The role of pancreatic lipase C2-like domain in enzyme interaction with a lipid-water interface

Human pancreatic lipase (HPL) consists of two functional domains: an N-terminal catalytic domain (N-HPL), and a beta-sandwich C-terminal domain (C-HPL) involved in the binding process between HPL and colipase. Structural similarities between C-HPL and C2 domains have suggested another function in lipase-lipid interactions. C2 domains occur in a wide range of proteins involved in phospholipid interactions, in signal transduction (phosphoinositide-specific phospholipase C, protein kinase C, cytosolic phospholipase A2), membrane traffic (synaptotagmin I, rabphilin), and membrane disruption (perforin). In this paper, we review experiments supporting an absolute requirement of C-HPL for interfacial binding of HPL. N-HPL and C-HPL were produced as individual polypeptides, and their partitioning between the water phase and a lipid interface was assessed using trioctanoin emulsions or the monomolecular film technique. Using monoclonal antibodies and site-directed mutagenesis, the contribution to lipase-lipid interactions of an exposed hydrophobic loop (beta5') present in C-HPL was investigated. The beta5'loop is located on the same side of HPL as the hydrophobic loops surrounding the active site, and has the same topology as the CBR3 loop found in C2 domains. Based on structural homology between C-HPL and C2 domains, a model of HPL orientation at a lipid interface is proposed.