Lipase

Anything in this section will appear adjacent to the 3D structure and will be scrollable.

Introduction

Lipase is a hydrolase that catalyzes the breakdown of lipids by hydrolyzing the esters of fatty acids. Lipases are important in digestion, promoting absorption of fats in the intestines. Lipase is primarily found in the pancreas but is also found in the mouth and the stomach. Pancreatic lipase (PDB ID: 1HPL) which is pictured below 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 ^[1]. The reaction catalyzed by this enzyme is shown below.

Further breakdown ultimately results in 2-monoacylglycerols and free fatty acids ^[2]. Pancreatic liapase is a 50 kDa protein, consisting of two identical, 449 residue chains ^[3]. 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. It wasn't until 1955 that Mattson and Beck demonstrated a high-specificity of pancreatic lipase for triglyceride primary esters ^[4]. In recent years, determination of the crystal structure of pancreatic lipase has become the focus and many scientists have worked to further this. See also Molecular Playground/Pancreatic Lipase.

Structure

The s 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%. Each chain contains two well defined . 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 ^[5]. Each monomer and dimer structure of lipase is held together by disulfide bonds, hydrogen bonds, and electrostatic interactions (salt bridges). Lipase has 12 total between Cys residues. are formed between the positively charge nitrogens (blue) in Arg and Lys, and negative oxygens (red) in Asp and Glu residues. also stabilize the enzyme and . Lipase has a distinct distribution of hydrophobic and hydrophilic residues. Hydrophobic collapse contributes to much of the secondary and tertiary structures, as the , shown in grey, point towards the interior of the protein. Conversely, the , in pink, point outwards ^[6]. In addition, lipase has two , 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 ^[7].

Lipase Catalytic Mechanism

Although a diverse array of lipase enzymes are found in nature, occupying diverse protein scaffolds, most are built upon an alpha/beta hydrolase fold^[8][9] and possess a chymotrypsin-like comprised of an acidic residue, a histidine, and a serine nucleophile. In the case of the images above of a horse pancreatic lipase, the catalytic triad is comprised of ^[10] This catalytic triad functions like most found in nature, first with the Aspartic acid forming 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 glycerol backbone of the lipid substrate. Upon attacking the lipid, a negatively charged tetrahedral intermediate is formed. It is stabilized in the oxyanion hole by two residues: . The carbonyl reforms. A water molecule then donates a proton to the histidine, creating a reactive hydroxyl anion, which can attack the carbonyl carbon of the lipid, forming another negatively charged tetrahedral intermediate which is stabilized in the oxyanion hole. Upon reformation of the carbonyl, the catalytic serine is released and monoglyceride and fatty acid monomers diffuse away.

Inhibition of Pancreatic Lipase

In this structure, only one of the two identical chains is shown for lipase and colipase to better visualize the interaction of substrates and ligands with the protein. , a C11 alkyl phosphonate, is a competitive inhibitor of pancreatic lipase which binds to the active site. It is highlighted in purple. There are also five B-octylglucoside molecules in association with lipase. They are shown in grey and red. MUP forms hydrogen bonds with : 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 ^[11].

added by biochem class

Reaction 1: Image:M0218.stg01.gif

Reaction 2:

Image:M0218.stg02.gif

Reaction 3:

Image:M0218.stg03.gif

Reaction 4:

Image:M0218.stg04.gif

Protein - Substrate Interactions

Seen here is the Candida rugosa lipase in

with two molecules of cholesteryl linoleate (grey).

Clinical Significance

Pancreatic lipase is secreted into the duodenum through the duct system of the pancreas. In a healthy individual, it is in 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.^[12]

Here, lipase and colipase can be seen

The N-terminal domain also contains a that blocks solvent from entering the active site.