Hydrolysis of casein-derived peptides alpha(S1)-casein(f1-9) and beta-casein(f193-209) by Lactobacillus helveticus peptidase deletion mutants indicates the presence of a previously undetected endopeptidase

Characterization of a New M13 Metallopeptidase from Deep-Sea Shewanella sp. E525-6 and Mechanistic Insight into Its Catalysis

Bacterial extracellular peptidases are important for bacterial nutrition and organic nitrogen degradation in the ocean. While many peptidases of the M13 family from terrestrial animals and bacteria are studied, there has been no report on M13 peptidases from marine bacteria. Here, we characterized an M13 peptidase, PepS, from the deep-sea sedimentary strain Shewanella sp. E525-6, and investigated its substrate specificity and catalytic mechanism. The gene pepS cloned from strain E525-6 contains 2085 bp and encodes an M13 metallopeptidase. PepS was expressed in Escherichia coli and purified. Among the characterized M13 peptidases, PepS shares the highest sequence identity (47%) with Zmp1 from Mycobacterium tuberculosis, indicating that PepS is a new member of the M13 family. PepS had the highest activity at 30°C and pH 8.0. It retained 15% activity at 0°C. Its half life at 40°C was only 4 min. These properties indicate that PepS is a cold-adapted enzyme. The smallest substrate for PepS is pentapeptide, and it is probably unable to cleave peptides of more than 30 residues. PepS prefers to hydrolyze peptide bonds with P1′ hydrophobic residues. Structural and mutational analyses suggested that His531, His535 and Glu592 coordinate the catalytic zinc ion in PepS, Glu532 acts as a nucleophile, and His654 is probably involved in the transition state stabilization. Asp538 and Asp596 can stablize the orientations of His531 and His535, and Arg660 can stablize the orientation of Asp596. These results help in understanding marine bacterial peptidases and organic nitrogen degradation.

Lactobacillus helveticus: the proteolytic system

Lactobacillus helveticus is one of the species of lactic acid bacteria (LAB) most commonly used in the production of fermented milk beverages and some types of hard cheese. The versatile nature of this bacterium is based on its highly efficient proteolytic system consisting of cell-envelope proteinases (CEPs), transport system and intracellular peptidases. Besides use of L. helveticus in cheese processing, the production of fermented milk preparations with health promoting properties has become an important industrial application. Studies have shown that fermented dairy products are able to decrease blood pressure, stimulate the immune system, promote calcium absorption, and exert an anti-virulent effect against pathogens. These beneficial effects are produced by a variety of peptides released during the hydrolysis of milk proteins by the proteolytic system of L. helveticus, which provides the bacterium with its nutritional requirements for growth. In recent years, studies have focused on understanding the factors that affect the kinetics of milk protein hydrolysis by specific strains and have concentrated on the effect of pH, temperature, growth phase, and matrix composition on the bacterial enzymatic system. This review focuses on the role of the proteolytic system of L. helveticus in the production of bioactive compounds formed during fermentation of dairy products. Taking advantage of the powerful proteolytic system of this bacterium opens up future opportunities to search for novel food-derived compounds with potential health promoting properties.

Characterization of immune-active peptides obtained from milk fermented byLactobacillus helveticus

The objectives of this research were to confirm the effect of compounds derived from milk fermented byLactobacillus helveticus(LH-2) on the nonspecific host defence system, and isolate and characterize the active peptides that mediate the immune response. The cell-free supernatant obtained from the fermented milk and its fractions were testedin vitrofor immuno-modulating activity using murine macrophages (RAW 264·7 cell line). Cytokine production (Interleukin-6 (IL-6), Tumor Necrosis Factor-α (TNF-α), and Interleukin-1β (IL1-β)), nitric oxide (NO) production and phagocytosis were used as biomarkers. Macrophages stimulated with cell-free supernatant of fermented milk showed higher production of cytokines and NO compared with macrophages stimulated with LPS (Lipopolysaccharide) and a commercial immunomodulator derived from β-casein (f54-59). Phagocytosis was observed by macrophages stimulated with the supernatant. Two of nine fractions collected from the supernatant using size exclusion chromatography produced the highest response when used to stimulate macrophages. The results of the dose-response study of the effect of the fraction with the highest stimulation effect on the production of TNF-α showed a direct correlation between protein concentration and TNF-α release. The fraction contained four novel peptides, three derived from the hydrolysis of β-casein and one from the hydrolysis of α-lactalbumin. These results confirm that fermentation of milk byLactobacillus helveticus(LH-2) results in the production of specific peptides capable of modulating macrophage activity.

Extraction and partial characterization of proteolytic activities from the cell surface of Lactobacillus helveticus Zuc2

Proteolytic activities were extracted from a dairy Lactobacillus helveticus strain and partially characterized. A first cell envelope proteinase (CEP) was extracted using a high ionic strength buffer, both in the presence and in the absence of Ca2+. Moreover, cell treatment by 5 M LiCl allowed for the selective removal of the S-layer protein and CEP, suggesting an enzyme ionic linkage to the cell envelope similar to that observed for the Slayer structure. The enzyme specificity against alpha(s1)-CN (f1-23) showed unusual activity on the Lys3-His4 bond compared with other proteinases of the same species. A second proteinase appeared to be linked to the cell membrane because it was extractable only after membrane disgregation by detergents. Its specificity against CN fractions and alpha(s1)-CN (f1-23) was different from that of the first CEP; moreover, the measured activity was lower than that of CEP.

Genome sequence of Lactobacillus helveticus, an organism distinguished by selective gene loss and insertion sequence element expansion

Mobile genetic elements are major contributing factors to the generation of genetic diversity in prokaryotic organisms. For example, insertion sequence (IS) elements have been shown to specifically contribute to niche adaptation by promoting a variety of genetic rearrangements. The complete genome sequence of the cheese culture Lactobacillus helveticus DPC 4571 was determined and revealed significant conservation compared to three nondairy gut lactobacilli. Despite originating from significantly different environments, 65 to 75% of the genes were conserved between the commensal and dairy lactobacilli, which allowed key niche-specific gene sets to be described. However, the primary distinguishing feature was 213 IS elements in the DPC 4571 genome, 10 times more than for the other lactobacilli. Moreover, genome alignments revealed an unprecedented level of genome stability between these four Lactobacillus species, considering the number of IS elements in the L. helveticus genome. Comparative analysis also indicated that the IS elements were not the primary agents of niche adaptation for the L. helveticus genome. A clear bias toward the loss of genes reported to be important for gut colonization was observed for the cheese culture, but there was no clear evidence of IS-associated gene deletion and decay for the majority of genes lost. Furthermore, an extraordinary level of sequence diversity exists between copies of certain IS elements in the DPC 4571 genome, indicating they may represent an ancient component of the L. helveticus genome. These data suggest a special unobtrusive relationship between the DPC 4571 genome and its mobile DNA complement.

Construction and evaluation of food-grade vectors for Lactococcus lactis using aspartate aminotransferase and alpha-galactosidase as selectable markers

AIMS

We report development of two food-grade cloning vectors for Lactococcus lactis, which utilize either a lactococcal aspartate aminotransferase gene (aspC), or Bifidobacterium longumalpha-galactosidase gene (aglL) as selectable markers.

METHODS AND RESULTS

The theta-replicon of lactococcal plasmid, pW563, was combined with aspC and a multiple cloning site. When electroporated into L. lactis JLS400 (AspC-), the resulting vector, pSUW611 (3.9 kbp), restores ability of the mutant to grow in milk thus allowing for selection of the transformants. The vector is stable during 100 generations of nonselective growth (0.2% loss per generation). The second vector, pSUW711 (5.1 kbp), was constructed by exchanging aspC with aglL under the control of usp45 promoter. Lactococcus lactis transformed with pSUW711 produced distinctive colonies within 48-72 h on melibiose-containing plates. Expression of two Lactobacillus helveticus peptidases was attempted using these new vehicles. Introduction of pepN on pSUW611 and pSUW711 into L. lactis led to a sixfold, or 27-fold increase in aminopeptidase activity, respectively. However, no changes in endopeptidase activity were recorded upon transformation with pSUW611 carrying pepO2 under control of three different promoters. Attempts were also made to construct high copy variants of pSUW711.

CONCLUSIONS

The aspC and aglL can be employed as food-grade genetic markers for L. lactis. The vectors, pSUW611 and pSUW711, were successfully used to express Lact. helveticus PepN in L. lactis.

SIGNIFICANCE AND IMPACT OF THE STUDY

Two novel food-grade vectors were developed which provide simple and convenient selection and maintenance in L. lactis.

Identification of endopeptidase genes from the genomic sequence of Lactobacillus helveticus CNRZ32 and the role of these genes in hydrolysis of model bitter peptides

Genes encoding three putative endopeptidases were identified from a draft-quality genome sequence of Lactobacillus helveticus CNRZ32 and designated pepO3, pepF, and pepE2. The ability of cell extracts from Escherichia coli DH5alpha derivatives expressing CNRZ32 endopeptidases PepE, PepE2, PepF, PepO, PepO2, and PepO3 to hydrolyze the model bitter peptides, beta-casein (beta-CN) (f193-209) and alpha(S1)-casein (alpha(S1)-CN) (f1-9), under cheese-ripening conditions (pH 5.1, 4% NaCl, and 10 degrees C) was examined. CNRZ32 PepO3 was determined to be a functional paralog of PepO2 and hydrolyzed both peptides, while PepE and PepF had unique specificities towards alpha(S1)-CN (f1-9) and beta-CN (f193-209), respectively. CNRZ32 PepE2 and PepO did not hydrolyze either peptide under these conditions. To demonstrate the utility of these peptidases in cheese, PepE, PepO2, and PepO3 were expressed in Lactococcus lactis, a common cheese starter, using a high-copy vector pTRKH2 and under the control of the pepO3 promoter. Cell extracts of L. lactis derivatives expressing these peptidases were used to hydrolyze beta-CN (f193-209) and alpha(S1)-CN (f1-9) under cheese-ripening conditions in single-peptide reactions, in a defined peptide mix, and in Cheddar cheese serum. Peptides alpha(S1)-CN (f1-9), alpha(S1)-CN (f1-13), and alpha(S1)-CN (f1-16) were identified from Cheddar cheese serum and included in the defined peptide mix. Our results demonstrate that in all systems examined, PepO2 and PepO3 had the highest activity with beta-CN (f193-209) and alpha(S1)-CN (f1-9). Cheese-derived peptides were observed to affect the activity of some of the enzymes examined, underscoring the importance of incorporating such peptides in model systems. These data indicate that L. helveticus CNRZ32 endopeptidases PepO2 and PepO3 are likely to play a key role in this strain's ability to reduce bitterness in cheese.

Identification and characterization of Lactobacillus helveticus PepO2, an endopeptidase with post-proline specificity

A post-proline endopeptidase (PepO2) was detected in cell extracts from a genomic library of Lactobacillus helveticus CNRZ32 by using the synthetic substrate N-acetyl-beta-casein-(f203-209)-rho-nitroanilide in a coupled reaction with aminopeptidase N. Isolates with activity for this substrate contained plasmids with visually indistinguishable restriction profiles. Nucleotide sequence analysis revealed a 1,947-bp open reading frame, designated pepO2, encoding a putative 71.4-kDa protein. Analysis of the predicted peptide sequence revealed that L. helveticus PepO2 contained the zinc-dependent metalloprotease motif HEXXH and exhibited levels of amino acid sequence similarity of 72, 61, 59, and 53% to L. helveticus PepO, Lactococcus lactis PepO2, L. lactis PepO, and Lactobacillus rhamnosus PepO, respectively. Northern hybridization results indicated that the transcript containing pepO2 was monocistronic. Despite the high degrees of amino acid similarity to PepO proteins from other lactic acid bacteria, the specificity of the L. helveticus PepO2 for post-proline bonds distinguishes it from other PepO-type endopeptidases characterized to date. The specificity for post-proline bonds also suggests that this enzyme may play a central role in the hydrolysis of casein-derived bitter peptides, such as beta-casein(f193-209).