Purification and Some Properties of a Membrane-Bound Aminopeptidase A from Streptococcus cremoris

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).

Purification and Characterization of an Aminopeptidase from Lactococcus lactis subsp. cremoris AM2

An aminopeptidase was purified from cell extracts of Lactococcus lactis subsp. cremoris AM2 by ion-exchange chromatography. After electrophoresis of the purified enzyme in the presence or absence of sodium dodecyl sulfate, one protein band was detected. The enzyme was a 300-kilodalton hexamer composed of identical subunits not linked by disulfide bridges. Activity was optimal at 40°C and pH 7 and was inhibited by classical thiol group inhibitors. The aminopeptidase hydrolyzed naphthylamide-substituted amino acids, as well as dipeptides and tripeptides. Longer protein chains such as the B chain of insulin were hydrolyzed, but at a much slower rate. The Michaelis constant ( K m ) and the maximal rate of hydrolysis ( V max ) were, respectively, 4.5 mM and 3,600 pkat/mg for the substrate l -histidyl-β-naphthylamide. Amino acid analysis showed that the enzyme contained low levels of hydrophobic residues. The partial N-terminal sequence of the first 19 residues of the mature enzyme was determined. Polyclonal antibodies were obtained from the purified enzyme, and after immunoblotting, there was no cross-reaction between these antibodies and other proteins in the crude extract.

Isolation and Characterization of Aminopeptidase-Deficient Lactobacillus bulgaricus Mutants

Lactobacillus bulgaricus CNRZ 397 is able to hydrolyze many amino-acyl- and dipeptidyl-β-naphthylamides. Analysis of heat inactivation kinetics, protease inhibitor effects, and the subcellular location of aminopeptidase (AP) activities from the parental strain and mutant derivatives dificient in alanyl- or leucyl-β-naphthylamide hydrolysis pointed out the existence of four APs. All mutants isolated were totally deficient in AP II, a cell wall metallo-enzyme with a broad substrate specificity but that is specifically responsible for lysyl-AP activity and is characterized by a molecular mass of 95,000 daltons. AP I and AP III are cytoplasmic enzymes that exhibit arginyl-AP activity; both enzymes are inducible during growth in rich peptide MRS medium (Difco Laboratories, Detroit, Mich.). The existence of a fourth AP (AP IV) that is involved in leucyl-AP activity was suggested. Moreover, we showed that X-prolyl-dipeptidyl-AP activity, which was not catalyzed by an AP, involved an enzyme(s) that is controlled by a regulatory mechanism that is common to that of AP II.

Efficient Implementation of Consecutive Reactions by Peptidases at the Periphery of the Streptococcus cremoris Membrane

Activities detectable in Streptococcus cremoris with the chymotrypsin substrate N -glutaryl- l -phenylalanine-4-nitroanilide and formerly designated endopeptidases P37 and P50 (F. A. Exterkate, Appl. Environ. Microbiol. 47:177-183, 1984) are both coupled peptidase reactions. These coupled reactions involve a membrane-bound, restricted l -α-glutamyl aminopeptidase which is responsible for the initial release of the glutaryl moiety. The subsequent reaction is catalyzed by either a so-called low-temperature or a high-temperature phenylalanyl aminopeptidase activity, both located at the outside surface of the membrane. Altered microenvironmental conditions created by the membrane-perturbing action of n -butanol or obtained by solubilization resulted in the removal of a restriction on the activity of l -α-glutamyl aminopeptidase and in a less efficient functioning of the coupled reactions; a long transient phase occurred before the steady state was reached. The results suggest that the in situ spatial organization is conducive to an efficient attuning of at least three peptidases which are located at the outer membrane surface and in the membrane. The possibility that peptidases in these locations exist as a cluster with physiological significance is discussed in relation to growth of S. cremoris in milk.