A PANCREATIC LIPASE WITH A PHOSPHOLIPASE A1 ACTIVITY - CRYSTAL-STRUCTURE OF A CHIMERIC PANCREATIC LIPASE-RELATED PROTEIN-2 FROM GUINEA-PIG

Background: The guinea pig pancreatic lipase-related protein 2 (GPLRP2 ) differs from classical pancreatic lipases in that it displays both l ipase and phospholipase A1 activities; classical pancreatic lipases ha ve no phospholipase activity. The sequence of GPLRP2 is 63% identical to that of human pancreatic lipase (HPL), but the so-called lid domain , is much reduced in GPLRP2. A phospholipase A1 from hornet venom (Dol ml PLA1) is very similar to HPL and GPLRP2 but is devoid of lipase act ivity; Dolml PLA1 also contains a reduced lid domain and lacks a regio n termed the beta 9 loop, which is located in the vicinity of the HPL and GPLRP2 active sites. The structure determination of a chimera of G PLRP2 and HPL and domain building of Dolml PLA1 were undertaken to gai n a better understanding of the structural parameters responsible for the differences in lipase versus phospholipase activity among these st ructurally related enzymes. Results: The crystal structure of a chimer ic mutant of GPLRP2, consisting of the catalytic domain of GPLRP2 and the C-terminal domain of HPL, has been solved and refined to 2.1 Angst rom resolution, This enzyme belongs to the alpha/beta hydrolase fold f amily and shows high structural homology with classical pancreatic lip ases. The active site is closely related to those of serine esterases, except for an unusual geometry of the catalytic triad, Due to the red uced size of the lid domain, the catalytic serine is fully accessible to solvent. Part of the beta 9 loop, which stabilizes the lid domain i n the closed conformation of the classical HPL, is totally exposed to the solvent and is not visible in the electron-density map. Conclusion s: The structures of the related enzymes, GPLRP2 and HPL and the model of Dolml PLA1, provide insights into the role played by the loops loc ated above the active site in controlling substrate selectivity toward s triglycerides or phospholipids. In GPLRP2, the lid domain is reduced in size compared to HPL, and hydrophilic residues are exposed to solv ent. GPLRP2 is thus able to accommodate the polar head of phospholipid s. The beta 9 loop is still present in GPLRP2, making it possible for this enzyme to still accommodate triglycerides. In Dolml PLA1, the bet a 9 loop is absent and this enzyme is unable to process triglycerides retaining only the phospholipase A1 activity. (C) Current Biology Ltd