Purification and characterization of a cell wall proteinase from Streptococcus lactis NCDO 763

Cell-envelope proteinases from lactic acid bacteria: Biochemical features and biotechnological applications

Proteins displayed on the cell surface of lactic acid bacteria (LAB) perform diverse and important biochemical roles. Among these, the cell-envelope proteinases (CEPs) are one of the most widely studied and most exploited for biotechnological applications. CEPs are important players in the proteolytic system of LAB, because they are required by LAB to degrade proteins in the growth media into peptides and/or amino acids required for the nitrogen nutrition of LAB. The most important area of application of CEPs is therefore in protein hydrolysis, especially in dairy products. Also, the physical location of CEPs (i.e., being cell-envelope anchored) allows for relatively easy downstream processing (e.g., extraction) of CEPs. This review describes the biochemical features and organization of CEPs and how this fits them for the purpose of protein hydrolysis. It begins with a focus on the genetic organization and expression of CEPs. The catalytic behavior and cleavage specificities of CEPs from various LAB are also discussed. Following this, the extraction and purification of most CEPs reported to date is described. The industrial applications of CEPs in food technology, health promotion, as well as in the growing area of water purification are discussed. Techniques for improving the production and catalytic efficiency of CEPs are also given an important place in this review.

Evidence of cell-associated proteinases from Virgibacillus sp. SK33 isolated from fish sauce fermentation

Cell-associated proteinases from Virgibacillus sp. SK33 isolated from fish sauce fermentation were extracted and characterized. Proteinases were effectively released when washed cells were incubated in 0.3 mg/mL lysozyme in 50 mM Tris-maleate (pH 7) at 37 °C for 2 h. Major cell-associated proteinases exhibited molecular mass of 17, 32, and 65 kDa, but only a 32-kDa proteinase showed strong amidolytic activity toward Suc-Ala-Ala-Pro-Phe-AMC. Activity of all cell-associated proteinases was completely inhibited by phenylmethanesulfonyl fluoride, indicating a characteristic of serine proteinase. In addition, a 65-kDa serine proteinase was also inhibited by ethylenediaminetetraacetic acid, implying a metal-dependent characteristic. Optimum activity toward a synthetic peptide substrate was at 50 °C and pH 8 and 11. Proteinases with molecular mass of 17 and 32 kDa exhibited caseinolytic activity at 25% NaCl and activity based on a synthetic peptide substrate increased with NaCl concentrations up to 25%, suggesting their role in hydrolyzing proteins at high salt concentrations. This is the first report of liberated cell-associated proteinases from a moderate halophile, Virgibacillus sp.

PRACTICAL APPLICATION

The cell-associated proteinases could be extracted from Virgibacillus sp. SK 33 using lysozyme. The extracted enzyme could be applied to hydrolyze food proteins at NaCl content as high as 25%. In addition, this study demonstrated that not only extracellular but also cell-associated proteinases are key factors contributing to protein-degrading ability at high salt environment of Virgibacillus sp. SK 33.

Intracellular proteinase ofLactococcus lactissubsp.lactisNCDO 763

An intracellular proteinase was purified fromLactococcus lactissubsp.lactisNCDO 763 after spheroplast formation from cell wall proteinase-deficient variants. The proteinase was active at pH 7·5 and 45 °C and affected by metalloenzyme inhibitors. Its specificity, determined on B-chain of insulin, was thermolysin-like. The B-chain of insulin was hydrolysed rapidly while hydrolysis of β-casein was slower. This enzyme has aMrof ∽ 93000.

The specificity of oligopeptide transport by Streptococcus thermophilus resembles that of Lactococcus lactis and not that of pathogenic streptococci

Peptide transport is a crucial step in the growth of Streptococcus thermophilus in protein- or peptide-containing media. The objective of the present work was to determine the specificity of peptide utilization by this widely used lactic acid bacterium. To reach that goal, complementary approaches were employed. The capability of a proteinase-negative S. thermophilus strain to grow in a chemically defined medium containing a mixture of peptides isolated from milk as the source of amino acids was analysed. Peptides were separated into three size classes by ultrafiltration. The strain was able to use peptides up to 3.5 kDa during growth, as revealed by liquid chromatography and mass spectrometry analyses. The same strain was grown in chemically defined medium containing a tryptic digest of casein, and the respective time-course consumption of the peptides during growth was estimated. The ability to consume large peptides (up to 23 residues) was confirmed, as long as they are cationic and hydrophobic. These results were confirmed by peptide transport studies. Extension of the study to 11 other strains revealed that they all shared these preferences.

Casein utilization by Streptococcus thermophilus results in a diauxic growth in milk

In milk, Streptococcus thermophilus displays two distinct exponential growth phases, separated by a nonexponential one, during which proteinase synthesis was initiated. During the second exponential phase, utilization of caseins as the source of amino acids resulted in a decrease in growth rate, presumably caused by a limiting peptide transport activity.

The proteolytic systems of lactic acid bacteria

Proteolysis in dairy lactic acid bacteria has been studied in great detail by genetic, biochemical and ultrastructural methods. From these studies the picture emerges that the proteolytic systems of lactococci and lactobacilli are remarkably similar in their components and mode of action. The proteolytic system consists of an extracellularly located serine-proteinase, transport systems specific for di-tripeptides and oligopeptides (> 3 residues), and a multitude of intracellular peptidases. This review describes the properties and regulation of individual components as well as studies that have led to identification of their cellular localization. Targeted mutational techniques developed in recent years have made it possible to investigate the role of individual and combinations of enzymes in vivo. Based on these results as well as in vitro studies of the enzymes and transporters, a model for the proteolytic pathway is proposed. The main features are: (i) proteinases have a broad specificity and are capable of releasing a large number of different oligopeptides, of which a large fraction falls in the range of 4 to 8 amino acid residues; (ii) oligopeptide transport is the main route for nitrogen entry into the cell; (iii) all peptidases are located intracellularly and concerted action of peptidases is required for complete degradation of accumulated peptides.

The physiology and biochemistry of the proteolytic system in lactic acid bacteria

The inability of lactic acid bacteria to synthesize many of the amino acids required for protein synthesis necessitates the active functioning of a proteolytic system in those environments where protein constitutes the main nitrogen source. Biochemical and genetic analysis of the pathway by which exogenous proteins supply essential amino acids for growth has been one of the most actively investigated aspects of the metabolism of lactic acid bacteria especially in those species which are of importance in the dairy industry, such as the lactococci. Much information has now been accumulated on individual components of the proteolytic pathway in lactococci, namely, the cell envelope proteinase(s), a range of peptidases and the amino acid and peptide transport systems of the cell membrane. Possible models of the proteolytic system in lactococci can be proposed but there are still many unresolved questions concerning the operation of the pathway in vivo. This review will examine current knowledge and outstanding problems regarding the proteolytic system in lactococci and also the extent to which the lactococcal system provides a model for understanding proteolysis in other groups of lactic acid bacteria.

Degradation and debittering of a tryptic digest from beta-casein by aminopeptidase N from Lactococcus lactis subsp. cremoris Wg2

The mode of action of purified aminopeptidase N from Lactococcus lactis subsp. cremoris Wg2 on a complex peptide mixture of a tryptic digest from bovine beta-casein was analyzed. The oligopeptides produced in the tryptic digest before and after aminopeptidase N treatment were identified by analysis of the N- and C-terminal amino acid sequences and amino acid compositions of the isolated peptides and by on-line liquid chromatography-mass spectrometry. Incubation of purified peptides with aminopeptidase N resulted in complete hydrolysis of many peptides, while others were only partially hydrolyzed or not hydrolyzed. The tryptic digest of beta-casein exhibits a strong bitter taste, which corresponds to the strong hydrophobicity of several peptides in the tryptic digest of beta-casein. The degradation of the "bitter" tryptic digest by aminopeptidase N resulted in a decrease of hydrophobic peptides and a drastic decrease of bitterness of the reaction mixture.

Characterization of a Cell Envelope-Associated Proteinase Activity from Streptococcus thermophilus H-Strains

The production and biochemical properties of cell envelope-associated proteinases from two strains of Streptococcus thermophilus (strains CNRZ 385 and CNRZ 703) were compared. No significant difference in proteinase activity was found for strain CNRZ 385 when cells were grown in skim milk medium and M17 broth. Strain CNRZ 703 exhibited a threefold-higher proteinase activity when cells were grown in low-heat skim milk medium than when grown in M17 broth. Forty-one percent of the total activity of CNRZ 385 was localized on the cell wall. The optimum pH for enzymatic activity at 37°C was around 7.0. Serine proteinase inhibitors, such as phenylmethylsulfonyl fluoride and diisopropylfluorophosphate, inhibited the enzyme activity in both strains. The divalents cations Ca 2+ , Mg 2+ , and Mn 2+ were activators, while Zn 2+ and Cu 2+ were inhibitors. β-Casein was hydrolyzed more rapidly than α s1 -casein. The results of DNA hybridization and immunoblot studies suggested that the S. thermophilus cell wall proteinase and the lactococcal proteinase are not closely related.

Stability and Specificity of the Cell Wall-Associated Proteinase from Lactococcus lactis subsp. cremoris H2 Released by Treatment with Lysozyme in the Presence of Calcium Ions

The cell wall-associated proteinase from Lactococcus lactis subsp. cremoris H2 (isolate number 4409) was released from the cells by treatment with lysozyme, even in the presence of 50 mM calcium chloride. Cell lysis during lysozyme treatment was minimal. The proteinase activity released by lysozyme treatment fractionated on ion-exchange chromatography as three main forms, the molecular masses of which were determined by gel exclusion chromatography and polyacrylamide gel electrophoresis. Two of the enzyme forms released, 137 and 145 kDa, were the same as those released by incubation of cells in calcium-free phosphate buffer. In the presence of calcium, lysozyme treatment also resulted in the release of a 180-kDa enzyme molecule. The total proteinase activity released by lysozyme treatment (in the presence or absence of calcium) was not only greater than that released by phosphate buffer but was also greater than that initially detectable on the surface of whole cells, suggesting an unmasking of enzyme on the cell surface. The presence of calcium during release treatment resulted in increased stability of the crude enzyme preparations. For the proteinase preparation released by using lysozyme with 50 mM CaCl 2 , the half-life of proteinase activity at 37°C was 39 h, compared with 0.22 h for the calcium-free phosphate buffer-released preparation. In all cases, maximum stability was observed at pH 5.5. Comparison of β-casein hydrolysis by the three forms of the enzyme showed that the products of short-term (5- to 30-min) digestions were very similar, although subtle differences were detected with the 180-kDa form.

Localization of Peptidases in Lactococci

The localization of two aminopeptidases, an X-prolyl-dipeptidyl aminopeptidase, an endopeptidase, and a tripeptidase in Lactococcus lactis was studied. Polyclonal antibodies raised against each purified peptidase are specific and do not cross-react with other peptidases. Experiments were performed by immunoblotting after cell fractionation and by electron microscopy of immunogold-labeled peptidases. All peptidases were found to be intracellular. However, immunogold studies showed a peripheral labeling of the X-prolyl-dipeptidyl aminopeptidase, the tripeptidase, and the endopeptidase. This peripheral location was further supported by the detection of these three enzymes in cell membrane fractions in which none of the two aminopeptidases was present.

Purification and characterization of an endopeptidase from Lactococcus lactis subsp. cremoris Wg2

An endopeptidase has been purified to homogeneity from a crude cell extract of Lactococcus lactis subsp. cremoris Wg2 by a procedure that includes diethyl-aminoethane-Sephacel chromatography, phenyl-Sepharose chromatography, hydroxylapatite chromatography, and fast protein liquid chromatography over an anion-exchange column and a hydrophobic-interaction column. Gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated a molecular mass of the purified enzyme of 70,000 Da. The endopeptidase can degrade several oligopeptides into various tetra-, tri-, and dipeptides. The endopeptidase has no aminopeptidase, carboxypeptidase, dipeptidase, or tripeptidase activity. It is optimally active at pH 6.0 to 6.5 and in the temperature range of 30 to 38 degrees C. The enzyme is inactivated by the chemical agents 1,10-phenanthroline, ethylenedinitrilotetraacetate, beta-mercaptoethanol, and phenylmethylsulfonyl fluoride and is inhibited by Cu2+ and Zn2+. The ethylenedinitrilotetraacetate- or 1,10-phenanthroline-treated enzyme can be reactivated by Co2+. Immunoblotting with specific antibodies raised against the purified endopeptidase indicated that the enzyme is also present in other Lactococcus spp., as well as in Lactobacillus spp. and Streptococcus salivarius subsp. thermophilus.

Autoproteolysis of the Extracellular Serine Proteinase of Lactococcus lactis subsp. cremoris Wg2

The molecular masses of purified extracellular serine proteinase of a number of Lactococcus lactis strains vary significantly, and these molecular mass values do not correspond to the values estimated on the basis of genetic data. The discrepancies can only partially be explained by N-terminal processing during maturation of the precursor enzyme and by C-terminal cleaving during the release from the cell envelope. With a monoclonal antibody that binds in the active site region of the L. lactis proteinase, the processing of the released proteinase was followed. At 30�C the proteinase was degraded with a concomitant loss of β-casein hydrolytic activity. In the presence of CaCl 2 , proteinase degradation was inhibited, and new degradation products were detected. The specific serine proteinase inhibitors phenylmethylsulfonyl fluoride and diisopropylfluorophosphate also inhibited proteinase degradation. Two major high-molecular-mass proteinase fragments (165 and 90 kDa) were found to have the same N-terminal amino acid sequence as the mature proteinase, i.e., [Asp-1-Ala-2-Lys-3-Ala-4-Asn-5-Ser-6, indicating that both fragments were formed by cleavage at the C terminus. The N terminus of a proteinase fragment with low molecular mass (58 kDa) started with Gln-215. In this fragment part of the active site region was eliminated, suggesting that it is proteolytically inactive. Unlike larger fragments, this 58-kDa fragment remained intact after prolonged incubations. These results indicate that autoproteolysis of the L. lactis subsp. cremoris Wg2 proteinase ultimately leads to inactivation of the proteinase by deletion of the active site region.

Genetics of the proteolytic system of lactic acid bacteria

The proteolytic system of lactic acid bacteria is of eminent importance for the rapid growth of these organisms in protein-rich media. The combined action of proteinases and peptidases provides the cell with small peptides and essential amino acids. The amino acids and peptides thus liberated have to be translocated across the cytoplasmic membrane. To that purpose, the cell contains specific transport proteins. The internalized peptides are further degraded to amino acids by intracellular peptidases. The world-wide economic importance of the lactic acid bacteria and their proteolytic system has led to an intensive research effort in this area and a considerable amount of biochemical data has been collected during the last two decades. Since the development of systems to genetically manipulate lactic acid bacteria, data on the genetics of enzymes and processes involved in proteolysis are rapidly being generated. In this review an overview of the latest genetic data on the proteolytic system of lactic acid bacteria will be presented. As most of the work in this field has been done with lactococci, the emphasis will, inevitably, be on this group of organisms. Where possible, links will be made with other species of lactic acid bacteria.

Purification and Characterization of a Tripeptidase from Lactococcus lactis subsp. cremoris Wg2

A tripeptidase from a cell extract of Lactococcus lactis subsp. cremoris Wg2 has been purified to homogeneity by DEAE-Sephacel and phenyl-Sepharose chromatography followed by gel filtration over a Sephadex G-100 SF column and a high-performance liquid chromatography TSK G3000 SW column. The enzyme appears to be a dimer with a molecular weight of between 103,000 and 105,000 and is composed of two identical subunits each with a molecular weight of about 52,000. The tripeptidase is capable of hydrolyzing only tripeptides. The enzyme activity is optimal at pH 7.5 and at 55°C. EDTA inhibits the activity, and this can be reactivated with Zn 2+ , Mn 2+ , and partially with Co 2+ . The reducing agents dithiothreitol and β-mercaptoethanol and the divalent cation Cu 2+ inhibit tripeptidase activity. Kinetic studies indicate that the peptidase hydrolyzes leucyl-leucyl-leucine with a K m of 0.15 mM and a V max of 151 μmol/min per mg of protein.

Purification and Characterization of an Aminopeptidase from Lactococcus lactis subsp. cremoris Wg2

An aminopeptidase was purified to homogeneity from a crude cell extract of Lactococcus lactis subsp. cremoris Wg2 by a procedure that included diethyl-aminoethane-Sephacel chromatography, phenyl-Sepharose chromatography, gel filtration, and high-performance liquid chromatography over an anion-exchange column. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the purified enzyme showed a single protein band with a molecular weight of 95,000. The aminopeptidase was capable of degrading several peptides by hydrolysis of the N-terminal amino acid. The peptidase had no endopeptidase or carboxypeptidase activity. The aminopeptidase activity was optimal at pH 7 and 40�C. The enzyme was completely inactivated by the p -chloromecuribenzoate mersalyl, chelating agents, and the divalent cations Cu 2+ and Cd 2+ . The activity that was lost by treatment with the sulfhydryl-blocking reagents was restored with dithiothreitol or β-mercapto-ethanol, while Zn 2+ or Co 2+ restored the activity of the 1,10-phenantroline-treated enzyme. Kinetic studies indicated that the enzyme has a relatively low affinity for lysyl- p -nitroanilide ( K m , 0.55 mM) but that it can hydrolyze this substrate at a high rate ( V max , 30 μmol/min per mg of protein).

Mechanism of Proteinase Release from Lactococcus lactis subsp. cremoris Wg2

The procedure generally used for the isolation of extracellular, cell-associated proteinases of Lactococcus lactis species is based on the release of the proteinases by repeated incubation and washing of the cells in a Ca 2+ -free buffer. For L. lactis subsp. cremoris Wg2, as many as five incubations for 30 min at 29°C are needed in order to liberate 95% of the proteinase. Proteinase release was not affected by chloramphenicol, which indicates that release is not the result of protein synthesis during the incubations. Ca 2+ inhibited, while ethylene glycol-bis(β-aminoethyl ether)- N,N,N′,N′ -tetraacetic acid (EGTA) stimulated, proteinase release from the cells. The pH optimum for proteinase release ranged between 6.5 and 7.5, which was higher than the optimum pH of the proteinase measured for casein hydrolysis (i.e., 6.4). Treatment of cells with the serine proteinase inhibitor phenylmethylsulfonyl fluoride prior to the incubations in Ca 2+ -free buffer reduced the release of the proteinase by 70 to 80%. The residual proteinase remained cell associated but could be removed by the addition of active L. lactis subsp. cremoris Wg2 proteinase. This suggests that proteinase release from cells of L. lactis subsp. cremoris Wg2 is the result of autoproteolytic activity. From a comparison of the N-terminal amino acid sequence of the released proteinase with the complete amino acid sequence determined from the nucleotide sequence of the proteinase gene, a protein of 180 kilodaltons would be expected. However, a proteinase with a molecular weight of 165,000 was found, which indicated that further hydrolysis had occurred at the C terminus.

Molecular characterization of a cell wall-associated proteinase gene from Streptococcus lactis NCDO763

Streptococcus lactis NCDO763 harbours a plasmid designated pLP763. The cells harbouring pLP763 are able to grow to a higher density in milk because of their proteinase-positive phenotype (Prt+). The 6.2 kb HindIII-PstI fragment from pLP763 was found to be responsible for the Prt+ phenotype. The DNA fragment contains an incomplete large open reading frame (ORF). Further sequence analysis downstream from the PstI site revealed that the ORF consists of 5706 bases. It was found that the deduced amino acid sequence consisting of 1902 amino acid residues was extremely similar to that of the Wg2 proteinase, a serine protease from Streptococcus cremoris, suggesting that both genes were derived from a common ancestral gene.

Monoclonal Antibodies to the Cell-Wall-Associated Proteinase of Lactococcus lactis subsp. cremoris Wg2

Twelve monoclonal antibodies directed to the cell-wall-associated proteinase of Lactococcus lactis subsp. cremoris Wg2 were isolated after immunization of BALB/c mice with a partially purified preparation of the proteinase. The monoclonal antibodies reacted with the 126-kilodalton proteinase band in a Western immunoblot. All but one of the monoclonal antibodies reacted with protein bands with a molecular weight below 126,000, possibly degradation products of the proteinase. The monoclonal antibodies could be divided into six groups according to their different reactions with the proteinase degradation products in the Western blot. Different groups of monoclonal antibodies reacted with different components of the L. lactis subsp. cremoris Wg2 proteinase. Crossed immunoelectrophoresis showed that monoclonal antibody groups I, II, and III react with proteinase component A and that groups IV, V, and VI react with proteinase component B. The isolated monoclonal antibodies cross-reacted with the proteinases of other L. lactis subspecies. Monoclonal antibodies of group IV cross-reacted with proteinase component C of other L. lactis subsp. cremoris strains. The molecular weight of the proteinase attached to the cells of L. lactis subsp. cremoris Wg2 was 200,000, which is different from the previously reported values. This could be analyzed by immunodetection of the proteinase on a Western blot. This value corresponds to the molecular weight calculated from the amino acid sequence of the cloned L. lactis subsp. cremoris Wg2 proteinase gene.