Lipase: Difference between revisions
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<StructureSection load=' | <StructureSection load='1akn' size='300' side='right' caption='Structure of glycosylated pancreatic lipase (PDB entry [[1akn]])' scene=''> | ||
{{TOC limit|limit=2}} | {{TOC limit|limit=2}} | ||
== '''Introduction''' == | == '''Introduction''' == | ||
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. | '''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 '''hormone-sensitive lipase''' (LIPE) hydrolyzes a variety of esters. For details see [[Hormone sensitive lipase]].<br /> | |||
* '''Monoacylglycerol lipase''' (MAGL) hydrolyzes intracellular triglycerides to fatty acid and glycerol. MAGL functions together with LIPE. For details see [[Monoglyceride lipase]]. | |||
The reaction catalyzed by the enzyme is shown below. | |||
[[Image:Picture 1.png]] | [[Image:Picture 1.png]] | ||
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. | 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 /> | |||
* [[Lipase lid morph]]<br /> | |||
* [[Hormone sensitive lipase]]<br /> | |||
* [[Lipase from Candida antarctica in closed state]]<br /> | |||
* [[Monoglyceride lipase]]<br /> | |||
* [[Human gastric lipase]]<br /> | |||
* [[Lipoprotein Lipase (LPL) complexed with GPIHBP1]]<br /> | |||
* [[Lipase (Hebrew)]]<br /> | |||
* [[Lipid metabolism]] | |||
== '''Structure''' == | == '''Structure''' == | ||
Pancreatic lipase is a 50 kDa protein. While the crystallographic [[asymmetric unit]] contains two identical chains, information (REMARK 350) in the data file [[1hpl]] indicates that the dimer is a crystallization artifact, and that the functional form (also called the [[biological assembly]]) is a single chain (monomer). The chain consists of 449 residues <ref name= "1HPL PDB">[http://www.pdb.org/pdb/explore/explore.do?structureId=1HPL] 1HPL PDB</ref>. The <scene name='Lipase/Secondary_structures/1'>secondary structure</scene>s of lipase (in 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 for 22% of the protein, while the beta sheets comprise 30%. Each chain contains two well defined <scene name='Lipase/N_and_c_terminus/1'>domains</scene>. The N terminal domain, shown in blue, is characterized by an alpha/beta hydrolase fold. While the C terminal domain, shown in green, contains a beta sheet sandwich which interacts with colipase <ref>http://www.pdb.org/pdb/explore/explore.do?structureId=1HPL</ref>. Each monomer and dimer structure of lipase are held together by disulfide bonds, hydrogen bonds, and electrostatic interactions (salt bridges). Lipase has 12 total <scene name='Lipase/Disulfide_bonds/2'>disulfide bonds</scene> between cysteine residues. <scene name='Lipase/Salt_bridges/1'>Salt bridges</scene> are formed between the positively charge nitrogens (blue) in Arg and Lys, and negative oxygens (red) in Asp and Glu residues. <scene name='Lipase/Hydrogen_bonds/2'>Hydrogen bonds</scene> (in yellow) also stabilize the enzyme between main chain and side chain atoms. Lipase has a distinct distribution of <scene name='Lipase/Hphobic_residues/3'>hydrophobic and hydrophilic</scene> residues (purple spacefill represents polar residues). Hydrophobic collapse contributes to much of the secondary and tertiary structures, as the <scene name='Lipase/Surface/1'>hydrophobic core residues</scene> (shown in white) make up the interior of the protein, while polar residues (transparent blue) are on the surface <ref>http://www.pdb.org/pdb/explore/remediatedSequence.do?structureId=1HPL</ref>. In addition, lipase has two <scene name='Lipase/Lipase_ligand/1'>calcium ligands</scene>. One is buried in each monomer subunit. The calcium ion is essential to protein folding and enzyme activity <ref>http://www.springerlink.com/content/g5h1613440115701/fulltext.pdf</ref>. The image shows the green calcium ion in subunit A, coordinated by Glu187, Arg190, Asp192, and Asp195 residues. The Ca(+2) charge is stabilized by negatively charged glutamate and aspartate residues, and the oxygen atoms from two water molecules (pink). | Pancreatic lipase is a 50 kDa protein. While the crystallographic [[asymmetric unit]] contains two identical chains, information (REMARK 350) in the data file [[1hpl]] indicates that the dimer is a crystallization artifact, and that the functional form (also called the [[biological assembly]]) is a single chain (monomer). The chain consists of 449 residues <ref name= "1HPL PDB">[http://www.pdb.org/pdb/explore/explore.do?structureId=1HPL] 1HPL PDB</ref>. The <scene name='Lipase/Secondary_structures/1'>secondary structure</scene>s of lipase (in 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 for 22% of the protein, while the beta sheets comprise 30%. Each chain contains two well defined <scene name='Lipase/N_and_c_terminus/1'>domains</scene>. The N terminal domain, shown in blue, is characterized by an alpha/beta hydrolase fold. While the C terminal domain, shown in green, contains a beta sheet sandwich which interacts with colipase <ref>http://www.pdb.org/pdb/explore/explore.do?structureId=1HPL</ref>. Each monomer and dimer structure of lipase are held together by disulfide bonds, hydrogen bonds, and electrostatic interactions (salt bridges). Lipase has 12 total <scene name='Lipase/Disulfide_bonds/2'>disulfide bonds</scene> between cysteine residues. <scene name='Lipase/Salt_bridges/1'>Salt bridges</scene> are formed between the positively charge nitrogens (blue) in Arg and Lys, and negative oxygens (red) in Asp and Glu residues. <scene name='Lipase/Hydrogen_bonds/2'>Hydrogen bonds</scene> (in yellow) also stabilize the enzyme between main chain and side chain atoms. Lipase has a distinct distribution of <scene name='Lipase/Hphobic_residues/3'>hydrophobic and hydrophilic</scene> residues (purple spacefill represents polar residues). Hydrophobic collapse contributes to much of the secondary and tertiary structures, as the <scene name='Lipase/Surface/1'>hydrophobic core residues</scene> (shown in white) make up the interior of the protein, while polar residues (transparent blue) are on the surface <ref>http://www.pdb.org/pdb/explore/remediatedSequence.do?structureId=1HPL</ref>. In addition, lipase has two <scene name='Lipase/Lipase_ligand/1'>calcium ligands</scene>. One is buried in each monomer subunit. The calcium ion is essential to protein folding and enzyme activity <ref>http://www.springerlink.com/content/g5h1613440115701/fulltext.pdf</ref>. The image shows the green calcium ion in subunit A, coordinated by Glu187, Arg190, Asp192, and Asp195 residues. The Ca(+2) charge is stabilized by negatively charged glutamate and aspartate residues, and the oxygen atoms from two water molecules (pink). | ||
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>. 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>. | 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 Coenzyme''' == | == '''Colipase Coenzyme''' == | ||
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== '''Lipase Catalytic Mechanism''' == | == '''Lipase Catalytic Mechanism''' == | ||
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>. | 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>. | ||
[[Image:M0218. | <!-- The next 4 images have been darkened and resized to 600px width. The original files remain with the same filenames minus the "r" at the end ("r" for reduced).--> | ||
[[Image:M0218.stg01r.gif|center|]] | |||
The carbonyl reforms with the glycerol backbone segment acting as the leaving group (Reaction 2). | The carbonyl reforms with the glycerol backbone segment acting as the leaving group (Reaction 2). | ||
[[Image:M0218. | [[Image:M0218.stg02r.gif|center|]] | ||
A water molecule then donates a proton to the histidine, creating a reactive hydroxyl anion. The hydroxyl anion can then attack the carbonyl carbon of the lipid, forming another negatively charged tetrahedral intermediate which is stabilized in the oxyanion hole (Reaction 3). | A water molecule then donates a proton to the histidine, creating a reactive hydroxyl anion. The hydroxyl anion can then attack the carbonyl carbon of the lipid, forming another negatively charged tetrahedral intermediate which is stabilized in the oxyanion hole (Reaction 3). | ||
[[Image:M0218. | [[Image:M0218.stg03r.gif|center|]] | ||
Upon reformation of the carbonyl, the catalytic serine is released and monoglyceride and fatty acid monomers diffuse away (Reaction 4). | Upon reformation of the carbonyl, the catalytic serine is released and monoglyceride and fatty acid monomers diffuse away (Reaction 4). | ||
[[Image:M0218. | [[Image:M0218.stg04r.gif|center|]] | ||
== '''Inhibition of Pancreatic Lipase''' == | == '''Inhibition of Pancreatic Lipase''' == | ||
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== '''Clinical Significance''' == | == '''Clinical Significance''' == | ||
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>. | 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>. | ||
== 3D Structures of Lipase == | == 3D Structures of Lipase == | ||
[[Lipase 3D Structures]] | |||
</StructureSection> | |||
==References== | ==References== | ||