Identification of amino acids involved in the hydrolytic activity of lipase LipBL from Marinobacter lipolyticus

Abstract

The lipolytic enzyme family VIII currently includes only seven members but represents a group of lipolytic enzymes with interesting properties. Recently, we identified a gene encoding the family VIII lipase LipBL from the halophilic bacterium Marinobacter lipolyticus. This enzyme, like most lipolytic enzymes from family VIII, possesses two possible nucleophilic serines located in an S-X-X-K β-lactamase motif and a G-X-S-X-G lipase motif. The serine in the S-X-X-K motif is a catalytic residue, but the role of serine within the common lipase consensus sequence G-X-S-X-G has not yet been systematically studied. Here, the previously reported time-intensive procedure for purification of recombinant LipBL was replaced by one-step metal-affinity chromatography purification in the presence of ATP. Heterologous co-expression of His(6)-tagged LipBL with the cytoplasmic molecular chaperones GroEL/GroES was necessary to obtain catalytically active LipBL. Site-directed mutagenesis performed to map the active site of LipBL revealed that mutation of serine and lysine in the β-lactamase motif (S(72)-M-T-K(75)) to alanine abolished the enzyme activity of LipBL, in contrast to mutation of the serine in the lipase consensus motif (S321A). Furthermore, mutagenesis was performed to understand the role of the G-X-S-X-G motif and other amino acids that are conserved among family VIII esterases. We describe how mutations in the conserved G-X-S-X-G motif altered the biochemical properties and substrate specificity of LipBL. Molecular modelling results indicate the location of the G-X-S(321)-X-G motif in a loop close to the catalytic centre of LipBL, presumably representing a substrate-binding site of LipBL.