[General properties and specificity of action of a neutral intracellular endopeptidase from Streptococcus thermophilus]

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.

Proteinase, peptidase and esterase activities of cell-free extracts from wild strains ofLactobacillus delbrueckiisubsp.bulgaricusandStreptococcus salivariussubsp.thermophilusisolated from traditional Greek yogurt

Proteinase, peptidase and esterase activities were detected in cell-free extracts of fourLactobacillus delbrueckiisubsp.bulgaricusand fourStreptococcus salivariussubsp.thermophilusstrains. Post-electrophoretic detection was based on hydrolysis of L-leucine-βnaphthylamide and α-naphthylacetate. The substrates L-leucine-p–nitroanilide,N-acetyl-L-alanine-p–nitroanilide, 2-nitrophenylbutyrate and 4-nitrophenylbutyrate were used for the spectrophotometric detection of the enzymes. Estimation of total proteolytic activity was based on hydrolysis of bovine whole casein. Interesting differences were observed between the two French strains and those isolated from traditional Greek yogurt.

Characterization of a Bifidobacterium longum BORI dipeptidase belonging to the U34 family

A dipeptidase was purified from a cell extract of Bifidobacterium longum BORI by ammonium sulfate precipitation and chromatography on DEAE-cellulose and Q-Sepharose columns. The purified dipeptidase had a molecular mass of about 49 kDa and was optimally active at pH 8.0 and 50 degrees C. The enzyme was a strict dipeptidase, being capable of hydrolyzing a range of dipeptides but not tri- and tetrapeptides, p-nitroanilide derivatives of amino acids, or N- or C-terminus-blocked dipeptides. A search of the amino acid sequence of an internal tryptic fragment against protein sequences deduced from the total genome sequence of B. longum NCC2705 revealed that it was identical to an internal sequence of the dipeptidase gene (pepD), which comprised 1,602 nucleotides encoding 533 amino acids with a molecular mass of 60 kDa, and thereby differed considerably from the 49-kDa mass of the purified dipeptidase. To understand this discrepancy, pepD was cloned into an Escherichia coli expression vector (pBAD-TOPO derivative) to generate the recombinant plasmids pBAD-pepD and pBAD-pepD-His (note that His in the plasmid designation stands for a polyhistidine coding region). Both plasmids were successfully expressed in E. coli, and the recombinant protein PepD-His was purified using nickel-chelating affinity chromatography and reconfirmed by internal amino acid sequencing. The PepD sequence was highly homologous to those of the U34 family of peptidases, suggesting that the B. longum BORI dipeptidase is a type of cysteine-type N-terminal nucleophile hydrolase and has a beta-hairpin motif similar to that of penicillin V acylase, which is activated by autoproteolytic processing.

Purification, characterisation, cloning and sequencing of the gene encoding oligopeptidase PepO from Streptococcus thermophilus A

The oligopeptidase PepO from Streptococcus thermophilus A was purified to protein homogeneity by a five-step chromatography procedure. It was estimated to be a serine metallopeptidase of 70 kDa, with maximal activity at pH 6.5 and 41 degrees C. PepO has endopeptidase activity on oligopeptides composed of between five and 30 amino acids. PepO was demonstrated to be active and stable at the pH, temperature and salt concentrations found in Swiss-type cheese during ripening. Using a battery of PCR techniques, the pepO gene was amplified, subcloned and sequenced, revealing an open reading frame of 1893 nucleotides. The amino acid sequence analysis of the pepO gene-translation product shows homology with PepO enzymes from other lactic acid bacteria and contains the signature sequence of the metallopeptidase family.

PepS from Streptococcus thermophilus. A new member of the aminopeptidase T family of thermophilic bacteria

The proteolytic system of lactic acid bacteria is essential for bacterial growth in milk but also for the development of the organoleptic properties of dairy products. Streptococcus thermophilus is widely used in the dairy industry. In comparison with the model lactic acid bacteria Lactococcus lactis, S. thermophilus possesses two additional peptidases (an oligopeptidase and the aminopeptidase PepS). To understand how S. thermophilus grows in milk, we purified and characterized this aminopeptidase. PepS is a monomeric metallopeptidase of approximately 45 kDa with optimal activity in the range pH 7.5-8.5 and at 55 degrees C on Arg-paranitroanilide as substrate. PepS exhibits a high specificity towards peptides possessing arginine or aromatic amino acids at the N-terminus. From the N-terminal protein sequence of PepS, we deduced degenerate oligonucleotides and amplified the corresponding gene by successive PCR reactions. The deduced amino-acid sequence of the PepS gene has high identity (40-50%) with the aminopeptidase T family from thermophilic and extremophilic bacteria; we thus propose the classification of PepS from S. thermophilus as a new member of this family. In view of its substrate specificity, PepS could be involved both in bacterial growth by supplying amino acids, and in the development of dairy products' flavour, by hydrolysing bitter peptides and liberating aromatic amino acids which are important precursors of aroma compounds.

Purification, characterization, gene cloning, sequencing, and overexpression of aminopeptidase N from Streptococcus thermophilus A

The general aminopeptidase PepN from Streptococcus thermophilus A was purified to protein homogeneity by hydroxyapatite, anion-exchange, and gel filtration chromatographies. The PepN enzyme was estimated to be a monomer of 95 kDa, with maximal activity on N-Lys-7-amino-4-methylcoumarin at pH 7 and 37 degrees C. It was strongly inhibited by metal chelating agents, suggesting that it is a metallopeptidase. The activity was greatly restored by the bivalent cations Co2+, Zn2+, and Mn2+. Except for proline, glycine, and acidic amino acid residues, PepN has a broad specificity on the N-terminal amino acid of small peptides, but no significant endopeptidase activity has been detected. The N-terminal and short internal amino acid sequences of purified PepN were determined. By using synthetic primers and a battery of PCR techniques, the pepN gene was amplified, subcloned, and further sequenced, revealing an open reading frame of 2,541 nucleotides encoding a protein of 847 amino acids with a molecular weight of 96,252. Amino acid sequence analysis of the pepN gene translation product shows high homology with other PepN enzymes from lactic acid bacteria and exhibits the signature sequence of the zinc metallopeptidase family. The pepN gene was cloned in a T7 promoter-based expression plasmid and the 452-fold overproduced PepN enzyme was purified to homogeneity from the periplasmic extract of the host Escherichia coli strain. The overproduced enzyme showed the same catalytic characteristics as the wild-type enzyme.

The occurrence of two intracellular oligoendopeptidases in Lactococcus lactis and their significance for peptide conversion in cheese

Two intracellular oligopeptide-preferring endopeptidases have been detected in Lactococcus lactis. A neutral thermolysin-like oligoendopeptidase (NOP) has been purified to homogeneity and an alkaline oligoendopeptidase has been partially purified. The specificity of the oligoendopeptidases towards important intermediary cheese peptides, produced by chymosin action on the caseins, clearly differs from that of the cell-envelope proteinase (CEP). NOP is active under conditions prevailing in cheese and contributes to initial proteolysis in a young cheese. It probably plays a crucial role in the degradation of an important bitter peptide in cheese, the beta-casein 193-209 fragment. The relatively low activity of the alkaline endopeptidase is further suppressed in cheese by the highly competitive actions of NOP and CEP.

Purification and characterization of a general aminopeptidase (St-PepN) from Streptococcus salivarius ssp. thermophilus CNRZ 302

A general aminopeptidase (St-PepN) was purified from an intracellular extract of Streptococcus salivarius ssp. thermophilus CNRZ 302 by ion-exchange chromatography and hydrophobic interaction chromatography. Gel electrophoresis of the purified enzyme in denaturing or nondenaturating conditions showed a single protein band. The enzyme is a monomer with a molecular mass of 97 kDa. Its activity is maximal at pH 7 and 36 degrees C and is completely abolished by CuCl2 and ZnCl2. The enzyme is strongly inhibited by metal-chelating reagents, such as EDTA and o-phenanthroline, which suggests that St-PepN is a metalloenzyme. The enzyme showed activity toward p-nitroanilide derivatives or dipeptides and tripeptides and showed a preference for hydrophobic or basic amino acids at the N-terminal position. Longer peptide chains, such as the B-chain of insulin, glucagon, or peptides generated by the hydrolysis of caseins, were degraded, too. The sequence of the first 21 residues of the mature enzyme was determined and showed high homology with that of the aminopeptidase PepN isolated from Lactococcus lactis ssp. cremoris Wg2. The properties of the enzyme are compared with those of corresponding enzymes of other species of lactic acid bacteria.

Purification and partial characterization of an intracellular aminopeptidase from Streptococcus salivarius subsp. thermophilus strain ACA-DC 114

An intracellular aminopeptidase from Streptococcus salivarius subsp. thermophilus strain ACA-DC 114, isolated from traditional Greek yoghurt, was purified by chromatography on DEAE-cellulose and Sephadex G-100. The enzyme had a molecular weight of 89,000. It was active over a pH range 4.5-9.5 and had optimum activity on L-lysyl-4-nitroanilide at pH 6.5 and 35 degrees C with Km = 1.80 mmol/l; above 55 degrees C the enzyme activity declined rapidly. The aminopeptidase was capable of degrading substrates by hydrolysis of the N-terminal amino acid; it had very low endopeptidase and no carboxypeptidase activity. The enzyme was strongly inactivated by EDTA. Serine and sulphydryl group reagents had no effect on enzyme activity.

Growth characteristics of enterococci isolated from Laban Rayeb

Eleven strains of Enterococcus (Streptococcus) faecalis isolated from Laban Rayeb, a type of fermented milk, had been studied when grew in milk. Enterococcus (Streptococcus) faecalis strains 19 and 22 were the most active strains in the production of lactic acid, acetaldehyde and diacetyl. Both strains exhibited a high proteolytic activity besides a good clean flavour. Based on the obtained results, Enterococcus (Streptococcus) faecalis 19 and 22 could be used to bring about the lactic fermentation during the manufacture of some cultured dairy products.

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.

Hydrolysis of beta-casein and peptides by intracellular neutral protease of Streptococcus diacetylactis

The endopeptidase activity of mesophilic streptococci was characterized further by investigating the specificity of an intracellular endopeptidase from Streptococcus diacetylactis for beta-casein, derived peptides, and bradykinin. The inhibitory action of phosphoramidon as well as direct determinations of metal content showed this enzyme was a metalloprotein. Hydrolysis of native beta-casein was relatively low. Peptides obtained from the fraction soluble at pH 4.6 led to the demonstration that Pro186-Ile187 and Ala189-Phe190 were hydrolyzed by the enzyme. Two peptides derived from beta-casein by the action of chymosin were hydrolyzed efficiently: we observed hydrolysis of Lys176-Ala177, Lys169-Val170, and Pro206-Ile207. The Pro7-Phe8 bond of bradykinin was hydrolyzed rapidly, showing that this enzyme was efficient for the hydrolysis of prolyl peptide bonds. The protease was slightly less sensitive to phosphoramidon than was thermolysin. Metal analyses showed the enzyme contained 580 microgram of zinc and 4,760 microgram of calcium per gram protein. This protease is thus a true metalloenzyme (E.C.3.4.24.4), and its action may complete the hydrolysis initiated by chymosin remaining active in cheese curd by hydrolyzing peptides released by chymosin.