Lipoprotein lipase

Lipoprotein lipase (LPL) is a member of the is_associated_with::lipase gene family, which includes is_associated_with::pancreatic lipase, is_associated_with::hepatic lipase, and is_associated_with::endothelial lipase. It is a water soluble is_associated_with::enzyme that is_associated_with::hydrolyzes is_associated_with::triglycerides in is_associated_with::lipoproteins, such as those found in is_associated_with::chylomicrons and very low-density lipoproteins (VLDL), into two free is_associated_with::fatty acids and one is_associated_with::monoacylglycerol molecule. It is also involved in promoting the cellular uptake of chylomicron remnants, cholesterol-rich lipoproteins, and free fatty acids. LPL requires is_associated_with::ApoC-II as a cofactor.

LPL is attached to the luminal surface of endothelial cells in is_associated_with::capillaries by heparin sulfated proteoglycans. It is most widely distributed in adipose, heart, and skeletal muscle tissue, as well as in lactating mammary glands.

Synthesis
In brief, LPL is secreted from is_associated_with::parenchymal cells as a glycosylated homodimer, after which it is translocated through the is_associated_with::extracellular matrix and across endothelial cells to the capillary lumen. After translation, the newly synthesized protein is glycosylated in the is_associated_with::endoplasmic reticulum. The glycosylation sites of LPL are Asn-43, Asn-257, and Asn-359. is_associated_with::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 is_associated_with::Golgi apparatus, the is_associated_with::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, however, the mechanism by which LPL travels across endothelial cells is still unknown.

Structure
The is_associated_with::crystal structure of LPL has not been discovered; however, there are substantial experimental evidence and structural homology between members of the lipase family to predict the likely structure and functional regions of the enzyme. LPL is composed of two distinct regions: the larger is_associated_with::N-terminus domain that contains the lipolytic is_associated_with::active site, and the smaller is_associated_with::C-terminus domain. These two regions are attached by a peptide linker. The N-terminus domain has an α/β hydrolase fold, which is a globular structure containing a central β sheet surrounded by α helices. The C-terminus domain is a β sandwich formed by two β sheet layers, and resembles an elongated cylinder.

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 is_associated_with::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 is_associated_with::LDL’s receptors. Both the N-and C-terminal domains contain is_associated_with::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 is_associated_with::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 is_associated_with::rate-limiting step in the reaction.

Function
LPL encodes lipoprotein lipase, which is expressed on endothelial cells in the heart, muscle, and adipose tissue. LPL functions as a homodimer, and has the dual functions of triglyceride hydrolase and ligand/bridging factor for receptor-mediated lipoprotein uptake. Through catalysis, VLDL is converted to IDL and then to LDL. Severe mutations that cause LPL deficiency result in type I hyperlipoproteinemia, while less extreme mutations in LPL are linked to many disorders of lipoprotein metabolism.

Regulation
LPL is controlled transcriptionally and posttranscriptionally. The is_associated_with::circadian clock may be important in the control of Lpl mRNA levels in peripheral tissues.

LPL is_associated_with::isozymes are regulated differently depending on the tissue. For example, is_associated_with::insulin is known to activate LPL in is_associated_with::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.

Consistent with this, dietary macronutrients differentially affect adipose and muscle LPL activity. After 16 days on a high-carbohydrate or a high-fat diet, LPL activity increased significantly in both tissues 6 hours after a meal of either composition, but there was a significantly greater rise in adipose tissue LPL in response to the high-carbohydrate diet compared to the high-fat diet. There was no difference between the two diets' effects on or insulin sensitivity, or on fasting LPL activity in either tissue.

The concentration of LPL displayed on endothelial cell surface cannot be regulated by endothelial cells, as they neither synthesize nor degrade LPL. Instead, this regulation occurs by managing the flux of LPL arriving at the lipolytic site and being released into circulation attached to lipoproteins. The typical concentration of LPL in plasma is in the nanomolar range.

Pathology
is_associated_with::Lipoprotein lipase deficiency leads to is_associated_with::hypertriglyceridemia (elevated levels of is_associated_with::triglycerides in the bloodstream). In mice, overexpression of LPL has been shown to affect insulin response and to promote obesity.

A high tissue LPL response to a high-carbohydrate diet may predispose toward fat gain. One study reported that subjects gained more body fat over the next four years if, after following a high-carbohydrate diet and partaking of a high-carbohydrate meal, they responded with an increase in adipose tissue LPL activity per adipocyte, or a decrease in skeletal muscle LPL activity per gram of tissue.

Interactions
Lipoprotein lipase has been shown to interact with is_associated_with::LRP1. It is also a ligand for is_associated_with::α2M, is_associated_with::GP330, and VLDL receptors. LPL has been shown to be a ligand for is_associated_with::LRP2, albeit at a lower affinity than for other receptors; however, most of the LPL-dependent VLDL degradation can be attributed to the LRP2 pathway. In each case, LPL serves as a bridge between receptor and lipoprotein. While LPL is activated by ApoC-II, it is inhibited by is_associated_with::ApoC-III.

In other organisms
The LPL gene is highly conserved across vertebrates. Lipoprotein lipase is involved in lipid transport in the placentae of live bearing lizards (is_associated_with::Pseudemoia entrecasteauxii).