The crystal structure of a family I.3 lipase from Pseudomonas sp. MIS38 in a closed conformation was determined at 1.5 Å resolution. This structure highly resembles that of Serratia marcescens LipA in an open conformation, except for the structures of two lids. Lid1 is anchored by a Ca 2+ ion (Ca1) in an open conformation, but lacks this Ca1 site and greatly changes its structure and position in a closed conformation. Lid2 forms a helical hairpin in an open conformation, but does not form it and covers the active site in a closed conformation. Based on these results, we discuss on the lid-opening mechanism.

Based on the classification of bacterial lipolytic enzymes, family I.3 lipase is a member of the large group of Gram-negative bacterial true lipases. This lipase family is distinguished from other families not only by the amino acid sequence, but also by the secretion mechanism. Lipases of family I.3 are secreted via the well-known type I secretion system. Like most of proteins secreted via this system, family I.3 lipases are composed of two domains with distinct yet related functions. Recent years have seen an increasing amount of research on this lipase family, in terms of isolation, secretion mechanism, as well as biochemical and biophysical studies. This review describes our current knowledge on the structure-function relationships of family I.3 lipase, with an emphasis on its secretion mechanism.

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The hormone‐sensitive lipase (HSL) family is comprised of carboxylesterases and lipases with similarity to mammalian HSL. Thermophilic enzymes of this family have a high potential for use in biocatalysis. We prepared and crystallized a carboxylesterase of the HSL family from Sulfolobus tokodaii (Sto‐Est), and determined its structures in the presence and absence of an inhibitor. Sto‐Est forms a dimer in solution and the crystal structure suggests the presence of a stable biological dimer. We identified a residue close to the dimer interface, R267, which is conserved in archaeal enzymes of HSL family and is in close proximity to the same residue from the other monomer. Mutations of R267 to Glu, Gly and Lys were conducted and the resultant R267 mutants were characterized and crystallized. The structures of R267E, R267G and R267K are highly similar to that of Sto‐Est with only slight differences in atomic coordinates. The dimerized states of R267E and R267G are unstable under denaturing conditions or at high temperature, as shown by a urea‐induced dimer dissociation experiment and molecular dynamics simulation. R267E is the most unstable mutant protein, followed by R267G and R267K, as shown by the thermal denaturation curve and optimum temperature for activity. From the data, we discuss the importance of R267 in maintaining the dimer integrity of Sto‐Est. Database The atomic coordinates and structural factors have been deposited in the Protein Data Bank with accession numbers of PDB: http://www.rcsb.org/pdb/search/structidSearch.do?structureId=3AIK for noninhibited Sto‐Est, PDB: http://www.rcsb.org/pdb/search/structidSearch.do?structureId=3AIL for DEP‐bound, PDB: http://www.rcsb.org/pdb/search/structidSearch.do?structureId=3AIM for R267E, PDB: http://www.rcsb.org/pdb/search/structidSearch.do?structureId=3AIN for R267G, and PDB: http://www.rcsb.org/pdb/search/structidSearch.do?structureId=3AIO for R267K Structured digital abstract  http://www.uniprot.org/uniprot/Q976W8 and http://www.uniprot.org/uniprot/Q976W8 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0407 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0807 (View Interaction: http://mint.bio.uniroma2.it/mint/search/interaction.do?interactionAc=MINT-8384556, http://mint.bio.uniroma2.it/mint/search/interaction.do?interactionAc=MINT-8384625)  http://www.uniprot.org/uniprot/Q976W8 and http://www.uniprot.org/uniprot/Q976W8 http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0407 by http://www.ebi.ac.uk/ontology-lookup/?termId=MI:0114 (http://mint.bio.uniroma2.it/mint/search/interaction.do?interactionAc=MINT-8384513)