Lipoprotein Lipase - Mechanism

Mechanism

The active site of LPL is composed of the conserved Ser-132, Asp-156, and His-241 triad. Other important regions of the N-terminal domain for catalysis includes an oxyanion hole (Trp-55, Leu-133), a lid region (residues 216-239), as well as a β5 loop (residues 54-64). The ApoC-II binding site is currently unknown, but it is predicted that residues on both N-and C-terminal domains are necessary for this interaction to occur. The C-terminal domain appears to confer LPL’s substrate specificity; it has a higher affinity for large triacylglyceride-rich lipoproteins than cholesterol-rich lipoproteins. The C-terminal domain is also important for binding to LDL’s receptors. Both the N-and C-terminal domains contain heparin binding sites distal to the lipid binding sites; LPL therefore serves as a bridge between the cell surface and lipoproteins. Importantly, LPL binding to the cell surface or receptors is not dependent on its catalytic activity.

The LPL non-covalent homodimer has a head-to-tail arrangement of the monomers. The Ser/Asp/His triad is contained in a hydrophobic groove that is blocked from solvent by the lid. Upon binding to ApoC-II and lipid in the lipoprotein, the C-terminal domain presents the lipid substrate to the lid region. The lipid interacts with both the lid region and the hydrophobic groove at the active site; this causes the lid to move, providing access to the active site. The β5 loop folds back into the protein core, bringing one of the electrophiles of the oxyanion hole into position for lipolysis. The glycerol backbone of the lipid is then able to enter the active site and is hydrolyzed.

Two molecules of ApoC-II can attach to each LPL dimer. It is estimated that up to forty LPL dimers may act simultaneously on a single lipoprotein. In regard to kinetics, it is believed that release of product into circulation is the rate-limiting step in the reaction.