Horse Pancreatic Lipase

Lipase, which is produced primarily in the pancreas, functions as a catalyst in the hydrolysis of ester bonds in lipid substrates. This makes lipases an essential molecule for fat digestion. Horse pancreatic lipase’s (HPL)function is to convert triacylglycerols into 2-monoacylglycerols and free fatty acids. These monomers are then able to be shuttled into the small intestine to be absorbed into the lymphatic system.

Structure

Horse pancreatic lipase contains two identical peptide chains containing 449 amino acid residues. The N to C terminal order is shown where the N terminus is in blue and can be followed to the C terminus in red. Each chain contains . The N-terminal domain is shown in blue and the C terminal domain is shown in green. The active site of HPL is highlighted in red to show its location in the N terminal domain of the A chain. Additionally, the can be seen here where negatively charged amino acid residues are seen in red, and positively charged amino acids are seen in blue.

The of HPL contains 13 alpha helices and 28 strands of beta sheets, representing 22% and 30%, respectively, of the protein's residues. Hydrophic collapse contributes to much of the secondary and tertiary structures, as the

shown in grey are mostly facing towards the interior of the protein. Conversely, the in pink point congregate more on the exterior and point outwards.

Additional tertiary stability is provided by between cysteine residues shown in yellow linkages. Cysteine residues not involved in disulfide bonds are shown as spheres.

The two chains of HPL are connected through , , , and . In the last scene, water molecules are shown as pink spheres.

ions are also present in the protein to further coordinate the structure of HPL. Each chain contains one calcium ion, each bound by the same residues. A more detailed view of the calcium coordination can be seen here:

Active Site and Catalytic Activity

The of HPL is characterized by side chain residues Ser 152, His 263, and Asp 176 shown in red. Additionally, the main chain amides of Phe 77 (blue) and Leu 153 (green) are shown.

HPL is used to hydrolyze triacylglycerol into carboxylate and diacylglycerol.

In the first step, His263 deprotonates Ser152. Ser152 is then free to attack the carboxy carbon of triacylglycerol through a nucleophilic addition reaction. Next, the diacylglycerol product is eliminated when the oxyaninion collapses. This deprotonates His263. In the third step, His263 deprotonates water, which can then attack the carboxyl carbon of Ser152 through a nucleophilic addition reaction. Finally, the carboxylate product and Ser152 are eliminated with the collapse of the oxyanion, and His263 is deprotonated.

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