CONFORMATIONAL LABILITY OF LIPASES OBSERVED IN THE ABSENCE OF AN OIL-WATER INTERFACE - CRYSTALLOGRAPHIC STUDIES OF ENZYMES FROM THE FUNGI HUMICOLA-LANUGINOSA AND RHIZOPUS-DELEMAR

Considerable controversy exists regarding the exact nature of the mole cular mechanism of interfacial activation, a process by which most lip ases achieve maximum catalytic activity upon adsorption to an oil wate r interface. X-ray cryslallographic studies show that lipases contain buried active centers and that displacements of entire secondary struc ture elements, or ''lids,'' take place when the enzymes assume active conformations [Derewenda, U., A. M. Brzozowski, D. M. Lawson, and Z. S . Derewenda. 1992. Biochemistry: 31: 1532-1541; van Tilbeurgh, H., M-P . Egloff, C. Martinet, N. Rugani, R. Verger, and C. Cambillau. 1993. N ature. 362: 814-820; Grochulski, P., L. Yunge, J. D. Schrag, F. Bouthi llier, P Smith, D. Harrison, B. Rubin, and M. Cygler. 1993. J. Biol. C hem. 268: 12843-12847]. A simple two-state model inferred from these r esults implies that the ''closed'' conformation is stable in an aqueou s medium, ren dering the active centers inaccessible to water soluble substrates. We now report that in crystals of the Humicola lanulginosa lipase the ''lid'' is significantly disordered irrespective of the io nic strength of the medium, while in a related enzyme from Rhizopus de lemar, crystallized in the presence of a detergent, the two molecules that form the asymmetric unit show different ''lid'' conformations. Th ese new results call into question the simplicity of the ''enzyme theo ry'' of interfacial activation.