Relationships among serine hydrolases: evidence for a common structural motif in triacylglyceride lipases and esterases

A detailed analysis of the highly refined (1.9 A resolution) molecular model of the fungal (Rhizomucor miehei) triglyceride lipase reveals a unique conformation of the oligopeptide containing the active serine (Ser 144) residue. It consists of a six-residue beta-strand (strand 4 of the central sheet), a four-residue turn of type II' with serine in the epsilon conformation, and a buried alpha-helix packed in a parallel way against strands 4 and 5 of the central beta-pleated sheet. It is shown that the invariant glycines in positions (1) and (5) of the so-called lipase consensus sequence (G-X-S-X-G) are in extended and helical conformations, respectively, and that they are conserved owing to the steric restrictions imposed on these residues by the packing stereochemistry of this beta-epsilon Ser-alpha motif, and not by secondary structure requirements, as is the case in serine proteinases. Sequence homologies indicate that this unique motif is likely to be found in serine esterases and other lipases, indicating a possible evolutionary link of these families of hydrolytic enzymes.