Lipase activation by nonionic detergents - The crystal structure of the porcine lipase-colipase-tetraethylene glycol monooctyl ether complex

The crystal structure of the ternary porcine lipase-colipase-tetra ethylene glycol monooctyl ether (TGME) complex has been determined at 2.8 Angstrom resolution, The crystals belong to the cubic space group F23 with a = 289.1 Angstrom and display a strong pseudo-symmetry corresponding to a P23 lattice, Unexpectedly, the crystalline two-domain lipase is found in its open configuration, This indicates that in the presence of colipase, pure micelles of the nonionic detergent TGME are able to activate the enzyme; a process that includes the movement of an N-terminal domain loop (the flap), The effects of TGME and colipase have been confirmed by chemical modification of the active site serine residue using diisopropyl p-nitrophenylphosphate (E600), In addition, the presence of a TGME molecule tightly bound to the active site pocket shows that TGME acts as a substrate analog, thus possibly explaining the inhibitory effect of this nonionic detergent on emulsified substrate hydrolysis at submicellar concentrations, A comparison of the lipase-colipase interactions between our porcine complex and the human-porcine complex (van Tilbeurgh, H., Egloff, M.-P., Martinez, C., Rugani, N., Verger, R., and Cambillau, C. (1993) Nature 362, 814-820) indicates that except for one salt bridge interaction, they are conserved, Analysis of the superimposed complexes shows a 5.4 degrees rotation on the relative position of the N-terminal domains excepting the flap that moves in a concerted fashion with the C-terminal domain, This flexibility may be important for the binding of the complex to the water-lipid interface.