.

Microbial α-galactosidases: Efficient biocatalysts for bioprocess technology

Galactomannans, abundantly present in plant biomass, can be used as renewable fermentation feedstock for biorefineries working for the production of bioethanol and other value-added products. The complete and efficient bioconversion of biomass to fermentable sugars for the generation of biofuels and other value-added products require the concerted action of accessory enzymes like α-galactosidases, which can work in cohesion with other carbohydrases in an enzyme cocktail. In the paper industry, α-galactosidases enhance the bleaching effect of endo-β-1,4-mannanases on softwood kraft pulp. Microbial α-galactosidases also find applications in the treatment of legume foods, recovery of sucrose from sugar beet syrup, improving the rheological properties of galactomannans, and synthesis of α-galactooligosaccharides to be used as functional food ingredients. Owing to their industrial applications, there is a surge in the research focused on α-galactosidases. The current review illustrates the diverse industrial applications of microbial α-galactosidases and their challenges and prospects.

Purification, biochemical and biophysical characterization of an acidic α-galactosidase from the seeds of Annona squamosa (custard apple)

Alpha galactosidase is an exoglycosidase that cleaves α-D-galactose and has numerous applications in medicine, biotechnology, food and pharma industries. In this study, a low molecular weight acidic α-galactosidase was identified from the seeds of custard apple. The purification of α-galactosidase from the crude extract of defatted seeds was achieved by employing ammonium sulphate fractionation, hydrophobic interaction and gel filtration chromatographic techniques. The purified custard apple α-galactosidase (CaG) migrated as a single band in native PAGE corresponding to molecular weight of ~67 kDa and cleaved chromogenic, fluorogenic and natural substrates. CaG was found to be a heterodimer with subunit masses of 40 and 30 kDa. The kinetic parameters such as K_M and Vmax were found to be 0.67 mM and 1.5 U/mg respectively with p-nitrophenyl α-D-galactopyranoside. Galactose, methyl α-D-galactopyranoside and D-galacturonic acid inhibited CaG activity in mixed mode. The CD spectral analysis at far UV region showed that purified CaG exists predominantly as helix (35%), beta sheets (16.3%) and random coils (32.3%) in its secondary structure. These biochemical and biophysical properties of CaG provide leads to understand its primary sequence and glycan structures which will eventually define its novel physiological roles in plants and potential industrial applications.

Characterization of a protease-resistant α-galactosidase from Aspergillus oryzae YZ1 and its application in hydrolysis of raffinose family oligosaccharides from soymilk

The α-galactosidase gene (galC) was cloned from Aspergillus oryzae YZ1 and expressed in Pichia pastoris. The galC (2319 bp) containing two introns encoded a protein of 726 amino acids. The activity of the α-galactosidase (GalC) increased 1-fold after coding sequence optimization. Purified GalC exhibited a single protein band (100 kDa) in SDS-PAGE. The optimum pH and temperature of GalC were pH 4.66 and 50 °C, respectively. Like many GH36 family α-galactosidases, GalC displayed its activities towards raffinose and stachyose. The Km values for pNPG, raffinose and stachyose were 2.16, 4.63 and 8.54 mM, respectively. The GalC retained about 90% activity within the pH range 3.0-8.0. The activity of GalC was inhibited by Cu2+, while Ca2+ increased the enzyme activity. Different concentrations of glucose, mannose, galactose, xylose and sucrose slightly affected the activity of GalC. The GalC displayed strong resistance to trypsin, α-chymotrypsin, and proteinase K. Under simulated gastric conditions, GalC maintained most of its native activity after pepsin treatment for 3 h. The GalC could also effectively degrade raffinose and stachyose in soymilk. The GalC with high hydrolysis efficiency towards raffinose family oligosaccharides (RFOs) and strong resistance to proteases is considered to have great potential in food and feed industries.

Good hydrolysis activity on raffinose family oligosaccharides by a novel α-galactosidase from Tremella aurantialba

An α-galactosidase designated as TAG was purified from the dried fruit bodies of Tremella aurantialba with 182.5-fold purification. The purification procedure involved ion exchange chromatography on Q-sepharose, DEAE-Cellulose, and Mono Q and gel filtration by FPLC on Superdex 75. The purified α-galactosidase was a monomeric protein with a molecular mass of 88 kDa. The optimal pH of TAG was 5.0 and more than 60% of the original enzyme activity remained at pH 2.0 and 3.0. Its optimal temperature was 54 °C with good thermo-stability, 30.8% of the original activity was retained after exposure to a temperature of 70 °C for 1 h. The metal ions Hg²⁺, Cu²⁺, Fe³⁺ and Mg²⁺ strongly inhibited the enzyme activity. The enzyme activity was found to be inhibited by N-bromosuccinimide indicating that tryptophan was essential to the catalytic activity of α-galactosidase. The enzyme completely hydrolysed stachyose and partially hydrolysed raffinose to galactose at 50 °C within 6 h as detected by thin layer chromatography and the dinitrosalicylic acid method and the content of reducing sugar reached 4.36 mg/mL.

Microbial production and biotechnological applications of α-galactosidase

α-Galactosidase, (E.C. 3.2.1.22) is an exoglycosidase that target galactooligosaccharides such as raffinose, melibiose, stachyose and branched polysaccharides like galactomannans and galacto-glucomannans by catalysing the hydrolysis of α-1,6 linked terminal galactose residues. The enzyme has been isolated and characterized from microbial, plant and animal sources. This ubiquitous enzyme possesses physiological significance and immense industrial potential. Optimization of the growth conditions and efficient purification strategies can lead to a significant increase in the enzyme production. To boost commercial productivity, cloning of novel α-galactosidase genes and their heterologous expression in suitable host has gained popularity. Enzyme immobilization leads to its greater reutilization, superior thermostability, pH tolerance and increased activity. The enzyme is well explored in food industry in the removal of raffinose family oligosaccharides (RFOs) in soymilk and sugar crystallization process. It also improves animal feed quality and biomass processing. Applications of the enzyme is in the area of biomedicine includes therapeutic advances in treatment of Fabry disease, blood group conversion and removal of α-gal type immunogenic epitopes in xenotransplantation. With considerable biotechnological applications, this enzyme has been vastly commercialized and holds greater future prospects.

Improvements of thermophilic enzymes: From genetic modifications to applications

Thermozymes (from thermophiles or hyperthermophiles) offer obvious advantages due to their excellent thermostability, broad pH adaptation, and hydrolysis ability, resulting in diverse industrial applications including food, paper, and textile processing, biofuel production. However, natural thermozymes with low yield and poor adaptability severely hinder their large-scale applications. Extensive studies demonstrated that using genetic modifications such as directed evolution, semi-rational design, and rational design, expression regulations and chemical modifications effectively improved enzyme's yield, thermostability and catalytic efficiency. However, mechanism-based techniques for thermozymes improvements and applications need more attention. In this review, stabilizing mechanisms of thermozymes are summarized for thermozymes improvements, and these improved thermozymes eventually have large-scale industrial applications.

Improving the Secretory Expression of an -Galactosidase from Aspergillus niger in Pichia pastoris

α-Galactosidases are broadly used in feed, food, chemical, pulp, and pharmaceutical industries. However, there lacks a satisfactory microbial cell factory that is able to produce α-galactosidases efficiently and cost-effectively to date, which prevents these important enzymes from greater application. In this study, the secretory expression of an Aspergillus niger α-galactosidase (AGA) in Pichia pastoris was systematically investigated. Through codon optimization, signal peptide replacement, comparative selection of host strain, and saturation mutagenesis of the P1’ residue of Kex2 protease cleavage site for efficient signal peptide removal, a mutant P. pastoris KM71H (Mut^s) strain of AGA-I with the specific P1’ site substitution (Glu to Ile) demonstrated remarkable extracellular α-galactosidase activity of 1299 U/ml upon a 72 h methanol induction in 2.0 L fermenter. The engineered yeast strain AGA-I demonstrated approximately 12-fold higher extracellular activity compared to the initial P. pastoris strain. To the best of our knowledge, this represents the highest yield and productivity of a secreted α-galactosidase in P. pastoris, thus holding great potential for industrial application.

Purification of thermostable α-galactosidase from Irpex lacteus and its use for hydrolysis of oligosaccharides

A monomeric α-galactosidase (ILGI) from the mushroom Irpex lacteus was purified 94.19-fold to electrophoretic homogeneity. ILGI exhibited a specific activity of 18.36 U mg(-1) and demonstrated a molecular mass of 60 kDa in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). ILGI was optimally active at 80 °C and pH 5.0, and it was stable over a temperature range of 4-70 °C and a wide pH range of 2.0-12.0. ILGI was completely inactivated by Ag(+) and Hg(2+) ions and N-bromosuccinimide (NBS). Moreover, ILGI exhibited good resistance to proteases. Galactose acted as a noncompetitive inhibitor with Ki and Kis of 3.34 and 0.29 mM, respectively. The α-galactosidase presented a broad substrate specificity, which included p-nitrophenyl α-D-galactopyranoside (pNPGal), melibiose, stachyose, and raffinose with Km values of 1.27, 3.24, 7.1, and 22.12 mM, correspondingly. ILGI exhibited efficient and complete hydrolysis to raffinose and stachyose. The aforementioned features of this enzyme suggest its potential value in food and feed industries.

Crystal structure and characterization of a novel L-serine ammonia-lyase from Rhizomucor miehei

L-serine ammonia-lyase, as a member of the β-family of pyridoxal-5'-phosphate (PLP) dependent enzymes, catalyzes the conversion of L-serine (L-threonine) to pyruvate (α-ketobutyrate) and ammonia. The crystal structure of L-serine ammonia-lyase from Rhizomucor miehei (RmSDH) was solved at 1.76 Å resolution by X-ray diffraction method. The overall structure of RmSDH had the characteristic β-family PLP dependent enzyme fold. It consisted of two distinct domains, both of which show the typical open twisted α/β structure. A PLP cofactor was located in the crevice between the two domains, which was attached to Lys52 by a Schiff-base linkage. Unique residue substitutions (Gly78, Pro79, Ser146, Ser147 and Thr312) were discovered at the catalytic site of RmSDH by comparison of structures of RmSDH and other reported eukaryotic L-serine ammonia-lyases. Optimal pH and temperature of the purified RmSDH were 7.5 and 40 °C, respectively. It was stable in the pH range of 7.0-9.0 and at temperatures below 40 °C. This is the first crystal structure of a fungal L-serine ammonia-lyase. It will be useful to study the catalytic mechanism of β-elimination enzymes and will provide a basis for further enzyme engineering.

A Fungal α-Galactosidase from Tricholoma matsutake with Broad Substrate Specificity and Good Hydrolytic Activity on Raffinose Family Oligosaccharides

An acidic α-galactosidase designated as TMG was purified from the fruiting bodies The purification protocol entailed ion exchange chromatography on Q-Sepharose and of Tricholoma matsutake with 136-fold purification and a specific activity of 909 units/mg. Mono-Q and fast protein liquid chromatography on Superdex 75. TMG is a monomeric protein exhibiting a molecular mass of 47 kDa in SDS-PAGE and gel filtration. The purified enzyme was identified by LC-MS/MS and three inner amino acid sequences were obtained. The optimum pH and temperature for TMG with pNPGal as substrate were pH 4.5 and 55 °C, respectively. The α-galactosidase activity was strongly inhibited by K+, Ca2+, Cd2+, Hg2+, Ag+ and Zn2+ ions. The enzyme activity was inhibited by the chemical modification agent N-bromosuccinimide (NBS), indicating the importance of tryptophan residue(s) at or near the active site. Besides hydrolyzing pNPGal, TMG also efficaciously catalyzed the degradation of natural substrates such as stachyose, raffinose, and melibiose. Thus TMG can be exploited commercially for improving the nutritional value of soy milk by degradation of indigestible oligosaccharides.