Introduction
Pancreatic lipase 1hpl (EC 3.1.1.3) is a an enzyme involved with the digestion of fat in the intestine;
specifically the breakdown of triacylglycerols. It is secreted by the pancreas into the duodenum where it participates in the initial stages of breaking down fats [1]. This is important because the enzyme actively digests at the lipid-water interface of the fatty micelles, requiring stability in both polar and non polar environments [2]. In the , the N-terminal domain (where the active site is located) has the hydrolase alpha/beta folding structure, consisting of an alpha-beta sheet consisting of 8 strands connected by helices. The C-terminal domain (where the enzyme colipase binds) has a beta-sheet sand which folding pattern [3] The of the lipase molecule is found in the N-terminal domain (residues 1-336) and contains a catalytic triad. The consists of a Ser-His-Arg for the ester hydrolysis reaction (similar to that of a serine protease). The active site is covered by a 25-residue helical 'flap' or 'lid' blocking the binding site unless appropriate conditions [4]. In the presence of fat micelles, the active site is uncovered and the 10-residue beta-5 loop changes confirmation so that an oxyanion hole and hydrophobic surface are formed near the active site [5]
Function
Pancreatic lipase initiates the breakdown of triacylglycerols into 2-monoacylglycerols and fatty acids releasing them into the intestinal lumen Consequently, lipase is hydrolyzes the ester bonds of the triglycerides. Lipase, as a digestive enzyme, is soluble in water but the triacylglycerides it hydrolyzes are not. Bile salts surround the fatty acid globules to make them more soluble. Therefore, lipase activity requires bile acids and its coenzyme to hydrolyze fats. Lipase is activated by the coenzyme colipase, which binds to the C-terminal non-catalytic domain. Upon binding, active lipase is stabilized for the hydrophobic interaction with the triacylglycerides [6]. Colipase must be present for activation of lipase when other amphiphilic molecules are present (such as phospholipids or bile salts). Colipase is also secreted in the pancreas, but in its inactive form which must be activated by trypsin before interacting with lipase [7]. Colipase and lipase are opposite of the active site on the C-terminal, connected by polar interactions such as salt bridges (stars on the image) and van der waals forces which stabilize the interaction [8]. shows that in the presence of colipase and a triacylglyceride, the lid over the active sight on the N-terminal is lifted and a 10 residue beta-5 loop changes confirmation to expose lipase's oxanion hole and hydrophobic surface. At the same time, hydrophobic loops from colipase interact with the hydrophobic atmosphere of the triacylglyceride initiating lipase active site binding to the lipid. Once the enzyme has bound the lipid, it initiates a serine-like hydrolysis mechanism involving the His, Asp and Ser residues in the catalytic triad. The hydrolysis reaction regenerates lipase and releases 2-monoacylglycerols and free fatty acids [9]
