Pancreatic
lipase (
triacylglycerol acyl
hydrolase) fulfills a key function in
dietary fat absorption by hydrolysing
triglycerides into
diglycerides and subsequently into
monoglycerides and
free fatty acids. We have determined the three-dimensional structure of the human
enzyme, a single-chain
glycoprotein of 449
amino acids, by X-ray crystallography and established its primary structure by sequencing
complementary DNA clones. Enzymatic activity is lost after chemical modification of Ser 152 in the porcine
enzyme, indicating that this residue is essential in catalysis, but other data are more consistent with a function in interfacial recognition. Our structural results are evidence that Ser 152 is the nucleophilic residue essential for catalysis. It is located in the larger N-terminal domain at the C-terminal edge of a doubly
wound parallel beta-sheet and is part of an Asp-
His-Ser triad, which is chemically analogous to, but structurally different from, that in the
serine proteases. This putative hydrolytic site is covered by a surface loop and is therefore inaccessible to
solvent. Interfacial activation, a characteristic property of lipolytic
enzymes acting on water-insoluble substrates at water-
lipid interfaces, probably involves a reorientation of this flap, not only in pancreatic lipases but also in the homologous hepatic and
lipoprotein lipases.