A model for the interfacial orientation and the mode of action of lipase is proposed. Lipase is oriented so that its active site is near the oil-water boundary. This orientation is achieved by oil-enzyme bonding at the "hydrophobic head" of the enzyme, a region free of electric charges and relatively resistant to unfolding. The measured K M is a complex constant including the dissociation constant of this oil-enzyme "complex". The interfacial orientation of lipase is further aided by hydrophilic negative charges on the "back" of the enzyme and by a hydrophilic carbohydrate "tail".
It is suggested that similar hydrophobic heads and hydrophilic tails and asymmetric charge distributions establish the orientation of many enzymes which act at interfaces. Many phospholipases, for instance, appear to be charge-oriented, and the carbohydrate residues of ribonucleases and many other glyeoproteins may be hydrophilic tails.
Lipase is probably a serine enzyme with a catalytic center similar to that of chymotrypsin, but more hindered, perhaps owing to the presence of a leucine residue, and there is no binding of substrate lipid chains in the "active complex". The substrate molecule is fixated on the enzyme in a two-dimensional orientation, because its leaving alkoxy group must be received by the serine hydroxyl hydrogen which is directed towards the imidazol ring of the reactive histidine through a hydrogen bond. The high turnover rate of lipolysis, 5 X 105/min, exceptional even for an enzyme, results from the extremely high substrate concentration near the active site, and from an almost complete extrusion of water because of the hydrophobicity of both the active site and the substrate. In addition, both substrate and enzyme, because of their polarity, are already so favorably positioned at the interface that the formation of the "active complex" requires only a proper two-dimensional alignment, perhaps with partial extraction of the substrate molecule from the lipid phase.