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 eth ylene 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 correspo nding to a P23 lattice, Unexpectedly, the crystalline two-domain lipas e is found in its open configuration, This indicates that in the prese nce 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 res idue using diisopropyl p-nitrophenylphosphate (E600), In addition, the presence of a TGME molecule tightly bound to the active site pocket s hows that TGME acts as a substrate analog, thus possibly explaining th e 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-porcin e complex (van Tilbeurgh, H., Egloff, M.-P., Martinez, C., Rugani, N., Verger, R., and Cambillau, C. (1993) Nature 362, 814-820) indicates t hat except for one salt bridge interaction, they are conserved, Analys is of the superimposed complexes shows a 5.4 degrees rotation on the r elative position of the N-terminal domains excepting the flap that mov es in a concerted fashion with the C-terminal domain, This flexibility may be important for the binding of the complex to the water-lipid in terface.