Lipase: Difference between revisions

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 is 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 buried in each monomer subunit.  The image shows the green calcium ion in subunit A, coordinated by Glu187, Arg190, Asp192, and Asp195.  The Ca(+2) charge is stabilized by negatively charged glutamate and aspartate residues, and the oxygen atoms from two water molecules (pink).  The calcium ion is essential to protein folding and enzyme activity <ref>http://www.springerlink.com/content/g5h1613440115701/fulltext.pdf</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 protects the oxyanion hole.  The lid (yellow) is especially important to substrate binding, as it undergoes a dramatic shift altering the secondary structure of 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 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>