Lipase, a dimer stabilized with hydrogen bonds arranged in a head to tail fasion, is an enzyme the body uses to break down fats during digestion. Primarily secreted by the pancreas, but also found in the mouth and stomach, this enzyme is just one of the many enzymes secreted into the duodenum to break down food. Due to its environment, this enzyme is water soluble and can work in various pHs, as shown by its mainly hydrophilic (pink) exterior and its stabilizing disulfide bonds. However, it is hydrophobic enough that it can bind to lipids. Shown here is the elucidated structure of horse pancreatic enzyme. It has been found to be very similar to human pancreatic enzyme and has been used to better understand the human enzyme.
Structure
Lipase has two domains that connect with interdomain hydrogen bonds: hydrogen bonds occur between the starred and un-starred molecules. The N-terminal domain, which contains the active site, has typical
alpha/beta hydrolase fold and makes up two thirds of the molecule. This means that it shares with several other enzymes such as acetylcholinesterase, dienelactone hydrolase, thioesterase, and serine carboxypeptidase and many others, the same structure around its active site (Holmquist, 2000). In this picture, the alpha helices are rose, the beta sheets yellow, and the turns are blue. The active site is representative of the serine proteases and made up of Ser152, Asp176, and His263. It is covered by a hydrophobic flap to protect it from solvent molecules. Opening of the flap allows access for the substrate to the catalytic site, the oxyanionic hole, and strengthened binding of collipase (Borne et al., 1994).
The C-terminal domain is responsible for binding to colipase, a small protein cofactor responsible for binding on to the inhibiting bile salt coated lipid-water interface. Without binding to colipase,lipase would not be able to catalyze the digestion of lipids because of the strong inhibitory effect of bile salts. This domain has a beta-sandwhich type morphology, made by four anti-parallel strands that allow for both hydrophobic and ionic binding of colipase. In the picture, lipase is complexed with pro-colipase, so no conformational change has been induced yet. With colipase, the flap covering the active site is moved to allow the binding of lipase to its substrate. This domain also has an active site that hydrolyzes p-nitrophenyl-acetate that is irreversibly acetylated on a lysine (Holmquist, 2000).
Lipase has one Metal ion, Calcium, with which it associates. The residues that associate with this calcium ion are indicated with a yellow halos. This ion is too far away from the active site to be a part of catalysis. No absolute requirement for Calcium has been found, but it is conserved faithfully in both horse and human papain, so it is likely in all of these types of enzymes. The shape of the calcium binding site is distorted pentagonal bipyramidal as it interactw with main chain O atoms of Glu187 and Arg190, a side chain O atom of Asp192, and both side chain O atoms of Asp195, as shown.
Lipase can be complexed with the inhibitor C11 alkyl phosphonate and have hydrogen bonded with these residues. Here lipase is pictured with colipase and its covalent inhibitor, C11 alkyl phosphonate. It fits in a hydrophobic groove to mimic that of the true substrate of lipase, triglycerides. Some are trying to find drugs that inhibit lipase in order to combat obesity. Although this inhibitor is not necessarily a candidate, its structure has aided researchers in developing a true inhibitor.
References
Holmquist, M. Alpha/Beta-hydrolase fold enzymes: structures, functions and mechanisms.Curr Protein Pept Sci. 2000 Sep;1(2):209-35.
Bourne, Y; Martinez, C; Kerfelec, B; Lombardo, D; Chapus, C; Cambillau, C. Horse pancreatic lipase. The crystal structure refined at 2.3 A resolution. J Mol Biol. 1994 May 20;238(5):709-32.
Egloff, MP; Marguet, F; Buono, G; Verger, R; Cambillau, C; van Tilbeurgh, H. The 2.46 A resolution structure of the pancreatic lipase-colipase complex inhibited by a C11 alkyl phosphonate. Biochemistry. 1995 Mar 7;34(9):2751-62.
