A calcium-gated lid and a large β-roll sandwich are revealed by the crystal structure of extracellular lipase from Serratia marcescens

Abstract

Lipase LipA from Serratia marcescens is a 613-amino acid enzyme belonging to family I.3 of lipolytic enzymes that has an important biotechnological application in the production of a chiral precursor for the coronary vasodilator diltiazem. Like other family I.3 lipases, LipA is secreted by Gram-negative bacteria via a type I secretion system and possesses 13 copies of a calcium binding tandem repeat motif, GGXGXDXUX (U, hydrophobic amino acids), in the C-terminal part of the polypeptide chain. The 1.8-Å crystal structure of LipA reveals a close relation to eukaryotic lipases, whereas family I.1 and I.2 enzymes appear to be more distantly related. Interestingly, the structure shows for the N-terminal lipase domain a variation on the canonical α/β hydrolase fold in an open conformation, where the putative lid helix is anchored by a Ca2+ ion essential for activity. Another novel feature observed in this lipase structure is the presence of a helical hairpin additional to the putative lid helix that exposes a hydrophobic surface to the aqueous medium and might function as an additional lid. The tandem repeats form two separated parallel β-roll domains that pack tightly against each other. Variations of the consensus sequence of the tandem repeats within the second β-roll result in an asymmetric Ca2+ binding on only one side of the roll. The analysis of the properties of the β-roll domains suggests an intramolecular chaperone function. © 2007 by The American Society for Biochemistry and Molecular Biology, Inc.