This family correspond to family I.8 of the classification of Arpigny and Jaeger (1999). These lipases differ from other bacterial lipases but groups putative psychrophilic esterases/lipases. The closest family is CarbLipBact_2. Features of the new family include: a conserved new active-site pentapeptide motif (LGG(F/L/Y)STG); the likely extra-cytoplasmic localization; the absence of a typical calcium-binding pocket; and the absence of a canonical lid.
The genome of the Antarctic bacterium Pseudoalteromonas haloplanktis TAC125 was searched for the presence of genes encoding ester-hydrolysing enzymes. Amongst the others, the gene PSHAa0051 coding for a putative secreted esterase/lipase was selected. The psychrophilic gene was cloned, functionally over-expressed in P. haloplanktis TAC125, and the recombinant product (after named PhTAC125 Lip1) was purified. PhTAC125 Lip1 was found to be associated to the outer membrane and exhibited higher enzymatic activity towards synthetic substrates with long acyl chains. A structural model was constructed using the structure of carboxylesterase Est30 from Geobacillus stearothermophilus as template. The model covered the central part of the protein with the exceptions of PhTAC125 Lip1 N- and C-terminal regions, where the psychrophilic protein displays extra-domains. The constructed model showed a typical alpha/beta-hydrolase fold, and confirmed the presence of a canonical catalytic triad consisting of Ser, Asp and His. The sequence analysis showed that PhTAC125 Lip1 is distantly related to other lipolytic enzymes, but closely related to other putative psychrophilic esterases/lipases. The aligned proteins share common features, such as: (1) a conserved new active-site pentapeptide motif (LGG(F/L/Y)STG); (2) the likely extra-cytoplasmic localization, (3) the absence of a typical calcium-binding pocket, and (4) the absence of a canonical lid. These observations strongly suggest that aligned proteins constitute a novel lipase family, typical of psychrophilic marine gamma-proteobacteria, and PhTAC125 Lip1 could be considered the first characterised member of this family.
