Structural basis for the substrate selectivity of pancreatic lipases and some related proteins

The classical human pancreatic lipase (HPL), the guinea pig pancreatic lipa se-related protein 2 (GPLRP2) and the phospholipase A1 from hornet venom (D olmI PLA1) illustrate three interesting steps in the molecular evolution of the pancreatic lipase gene family towards different substrate selectivitie s. Based on the known 3D structures of HPL and a GPLRP2 chimera, as well as the modeling of DolmI PLA1, we review here the structural features and the kinetic properties of these three enzymes for a better understanding of th eir structure-function relationships. HPL displays significant activity onl y on triglycerides, whereas GPLRP2 displays high phospholipase and galactol ipase activities, together with a comparable lipase activity. GPLRP2 shows high structural homology with HPL with the exception of the lid domain whic h is made of five amino acid residues (mini-lid) instead of 23 in HPL. The lid domain deletion in GPLRP2 allows the free access to the active site and reduces the steric hindrance towards large substrates, such as galactolipi ds. The role of the lid domain in substrate selectivity has been investigat ed by site-directed mutagenesis and the substitution of HPL and GPLRP2 lid domains. The addition of a large-size lid domain in GPLRP2 increases the su bstrate selectivity for triglycerides by depressing the phospholipase activ ity. The phospholipase activity is, however, not induced in the case of the HPL mutant with GPLRP2 mini-lid. Therefore, the presence of a full-length lid domain is not the unique structural feature explaining the absence of p hospholipase activity in HPL. The 3D structure of the GPLRP2 chimera and th e model of DolmI PLA1 reveal a higher hydrophilic/lipophilic balance (HLB) of the surface loops (beta 5 loop, beta 9 loop, lid domain) surrounding the active site, as compared to the homologous loops in HPL. This observation provides a potential explanation for the ability of GPLRP2 and DolmI PLA1 t o hydrolyze polar lipids, such as phospholipids. In conclusion, the beta 5 loop, the beta 9 loop, and the lid domain play an essential role in substra te selectivity towards triglycerides, phospholipids and galactolipids. (C) 1998 Elsevier Science B.V. All rights reserved.