Effect of salt on the activity of Streptomyces prolyl aminopeptidase

Functional expression and characterization of a novel aminopeptidase B from Aspergillus niger in Pichia pastoris

A novel aminopeptidase B (APB-AN) was identified from Aspergillus niger CGMCC 3.1454 for the first time and was cloned and expressed in Pichia pastoris. The mature enzyme of approximately 100 kDa was purified for characterization. The optimum pH and temperature of the recombinant APB-AN were determined to be 7.0 and 40 °C, respectively. The enzyme was stable below 40 °C and at pH values from 5.0 to 8.0. The K _m and V _max values were determined to be 0.61 mmol/L and 11.45 mmol/L/min, respectively, using Arg-pNA as the substrate. APB-AN was inhibited by Cu²⁺ and Fe²⁺ and activated by Co²⁺ and Na⁺. Most metal chelators (Ca²⁺, Mg²⁺ and Mn²⁺) and aminopeptidase inhibitors (bestatin and puromycin) suppressed its activity. APB-AN was found to be active towards 13 kinds of amino acid p-nitroanilide (pNA) substrates:Arg-pNA, Lys-pNA, Tyr- pNA, Trp-pNA, Phe-pNA, His-pNA, Ala-pNA, Met-pNA, Leu-pNA, Glu-pNA, Val-pNA, Pro-pNA and Ile-pNA, and the most preferred N-terminal amino acids were arginine and lysine. APB-AN also hydrolyzed 4 natural proteins: casein, bovine serum albumin, soy protein isolate and water-soluble wheat protein. It is expected that APB-AN has potential food processing applications.

Novel activity of Streptomyces aminopeptidase P

Streptomyces aminopeptidase P enzymes are proline-specific peptidases that belong to the peptidase M24 family. To evaluate the activity of a commercial Streptomyces aminopeptidase P, named ‘XPO DUET’, we performed three experiments involving degradation of tryptic casein, production of free amino acids from casein hydrolysate, and hydrolysis of synthetic peptides. Using an ion-trap liquid chromatography–mass spectrometry (LC–MS) apparatus, we demonstrate that XPO DUET could degrade FFVAPFPEVFGK, an allergic and bitter peptide, VAPFPEVFGK, and PEVFGK from tryptic casein. All amino acids, except Ala, Asp, Glu, and Tyr, were released in an XPO DUET activity-dependent manner during the hydrolysis of casein hydrolysate. LC–MS analysis also revealed the ability of XPO DUET to completely hydrolyze Phe-Phe-Phe into free Phe. Thus, we confirm that XPO DUET possesses broader specificity than its known activity toward Xaa-Pro peptides. Because XPO DUET is a food-grade peptidase, it is useful in the bioprocessing of protein hydrolysates through its combination with other food-grade peptidases.

Mutagenesis for Improvement of Activity and Stability of Prolyl Aminopeptidase from Aspergillus oryzae

In this study, the prokaryotic expression system of Escherichia coli was used to modify prolyl aminopeptidase derived from Aspergillus oryzae JN-412 (AoPAP) via random mutagenesis and site-directed saturation mutagenesis. A random mutant library with a capacity of approximately 3000 mutants was compiled using error-prone polymerase chain reaction, and nonconservative amino acids within 3 Å of the substrate L-proline-p-nitroaniline were selected as site-directed saturation mutagenesis sites via homologous simulation and molecular docking of AoPAP. Variants featuring high catalytic efficiency were screened by a high-throughput screening method. The specific activities of the variants of 3D9, C185V, and Y393W were 127 U mg^-1, 156 U mg^-1, and 120 U mg^-1, respectively, which were 27%, 56%, and 20% higher than those of the wild type, with a value of 100 U mg^-1. The half-life of thermostability of the mutant 3D9 was 4.5 h longer than that of the wild type at 50 °C. The mutant C185V improved thermostability and had a half-life 2 h longer than that of the wild type at a pH of 6.5. Prolyl aminopeptidase had improved stability within the acidic range and thermostability after modification, making it more suitable for a synergistic combination with various acidic and neutral endoproteases.

Optimized expression of prolyl aminopeptidase in Pichia pastoris and its characteristics after glycosylation

Prolyl aminopeptidases are specific exopeptidases that catalyze the hydrolysis of the N-terminus proline residue of peptides and proteins. In the present study, the prolyl aminopeptidase gene (pap) from Aspergillus oryzae JN-412 was optimized through the codon usage of Pichia pastoris. Both the native and optimized pap genes were inserted into the expression vector pPIC9 K and were successfully expressed in P. pastoris. Additionally, the activity of the intracellular enzyme expressed by the recombinant optimized pap gene reached 61.26 U mL(-1), an activity that is 2.1-fold higher than that of the native gene. The recombinant enzyme was purified by one-step elution through Ni-affinity chromatography. The optimal temperature and pH of the purified PAP were 60 °C and 7.5, respectively. Additionally, the recombinant PAP was recovered at a yield greater than 65 % at an extremely broad range of pH values from 6 to 10 after treatment at 50 °C for 6 h. The molecular weight of the recombinant PAP decreased from 50 kDa to 48 kDa after treatment with a deglycosylation enzyme, indicating that the recombinant PAP was completely glycosylated. The glycosylated PAP exhibited high thermo-stability. Half of the activity remained after incubation at 50 °C for 50 h, whereas the remaining activity of PAP expressed in E. coli was only 10 % after incubation at 50 °C for 1 h. PAP could be activated by the appropriate salt concentration and exhibited salt tolerance against NaCl at a concentration up to 5 mol L(-1).

Comparative transcriptomic analysis reveals novel genes and regulatory mechanisms of Tetragenococcus halophilus in response to salt stress

Tetragenococcus halophilus, a moderately halophilic Gram-positive bacterium, was isolated from Chinese style soy sauce. This species is a valuable resource for investigating salt tolerance mechanisms and improving salinity resistance in microorganisms. RNA-seq was used to sequence T. halophilus samples treated with 0 M (T1), 1 M (T2), and 3.5 M NaCl (T3). Comparative transcriptomic analyses of the different treatments were performed using gene ontology and Kyoto encyclopedia of genes and genome. The comparison of T1 and T2 by RNA-seq revealed that genes involved in transcription, translation, membrane system, and division were highly up-regulated under optimum salt condition. The comparison of T2 and T3 showed that genes related to heat shock proteins or the ATP-binding cassette transport systems were significantly up-regulated under maximum-salt condition. In addition, a considerable proportion of the significantly differently expressed genes identified in this study are novel. These data provide a crucial resource that may determine specific responses to salt stress in T. halophilus.

Characterization of a (D)-stereoselective aminopeptidase (DamA) exhibiting aminolytic activity and halophilicity from Aspergillus oryzae

β-Aminopeptidases exhibit both hydrolytic and aminolytic (peptide bond formation) activities and have only been reported in bacteria. We identified a gene encoding the β-aminopeptidase homolog from a genome database of the filamentous fungus Aspergillus oryzae. The gene was overexpressed in A. oryzae, and the resulting recombinant enzyme was purified. Apart from bacterial homologs [β-Ala-para-nitroanilide (pNA)], the enzyme preferred D-Leu-pNA and D-Phe-pNA as substrates. Therefore, we designated this gene as d-stereoselective aminopeptidase A (damA). The purified recombinant DamA was estimated to be a hexamer and was composed of two subunits with molecular masses of 29.5 and 11.5 kDa, respectively. Optimal hydrolytic activity of DamA toward D-Leu-pNA was observed at 50 °C and pH 8.0. The enzyme was stable up to 60 °C and from pH 4.0-11.0. DamA also exhibited aminolytic activity, producing D-Leu-D-Leu-NH2 from D-Leu-NH2 as a substrate. In the presence of 3.0 M NaCl, the amount of pNA liberated from D-Leu-pNA by DamA was 3.1-fold higher than that in the absence of NaCl. Thus, DamA is a halophilic enzyme. The enzyme was utilized to synthesize several hetero-dipeptides containing a D-amino acid at the N-terminus as well as physiologically active peptides.

Biochemical characterisation of prolyl aminopeptidase from shoots of triticale seedlings and its activity changes in response to suboptimal growth conditions

Prolyl aminopeptidase (PAP) was isolated from the shoots of three-day-old triticale seedlings and was purified using a five-step purification procedure (acid precipitation, gel filtration, anion-exchange chromatography, hydrophobic chromatography and rechromatography). The enzyme was purified 460-fold with a recovery of 6%. Prolyl aminopeptidase appears to be a tetramer consisting of four subunits, each with a molecular weight of approximately 54kDa. Its pH and temperature optimum are pH 7.5 and 37°C, respectively. The enzyme prefers substrates with Pro and Hyp at the N-terminus, but is also capable of hydrolysing β-naphthylamides (β-NA) of Ala, Phe, and Leu. The K(m) value of PAP against Pro-β-NA was the lowest among the substrates tested and it was 1.47×10(-5)M. The activity of PAP was not inhibited by EDTA, 1,10-phenantroline, or pepstatin A. The most effective inhibitors were DFP, Pefabloc, and PMSF, which are serine protease inhibitors. However, significant inhibition was also observed in the presence of E-64, which modifies sulfhydryl groups. A significant increase of the aminopeptidase activity against Pro-β-NA was observed in shoots of triticale plants grown under salinity, drought stress, and in the presence of cadmium and aluminium ions in the nutrient solution.

Characterization of recombinant prolyl aminopeptidase from Aspergillus oryzae

AIMS

Prolyl aminopeptidase (PAP) degrades only amino-terminal proline from peptides. The food-grade fungus Aspergillus oryzae produces this enzyme only in small amounts. In this paper, we present efficient production of recombinant PAP with an overexpression system of A. oryzae and characterization of its biochemical properties.

METHODS AND RESULTS

The gene encoding PAP was overexpressed as a His-tag fusion protein under a taka-amylase gene (amyB) promoter with a limited expressing condition in A. oryzae. The PAP activity in the mycelia grown in rich medium containing glucose (repressing condition) was twice that in starch (inducing condition). The enzyme prepared as cell-free extract was partially purified through two-step column chromatography. The PAP was estimated to be a hexameric protein and exhibited salt tolerance against NaCl of up to 4 mol l(-1).

CONCLUSIONS

Aspergillus oryzae PAP was produced under the repressing condition of amyB promoter in a PAP-overexpressing strain and purified 1800-folds. Overproduction of PAP under promoter-inducing conditions led to an increase in inactive PAP, possibly because of irregular folding.

SIGNIFICANCE AND IMPACT OF THE STUDY

PAP with a high specific activity and salt tolerance may be used effectively in the manufacturing processes of fermented foods.

Activation of oligopeptidase B from Streptomyces griseus by thiol-reacting reagents is independent of the single reactive cysteine residue

Oligopeptidase B from Streptomyces griseus was cloned and characterized to clarify the substrate recognition mechanism and the role of a reactive cysteine residue in family S9 prolyl oligopeptidases (POPs). The cloned enzyme, SGR-OpdB, was annotated as a putative family S9 prolyl oligopeptidase based on its deduced amino acid sequence, in which a sole cysteine residue Cys(544) is present close to the catalytic Asp residue in the C-terminal region. The protein was identified as oligopeptidase B, a member of the subfamily S9a of the family S9 POPs, as judged by its substrate specificity and enzymatic characteristics. Its enzymatic activity was markedly enhanced by high NaCl concentration and the reducing reagents dithiothreitol (DTT) and reduced glutathione (GSH). It is particularly interesting that oxidized glutathione (GSSG) also enhanced SGR-OpdB activity. The SGR-OpdB C544A mutant was constructed and characterized to clarify the role of the putative reactive Cys residue, Cys(544). Surprisingly, the enzymatic activity of the Cys-free mutant was also markedly activated by the general thiol-reacting reagent DTT, GSH, and GSSG. To our knowledge, this is the first report of activity-enhancing effects of thiol-reacting reagents toward Cys-free enzymes. Results clarified the role of additives in inducing conformational change of SGR-OpdB into active peptidase.