Lipase, as its name suggests, is an enzyme responsible for the cleavage of types of lipid molecules. The mechanism by which it completes this task designates it as a hydrolase. There are different types of lipases, many of which work in similar ways. For instance, Human Pancreatic Lipase, or HPL, splits triglycerides, the main lipids in the human diet, into glycerol and three fatty acids. The structure shown at right is that of Horse Pancreatic Lipase ^[1]. It consists of two identical subunits, totaling 449 amino acids each, and totals 50 kDA. To better visualize the directionality of the subunits with respect to each other we can use a . This diagram shows the N-terminus of each subunit in blue, the follows the spectrum through green, yellow, orange, and finally the C-terminus is shown in red.

Basic Structure

The of lipase (just one subunit) include 102 residues which create 13 alpha helices, shown in red, and 139 residues involved in beta sheets totaling 28 strands, shown in gold. The alpha helices account fot 22% of the proteing, while the beta sheets comprise 30%.^[2] The can show much about the structure of the protein. The negatively charged residues in this scene are shown in a red shade, and the positively charged residues are in a dark blue. Notice how the negatively charged residues are oriented around the positive calcium ligands. These residues are represented in ball and stick model form to show their interaction with the ligands. All other residues are colored black in this scene. If we analyze together the we notice that the majority of the nonpolar residues, in green, are buried in the center of each subunit. This is a good illustration of the hydrophobic collapse that contributes to the protein folding into its tertiary structure. The polar residues, colored blue, are exposed to the aqueous environment. The tertiary stucture of the molecule is stabilized by 6 ^[3] and ionic interactions with a calcium ligand within each subunit. Finally, the quaternary structure is completed by the adjoining of the two identical subunits.The include hydrogen bonds, hydrophobic interactions, salt bridges, and other interactions. As the figure shows, most of the interactions between the subunits occur between the active site of one subunit and the inactive portion of the other. This leaves an empty cavity-like area in the center of the protein.

The Calcium Ligand

The are two calcium ions, one buried within each subunit. This scene shows the interactions between the calcium ion (shown in green) in subunit A and the following residues from subunit A: GLU187, ARG190, ASP192, and ASP195. In addition to interactions with these molecules, the calcium ion is also stabilized by the oxygens from two water molecules shown in pink. These interactions between the amino acid residues and the ligand are crucial for proper protein folding, and subsequently protein function.

The Mechanism

Evolutionary Conservation

This scale is used to identify the evolutionary conservation of certain residues. The red residues are the most conserved and the blue tend to be variable between variations of the protein.

The consistency of residues between variations of lipases can be described as the of the protein. As it can be observed from the conservation of individual residues, the residues near the active site and close to the calcium ligand have the highest average conservation. The conservation of residues becomes more variable the farther away from each active site.

Optimization of Action

In order for the action of Pancreatic Lipase to be optimized in vivo, a coenzyme called colipase must be secreted by the pancreas and activated by intestinal trypsin. Colipase prevents bile salts from inhibiting the regular action of Pancreatic Lipase. Colipase binds to the C-terminal of a subunit of Pancreatic Lipase, stabilising the active conformation. Shown here is bonded to a subunit of Pancreatic Lipase. This view shows the many between colipase and Pancreatic Lipase.

References

1. ↑ http://www.pdb.org/pdb/explore/explore.do?structureId=1hpl
2. ↑ http://www.pdb.org/pdb/explore/remediatedSequence.do?structureId=1HPL#DSSPRefAnchor
3. ↑ http://molvis.sdsc.edu/fgij/fg.htm?mol=1hpl
4. ↑ http://www.engin.umich.edu/dept/che/research/savage/energy.html