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Pancreatic lipase is a soluble globular protein that must undergo structural modifications before it can hydrolyze oil droplets coated with bile salts. The binding of colipase and movement of the lipase lid open access to the active site. Mechanisms triggering lid mobility are unclear. The *KNILSQIVDIDGI* fragment of the lid of the human pancreatic lipase is predicted by molecular modeling to be a tilted peptide. Tilted peptides are hydrophobicity motifs involved in membrane fusion and more globally in perturbations of hydrophobic/hydrophilic interfaces. Analysis of this lid fragment predicts no clear consensus of secondary structure that suggests that its structure is not strongly sequence determined and could vary with environment. Point mutations were designed to modify the hydrophobicity profile of the [240 -252] fragment and their consequences on the lipase-mediated catalysis were tested. Two mutants, in which the tilted peptide motif was lost, also have poor activity on bile saltcoated oil droplets and cannot be reactivated by colipase. Conversely, one mutant in which a different tilted peptide is created retains colipase dependence. These results suggest that the tilted hydrophobicity pattern of the [240 -252] fragment is neither important for colipase binding to lipase, nor for interfacial binding but is important to trigger the maximal catalytic efficiency of lipase in the presence of bile salt.The pancreatic lipase-colipase complex plays a key role in dietary fat absorption in the intestine by converting triglycerides into more polar products able to cross the brush-border membrane of enterocytes. The lipase is fully active on water-insoluble substrates that form lipid/water interfaces, a phenomenon called interfacial activation. In vivo, oil droplets consist of a bulk substrate phase surrounded by a monolayer of amphiphilic compounds, mainly biliary lipids (phosphatidylcholine, cholesterol and bile salts) that prevent lipase adsorption. To counteract the inhibitory effect of biliary lipids, the pancreas secretes a small protein, colipase, which anchors lipase to the biliary lipid-coated water/ lipid interface. The water-soluble lipase must therefore partition between aqueous and lipid phases before lipolysis can occur.Structural studies (1-3) have shown that lipases possess a two-domain organization, the N-terminal domain bearing the active site and the C-terminal domain bearing the colipase binding site. One specific feature of lipase is the shielding of its catalytic site by a surface loop (lid) controlling the access of the substrate. Therefore, movement of the lid domain is an absolute requirement for the lipase to adopt an active conformation.The role of the lid in lipolysis has been investigated by site-directed mutagenesis, lid exchange, or lid deletion (4 -6). It was first thought to account for the interfacial activation of pancreatic lipase but the presence of a full-length lid in most pancreatic lipase-related proteins that display no interfacial activation rules out this explanation. Actua...

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