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.