Sandbox 50
From Proteopedia
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Horse Pancreatic LipaseIntroductionLipase, 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 (EC 3.1.1.3) 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.[1] StructureHorse 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 N terminal domain is characterized by an expected alpha/beta hydrolase fold, while the C terminal domain contains a beta sheet sandwich that is involved in colipase binding.[2] 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. [3] Disulfide BondsAdditional tertiary stability is provided by between cysteine residues shown in yellow linkages. Cysteine residues not involved in disulfide bonds are shown as spheres. Chain ContactsThe two chains of HPL are connected through , , , and . In the last scene, water molecules are shown as pink spheres. Calcium IonsTwo calcium ions are also present in the protein to further the structure of HPL. Each chain contains one calcium ion, each bound by the same residues. While the calcium ligands are no involved in the manipulating the substrate in the active site, enzymatic activity has been shown to be related to free calcium concentration. At lower calcium concentrations, lipases show reduced activity.[4] This is most likely due to reduced structural coordination. A more detailed view of the calcium coordination can be seen here: Image:Calcium binding.PNG Calcium Ion Coordination[5] Colipase BindingWhen bile salts accumulate at the lid/water interface, adsoprtion of the enzyme on its substrates is prohibited. In order to overcome this inhibitory effect, colipase binds to HPL and anchors lipase at the interface coated with bile salts.[6] When bound to colipase,HPL exists in its active, .[7] Here chains A and C (blue and pink) are the chians from the original HPL protein, and chains B and D (green and yellow) are colipase peptides. Rate studies show that the disassociation constant of the lipase-colipase complex is 101.1x10^−9 M. When the substrate is absent, the disassociation constant increases by several orders, indicating that disassociation of colipase from lipase increases when no substrates are present. These studies confirm that pancreatic lipase has many highly , as horse, ox, pig, rat, dog, and chicken colipases all can activate HPL and have similar disassociation constants. [8]
In the presence of colipase, the enzyme is activated which moves the (shown in red) 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.[9] The reorganization of the flap also induces a second conformational change that creates the oxyanion hole.[10] Active Site and MechanismImage:Mech..PNG HPL hydrolysis reaction[11] This active site in HPL is used to hydrolyze triacylglycerol into carboxylate and diacylglycerol.
Image:Lipase mech.gif lipase-catalyzed hydrolysis of esters[13]
Ligand Binding / Inhibition
References
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