Purification, molecular cloning, and expression of lipase from Pseudomonas aeruginosa

Bacterial lipases

Many different bacterial species produce lipases which hydrolyze esters of glycerol with preferably long-chain fatty acids. They act at the interface generated by a hydrophobic lipid substrate in a hydrophilic aqueous medium. A characteristic property of lipases is called interfacial activation, meaning a sharp increase in lipase activity observed when the substrate starts to form an emulsion, thereby presenting to the enzyme an interfacial area. As a consequence, the kinetics of a lipase reaction do not follow the classical Michaelis-Menten model. With only a few exceptions, bacterial lipases are able to completely hydrolyze a triacylglycerol substrate although a certain preference for primary ester bonds has been observed. Numerous lipase assay methods are available using coloured or fluorescent substrates which allow spectroscopic and fluorimetric detection of lipase activity. Another important assay is based on titration of fatty acids released from the substrate. Newly developed methods allow to exactly determine lipase activity via controlled surface pressure or by means of a computer-controlled oil drop tensiometer. The synthesis and secretion of lipases by bacteria is influenced by a variety of environmental factors like ions, carbon sources, or presence of non-metabolizable polysaccharides. The secretion pathway is known for Pseudomonas lipases with P. aeruginosa lipase using a two-step mechanism and P. fluorescens lipase using a one-step mechanism. Additionally, some Pseudomonas lipases need specific chaperone-like proteins assisting their correct folding in the periplasm. These lipase-specific foldases (Lif-proteins) which show a high degree of amino acid sequence homology among different Pseudomonas species are coded for by genes located immediately downstream the lipase structural genes. A comparison of different bacterial lipases on the basis of primary structure revealed only very limited sequence homology. However, determination of the three-dimensional structure of the P. glumae lipase indicated that at least some of the bacterial lipases will presumably reveal a conserved folding pattern called the alpha/beta-hydrolase fold, which has been described for other microbial and human lipases. The catalytic site of lipases is buried inside the protein and contains a serine-protease-like catalytic triad consisting of the amino acids serine, histidine, and aspartate (or glutamate). The Ser-residue is located in a strictly conserved beta-epsilon Ser-alpha motif. The active site is covered by a lid-like alpha-helical structure which moves away upon contact of the lipase with its substrate, thereby exposing hydrophobic residues at the protein's surface mediating the contact between protein and substrate.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of a catalytic antibody for stereoselective ester hydrolysis--a catalytic residue and mode of product inhibition

A catalytic antibody which catalyzes stereoselective ester hydrolysis was characterized, and the role of a catalytic Arg residue is discussed in terms of product inhibition. A monoclonal antibody 1C7 generated against the phosphonate 1 was highly stereoselective for (R)-isomer in hydrolyzing racemic ester 2. However, the reaction was almost stoichiometric due to strong inhibition by the product acid 3. One Arg residue in the antibody combining site was essential to the catalysis, and the same Arg was expected to play a dominant role in product inhibition by charge interaction with the negatively charged product acid. Indeed, the antibody experienced much less product inhibition with the hydrolysis of a carbonate ester 7, which yields a neutral alcohol 8 devoid of a negative charge, and exhibited at least 100 turnovers without any loss of activity. In addition, high stereoselectivity for (R)-isomer was still retained. The amino acid sequence and computer modeling of the variable domain of 1C7 suggested that Arg97 in the complementarity-determining region (CDR) of heavy chain was the putative catalytic residue.

Pseudomonas lipases: molecular genetics and potential industrial applications

Lipases are esterases able to hydrolyze water-insoluble esters such as long-chain triglycerides. These enzymes also catalyze the formation of esters (esterification) and the exchange of ester bonds (transesterification) when present in nonaqueous media. Lipases display a high degree of specificity and enantioselectivity for esterification and transesterification reactions, and thus their potential uses in industry are very wide. These potential industrial applications have been an important driving force for lipase research during the last several years, and in particular for the study of lipases produced by microorganisms. Pseudomonas lipases are very interesting because they display special biochemical characteristics not common among the lipases produced by other microorganisms, such as their thermoresistance and activity at alkaline pHs. Recently, several Pseudomonas genes have been cloned and sequenced, and the regulation of their expression is beginning to be understood. The molecular genetic approach to the study of Pseudomonas lipases will permit the construction of recombinant strains with increased lipase productivity and will provide the opportunity to modify these enzymes to suit particular industrial applications.

A periplasmic intermediate in the extracellular secretion pathway of Pseudomonas aeruginosa exotoxin A

Pseudomonas aeruginosa exotoxin A is synthesized with a secretion signal peptide typical of proteins whose final destination is the periplasm. However, exotoxin A is released from the cell without a detectable periplasmic pool, suggesting that additional determinants in this protein are important for recognition by a specialized machinery of extracellular secretion. The role of the N terminus of the mature exotoxin A in this recognition was investigated. A series of exotoxin A proteins with amino acid substitutions for the glutamic acid pair at the +2 and +3 positions were constructed by mutagenesis of the exotoxin A gene. These N-terminal acidic residues of the mature exotoxin A protein were found to be important not only for efficient processing of the precursor protein but also for extracellular localization of the toxin. The mutated exotoxin A proteins, in which a glutamic acid at the +2 position was replaced by a lysine or a double substitution of lysine and glutamine for the pair of adjacent glutamic acids, accumulated in precursor forms in the mixed cytoplasmic and membrane fractions, which was not seen with the wild-type exotoxin A. The processing of the precursor form of one exotoxin A mutant, in which the glutamic acid at the +2 position was replaced with a glutamine, was not affected. Moreover, a substantial fraction of the mature forms of all three mutants of exotoxin A accumulated in the periplasm, while wild-type exotoxin A could be detected only extracellularly. The periplasmic pools of these variants of exotoxin A could therefore represent the intermediate state during extracellular secretion. The signal for extracellular localization may be located in a small region near the amino terminus of the mature protein or could consist of several regions that are brought together after the polypeptide has folded. Alternatively, the acidic residues may be important for ensuring a conformation essential for exotoxin A to traverse the outer membrane.

Purification and characterization of Pseudomonas fluorescens SIK W1 lipase expressed in Escherichia coli

Pseudomonas fluorescens SIK W1 lipase was expressed as a form of inclusion bodies in Escherichia coli, which was equivalent to 46% of total cell protein. The inclusion bodies isolated from other cell components were solubilized in the buffer containing 8 M urea and then refolded by diluting urea. The lipase with active conformation was purified by hydrophobic interaction chromatography, gel filtration, anion-exchange chromatography and hydroxyapatite chromatography from the refolded sample. By these purification steps, a single band for active lipase was detected on non-reducing SDS-PAGE and 10-fold purification was attained on the basis of specific activity. Specific activity of the purified lipase toward olive oil emulsion was found to be 7395 units per mg protein. The optimum pH and temperature of the lipase were pH 8.5 and 45-55 degrees C, respectively. The lipase showed higher lipolytic activity toward tricaproin (C6) and tricaprylin (C8) among the triacylglycerols examined and preferentially hydrolyzed ester bond of 1- and 3-position of triolein. Lipase activity was greatly increased by approx. 6-fold and stability for pH was shifted to alkaline pH by Ca2+ ion. The lipase was inhibited by Hg2+, Ag2+, p-chloromercuribenzoate, diethylpyrocarbonate and sodium dodecyl sulfate.

An accessory gene, lipB, required for the production of active Pseudomonas glumae lipase

Pseudomonas glumae PG1 is able to secrete lipase into the extracellular medium. The lipase is produced as a precursor protein, with an N-terminal signal sequence. A second open reading frame (ORF) was found immediately downstream of the lipase structural gene, lipA, a situation found for the lipases of some other Pseudomonas species. Inactivation of this ORF resulted in a lipase-negative phenotype, indicating its importance in the production of active extracellular lipase. The ORF, lipB, potentially encodes a protein of 353-amino-acid residues, having a hydrophobic N-terminal (amino acids 1 to 90) and a hydrophilic C-terminal part. As a first step in determining the role of LipB, its subcellular location was determined. The protein was found to fractionate with the inner membranes. The expression of fusions of lipB fragments with phoA revealed an N(in)-C(out) topology for the LipB protein, which was confirmed by protease accessibility studies on EDTA-permeabilized cells and on inverted inner membrane vesicles. These and other results indicate that most of the LipB polypeptide is located in the periplasm and anchored to the inner membrane by an N-terminal transmembrane helix, located between amino acids 19 and 40.

Role of the lipB gene product in the folding of the secreted lipase of Pseudomonas glumae

The LipB protein of Pseudomonas glumae is essential for the production of active extracellular lipase encoded by the lipA gene. When lipase is overproduced in P. glumae in the absence of a functional lipB gene, the enzyme accumulates intracellularly in an inactive conformation. Heterologous expression of the lipase in Pseudomonas aeruginosa, Bacillus subtilis and Escherichia coli indicated that LipB is not directly involved in the translocation of the lipase across the inner or outer membrane. However, the presence of LipB was essential for obtaining active lipase and had a profound influence on the stability of the protein to proteolytic degradation. Inactive lipase, produced in the absence of LipB could be activated in vitro by unfolding and refolding, which demonstrates that LipB activity is not responsible for an essential covalent modification of the enzyme. We propose that LipB is a lipase-specific foldase. Furthermore, proper folding of the lipase in the periplasm appears to be essential for Xcp-mediated translocation across the outer membrane.

Pseudomonas lipases: biochemical properties and molecular cloning

Lipases (glycerol ester hydrolases; EC 3.1.1.3) are important enzymes which, due to their ability to catalyze a number of reactions, are receiving considerable interest from both academia and industry. The bacterial genus Pseudomonas is a prolific producer of a number of extracellular enzymes including lipase. This review summarizes the biochemical properties and recent advances in the molecular genetic analysis of a wide variety of Pseudomonas lipases. In particular, a comparison is made between the amino acid sequences of the various lipases as well as their secondary gene products, which are thought to be essential for secretion of the enzyme.

Lipase from Pseudomonas aeruginosa. Production in Escherichia coli and activation in vitro with a protein from the downstream gene

The lipase gene from Pseudomonas aeruginosa TE3285 is followed by another gene, lipB. The lipase gene was expressed in Escherichia coli BL21(DE3)pLysS using the T7 RNA polymerase expression system. The mature lipase was accumulated as inclusion bodies at 42% of the total cell proteins. The inclusion bodies were solubilized with 8 M urea, but lipase activity was not detected in the solubilized preparation containing 85% lipase protein even after removing urea by dialysis. The lipB gene, positioned downstream of the lipase gene and thought to be necessary for the expression of the lipase gene, was expressed in Escherichia coli JM109 as a fusion with the glutathione transferase gene from Schistosoma japonicum. The fusion protein was partially purified on glutathione-agarose beads to 36% purity. Incubated with the fusion protein at a molar ratio of 1:1 at 4 degrees C for 24 h, the solubilized lipase showed lipase activity of about a tenth that of the purified lipase prepared from Pseudomonas aeruginosa TE3285. Magnesium ions and ATP were not essential but increased the activation. When the fusion protein was treated with thrombin to release the glutathione transferase part, it retained its activity. The lipase activation with lipB protein probably proceeds to form a 1:1 complex with the inactive, solubilized lipase protein but by a different mode from known chaperones.

Activation of a bacterial lipase by its chaperone

The gene lipA of Pseudomonas cepacia DSM 3959 encodes a prelipase from which a signal peptide is cleaved during secretion, producing a mature extracellular lipase. Expression of lipase in several heterologous hosts depends on the presence of another gene, limA, in cis or in trans. Lipase protein has been overproduced in Escherichia coli in the presence and absence of the lipase modulator gene limA. Therefore, limA is not required for the transcription of lipA or for the translation of the lipA mRNA. However, no lipase activity is observed in the absence of limA. limA has been overexpressed and encodes a 33-kDa protein, Lim. If lipase protein is denatured in 8 M urea and the urea is removed by dialysis, lipase activity is quantitatively recovered provided Lim protein is present during renaturation. Lip and Lim proteins form a complex precipitable either by an anti-lipase or anti-Lim antibody. The Lim protein has therefore the properties of a chaperone.