Lipase: Difference between revisions

<StructureSection load='1akn' size='300' side='right' caption='Structure of glycosylated pancreatic lipase (PDB entry [[1akn]])' scene=''>

'''Lipase''' catalyzes the breakdown of lipids by hydrolyzing the esters of fatty acids.  Its function is important for digestion and promoting absorption of fats in the intestines.  Lipase is primarily found in and secreted by the pancreas, but is also found in the saliva and stomach.<br />

* '''Pancreatic lipase''' (PDB ID:  [[1hpl]]) which is pictured to the right, is a carboxylic ester hydrolase.  It is also commonly called '''pancreatic triacylglycerol lipase''' and its enzyme class number is E.C. 3.1.1.3 <ref name="1HPL PDB SUM">[http://www.ebi.ac.uk/thornton-srv/databases/cgi-bin/pdbsum/GetPage.pl?pdbcode=1hpl&template=main.html] 1HPL PDB SUM </ref>.<br />

* The '''bile salt-stimulated lipase''' (BSSL) is found in breast milk.<br />

 

The reaction catalyzed by the enzyme is shown below.  

Further breakdown ultimately results in 2-monoacylglycerols and free fatty acids <ref name= "A cross-linked complex between horse pancreatic lipase and colipase">[http://www.sciencedirect.com/science/article/pii/0014579389815923] A cross-linked complex between horse pancreatic lipase and colipase</ref>. An in depth discussion of the mechanism can be found in the Lipase Catalytic Mechanism section. The determination of the structure and function of lipase was a gradual process.  Lipase activity was first demonstrated in the pancreas by Claude Bernard in 1846.  However, it wasn't until 1955 that Mattson and Beck demonstrated a high-specificity of pancreatic lipase for triglyceride primary esters <ref name= "History of Lipids">[http://www.cyberlipid.org/history/history1.htm] History of Lipids</ref>.  In recent years, determination of the crystal structure of pancreatic lipase has become the primary focus as many scientists have worked to further this.<br />

 

==See also==

* [[Molecular Playground/Pancreatic Lipase]]<br />

In addition, lipase has a unique <scene name='Lipase/Lid/2'>lid</scene> (green) that blocks solvent from entering the active site (red).   The lid is a 25-residue helical structure that protects the oxyanion hole.  The lid (yellow) is especially important to substrate binding as it undergoes a dramatic shift that alters the secondary structure of the lipase binding site from a <scene name='Lipase/Closed_lid/1'>closed lid structure</scene> (active site in red) to an <scene name='Lipase/Open_ring/1'>open ring structure</scene> (active site in blue, triacylglyceride in spacefill) <ref>Fundamentals of Biochemistry...</ref> (see [[Lipase lid morph]] for an animation of this transition). The lid opening is accompanied by a change in secondary structure from a mostly beta-extended confirmation to a structure where more than half the active site is formed from alpha helices <ref>Thomas, A. etc. "Role of the Lid Hydrophobicity Pattern in Pancreatic Lipase Activity", The Journal of Biological Chemistry, 2005 September 22; 270 (48): 40074-40083. </ref>.

Colipase must be present for activation of lipase and acts as a bridge between lipase and the lipid. When colipase binds, active lipase is stabilized for the hydrophobic interaction with triacylglycerides <ref>Fundamentals of Biochemistry...</ref>.  Without colipase present, the accumulation of amphiphiles at the oil/water interface in the duodenum would prevent pancreatic lipase from binding to its substrate. <ref>Crandall,W., Lowe, M. "Colipase Residues Glu64 and Arg65 Are Essential for Normal Lipase-mediated Fat Digestion in the Presence of Bile Salt Micelles" Journal of Biological Chemistry, 2001, (276) 12505-12512</ref>. Colipase and lipase <scene name='Lipase/Contacts/2'>contacts</scene> are opposite of the active site on the C-terminal (contacts are regions of pink and yellow, with water molecules shown in darker blue).  The enzymes are bound by polar interactions such as <scene name='Lipase/Salt_bridges/2'>salt bridges</scene>, <scene name='Lipase/Hphobic_interactions/1'>hydrophobic interactions</scene> and <scene name='Lipase/Hydrogen_bonds_non_water/1'>hydrogen bonds</scene> <ref>van Tilbeurgh H, etc."Structure of the pancreatic lipase-procolipase complex",  1992 Sep 10;359(6391):159-62. PMID:1522902.[http://www.proteopedia.org/wiki/index.php/1n8s]</ref>.

 

In the presence of colipase, the enzyme is activated which moves the <scene name='Lipase/N-terminal_flap/1'>N-terminal flap</scene>(shown in red, active site in green) which is composed of amino acids 216-239. The N-terminal flap moves in a concerted fashion along with the C-terminal domain to reveal the active site (green), allowing it to bind with a substrate. It is hypothesized that this flexibility may have significance in binding the colipase-lipase complex with the water-lipid interface.<ref>http://www.pdb.org/pdb/explore/explore.do?structureId=1ETH</ref> The reorganization of the flap also induces a second conformational change that creates the oxyanion hole.<ref>http://www.nature.com/nature/journal/v362/n6423/abs/362814a0.html</ref>

Lipase activation at the lipid-water interface of triacylglycerides, in the presence of colipase and bile salts, is known as interfacial activation.  For the hydroloysis reaction to take place, colipase anchors lipase to the lipid-water membrane of the micelle which causes a surface change on lipase.  Colipase's four hydrophobic loops interact with the hydrophobic atmosphere of the triacylglyceride. This initiates active site binding to the lipid, and lid opening to reveal a more hydrophobic environment for the triacylglycerol.  This in turn, allows the triacylglycerol to interact with key active site residues like the catalytic triad.  A diverse array of lipase enzymes can be found in nature. Though the different forms occupy diverse protein scaffolds, most are built upon an alpha/beta hydrolase fold<ref>PMID: 1678899</ref><ref>PMID:1409539 </ref>  and possess a [[chymotrypsin]]-like <scene name='Lipase/Catalytic_site_outerview/1'>catalytic triad </scene>comprised of an acidic residue, a histidine, and a serine nucleophile. In the case of horse pancreatic lipase, the catalytic triad is comprised of <scene name='Lipase/Catalytic_triad/4'>Ser 152, Asp 176 and His 263. </scene><ref>PMID:8182745</ref>.  This catalytic triad functions like most found in nature. First, aspartic acid forms a hydrogen bond with His 263, increasing the pKa of the histidine imidazole nitrogen. This allows the histidine to act as a powerful general base and deprotonate the serine. The deprotonated serine then can serve as a nucleophile and attack the ester carbonyl of one of the fatty acids on the 1 or 3 carbons of the glycerol backbone of the lipid substrate.  Upon attacking the lipid, a negatively charged tetrahedral intermediate is formed (Reaction 1).  It is stabilized in the oxyanion hole by two residues:  <scene name='Lipase/Catalytic_triad_with_oxyanion/2'>Phe 77 and Leu 153</scene>.   

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[[Image:M0218.stg01r.gif|center|]]

The carbonyl reforms with the glycerol backbone segment acting as the leaving group (Reaction 2).

[[Image:M0218.stg02r.gif|center|]]

[[Image:M0218.stg03r.gif|center|]]

[[Image:M0218.stg04r.gif|center|]]

<scene name='Lipase/Lipase_colipase_inhibitor/1'>Methoxyundecylphosphinic acid (MUP)</scene> (purple), a C11 alkyl phosphonate, is a competitive inhibitor of pancreatic lipase.  It binds directly in the active site pocket.  There are also five B-octylglucoside (gray and red) molecules which associate with lipase.  MUP forms hydrogen bonds with <scene name='Lipase/Inhibitor_interaction/1'>four residues</scene>:  Ser 152 and His 263, which are part of the catalytic triad, and Phe 77 and Leu 153 which are the stabilizing residues located in the oxyanion hole <ref name= "1LPB PDB SUM">[http://www.ebi.ac.uk/thornton-srv/databases/cgi-bin/pdbsum/GetPage.pl?pdbcode=1lpb&template=main.html] 1LPB PDB SUM</ref>.

MUP was shown to be <scene name='Lipase/C11p_bound_h_phobics/2'>further stabilized</scene> by van der Waals contacts with hydrophobic side chains Ala 178, Phe 215, Pro l80, Tyr ll4, Leu 213 (shown in blue).

Shown in this scene is lipase from the yeast ''Candida rugosa'' in <scene name='Lipase/Complex_2/1'>complex</scene> with two molecules of cholesteryl linoleate (grey).  The active site residues including Ser152, Asp176, and His263 are shown in red stick representation.  Lipase can accommodate two lipid molecules due to the fact that it's two identical subunits catalyze an identical reaction.  One lipase molecule can catalyze two lipolysis reactions at a time. 

Pancreatic lipase is secreted into the duodenum through the duct system of the pancreas. In a healthy individual, it is at very low concentration in serum. Under extreme disruption of pancreatic function, such as pancreatitis or pancreatic cancer, the pancreas may begin to digest itself and release pancreatic enzymes including pancreatic lipase into serum. Measurement of serum concentration of pancreatic lipase can therefore aid in diagnosis of acute pancreatitis.<ref>"Pancreatic lipase". Wikipedia: The Free Encyclopedia. 7 Nov 2011 [http://en.wikipedia.org/wiki/Pancreatic_lipase]</ref>.  Due to lipase's activity in the digestion and absorption of fat, there has been a growing market for lipase inhibitors for weight loss pharmaceuticals.  The most popular is Orlistat (or Xenical®) which is a natural product from ''Streptomyces toxytricini'' and is the hydrogenation product of lipostation- an irreversible lipase inhibitor.  This inhibitor also acts by binding Ser152, producing an ester which hydrolyzes so slow that it is practically irreversible <ref>Kordik, C., Reitz, A. "Pharmacological Treatment of Obesity: Therapeutic Strategies" Journal of Medicinal Chemistry, 1999 (42).</ref>.