BACTERIAL LIPASES

Many different bacterial species produce lipases which hydrolyze ester s 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 interfa cial activation, meaning a sharp increase in lipase activity observed when the substrate starts to form an emulsion, thereby presenting to t he enzyme an interfacial area. As a consequence, the kinetics of a lip ase reaction do not follow the classical Michaelis-Menten model. With only a few exceptions, bacterial lipases are able to completely hydrol yze a triacylglycerol substrate although a certain preference for prim ary ester bonds has been observed. Numerous lipase assay methods are a vailable using coloured or fluorescent substrates which allow spectros copic and fluorimetric detection of lipase activitiy. Another importan t assay is based on titration of fatty acids released from the substra te. 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 bacter ia is influenced by a variety of environmental factors like ions, carb on sources, or presence of non-metabolizable polysaccharides. The secr etion pathway is known for Pseudomonas lipases with P. aeruginosa lipa se using a two-step mechanism and P. fluorescens lipase using a one-st ep mechanism. Additionally, some Pseudomonas lipases need specific cha perone-like proteins assisting their correct folding in the periplasm. These lipase-specific foldases (Lif-proteins) which show a high degre e of amino acid sequence homology among different Pseudomonas species are coded for by genes located immediately downstream the lipase struc tural genes. A comparison of different bacterial lipases on the basis of primary structure revealed only very limited sequence homology. How ever, determination of the three-dimensional structure of the P. gluma e lipase indicated that at least some of the bacterial lipases will pr esumably reveal a conserved folding pattern called the alpha/beta-hydr olase fold, which has been described for other microbial and human lip ases. The catalytic site of lipases is buried inside the protein and c ontains a serine-protease-like catalytic triad consisting of the amino acids serine, histidine, and aspartate (or glutamate). The Ser-residu e is located in a strictly conserved beta-epsilon Ser-alpha motif. The active site is covered by a lid-like alpha-helical structure which mo ves away upon contact of the lipase with its substrate, thereby exposi ng hydrophobic residues at the protein's surface mediating the contact between protein and substrate. This movable lid-like alpha-helix expl ains at a molecular level the lipase-specific phenomenon of interfacia l activation. At least some of the pathogenic bacterial species produc e a lipase which has been studied with respect to its role as a virule nce factor. Lipases of Propionibacterium acnes and Staphylococcus epid ermidis may be involved in colonization and persistence of these bacte ria on the human skin. Lipases of S. aureus and P. aeruginosa are prod uced during the bacterial infection process and, at least in vitro, co nsiderably impair the function of different cell types involved in the human immune response like macrophages or platelets. The present stat e of knowledge suggests to classify the lipases as important bacterial virulence factors which exert their harmful effects in combination wi th other bacterial enzymes, in particular the phospholipases C. Most o f the steadily increasing interest in bacterial lipases is based on th eir biotechnological applications which are partly based on their pote ntial to catalyze not only hydrolysis but also synthesis of a variety of industrially valuable products. Optically active compounds, various esters and lactones are among the substances synthesized using bacter ial lipases. Recently, an important application emerged with the addit ion of bacterial lipases to household detergents in order to reduce or even replace synthetic detergent chemicals which pose considerable en vironmental problems. As a main conclusion, lipases represent an extre mely versatile group of bacterial extracellular enzymes that are capab le of performing a variety of important reactions, thereby presenting a fascinating field for future research.