Lipid Metabolism

Fatty acids have four major physiologic roles in the cell:
? Building blocks of phospholipids and glycolipids
? Added onto proteins to create lipoproteins, which targets them to membrane
locations
? Fuel molecules - source of ATP
? Fatty acid derivatives serve as hormones and intracellular messengers
Absorption and Mobilization of Fatty Acids
? Most lipids are triacylglycerols, some are phospholipids and cholesterol.
? Digestion occurs primarily in the small intestine.
? Fat particles are coated with bile salts (amphipathic) from gall bladder.
? Degraded by pancreatic lipase (hydrolyzes C-1 and C-3 ---> 2 fatty acids and 2-
monoacylglycerol).
? Can then be absorbed by intestinal epithelial cells; bile salts are recirculated after
being absorbed by the intestinal epithelial cells.
? In the cells, fatty acids are converted by fatty acyl CoA molecules.
? Phospholipids are hydrolyzed by pancreatic phospholipases, primarily phospholipase
A2.
? Cholesterol esters are hydrolyzed by esterases to form free cholesterol, which is
solubilized by bile salts and absorbed by the cells.
? Lipids are transported throughout the body as lipoproteins.
? Lipoproteins consist of a lipid (tryacylglycerol, cholesterol, cholesterol ester) core
with amphipathic molecules forming layer on outside.
Lipoproteins
Both transported in form of lipoprotein particles, which solubilize hydrophobic lipids
and contain cell-targeting signals.
Lipoproteins classified according to their densities:
o chylomicrons - contain dietary triacylglycerols
o chylomicron remnants - contain dietary cholesterol esters
o very low density lipoproteins (VLDLs) - transport endogenous
triacylglycerols, which are hydrolyzed by lipoprotein lipase at capillary
surface
o intermediate-density lipoproteins (IDL) - contain endogenous cholesterol
esters, which are taken up by liver cells via receptor-mediated endocytosis
and converted to LDLs
o low-density lipoproteins (LDL) - contain endogenous cholesterol esters,
which are taken up by liver cells via receptor-mediated endocytosis; major
carrier of cholesterol in blood; regulates de novo cholesterol synthesis at
level of target cell
o high-density lipoproteins(HDL) - contain endogenous cholesterol esters
released from dying cells and membranes undergoing turnover