NEUTRON CRYSTALLOGRAPHIC EVIDENCE OF LIPASE-COLIPASE COMPLEX ACTIVATION BY A MICELLE

The concept of lipase interfacial activation stems from the finding th at the catalytic activity of most lipases depends on the aggregation s tate of their substrates. It is thought that activation involves the u nmasking and structuring of the enzyme's active site through conformat ional changes requiring the presence of oil-in-water droplets, Here, w e present the neutron structure of the activated lipase-colipase-micel le complex as determined using the D2O/H2O contrast variation low reso lution diffraction method, In the ternary complex, the disk-shaped mic elle interacts extensively with the concave face of colipase and the d istal tip of the C-terminal domain of lipase. Since the micelle-and su bstrate-binding sites concern different regions of the protein complex , we conclude that lipase activation is not interfacial but occurs in the aqueous phase and is mediated by colipase and a micelle.