Lipoprotein lipase (LPL) (EC 3.1.1.34) is a member of the lipase gene family, which includes pancreatic lipase, hepatic lipase, and endothelial lipase. It is a water-soluble enzyme that hydrolyzes triglycerides in lipoproteins, such as those found in chylomicrons and very low-density lipoproteins (VLDL), into two free fatty acids and one monoacylglycerol molecule. It is also involved in promoting the cellular uptake of chylomicron remnants, cholesterol-rich lipoproteins, and free fatty acids. LPL requires ApoC-II as a cofactor. LPL is attached to the luminal surface of endothelial cells in capillaries by the protein glycosylphosphatidylinositol HDL-binding protein 1 (GPIHBP1) and by heparan sulfated peptidoglycans. It is most widely distributed in adipose, heart, and skeletal muscle tissue, as well as in lactating mammary glands. In brief, LPL is secreted from heart, muscle and adipose parenchymal cells as a glycosylated homodimer, after which it is translocated through the extracellular matrix and across endothelial cells to the capillary lumen. After translation, the newly synthesized protein is glycosylated in the endoplasmic reticulum. The glycosylation sites of LPL are Asn-43, Asn-257, and Asn-359. Glucosidases then remove terminal glucose residues; it was once believed that this glucose trimming is responsible for the conformational change needed for LPL to form homodimers and become catalytically active. In the Golgi apparatus, the oligosaccharides are further altered to result in either two complex chains, or two complex and one high-mannose chain. In the final protein, carbohydrates account for about 12% of the molecular mass (55-58 kDa). Homodimerization is required before LPL can be secreted from cells. After secretion, LPL is carried across endothelial cells and presented into the capillary lumen by the protein glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1. 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 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. LPL isozymes are regulated differently depending on the tissue. For example, insulin is known to activate LPL in adipocytes and its placement in the capillary endothelium. By contrast, insulin has been shown to decrease expression of muscle LPL. Muscle and myocardial LPL is instead activated by glucagon and adrenaline. This helps to explain why during fasting, LPL activity increases in muscle tissue and decreases in adipose tissue, whereas after a meal, the opposite occurs.

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