Lipid Metabolism
Prof. Dr. Zeliha Büyükbingöl
Lipids
The term lipids includes a divers group of structurally distinct hydrophobic and amphipathic molecules.
Most of the lipids found in the body fall into the categories of:
Fatty acids
Triacylglycerols
Glycerophospholipids and sphingolipids Eicosanoids
Cholesterol Bile salts
Steroid hormones
Fat-soluble vitamins
Biological Functions
Lipids are synthesized in the body using complex biosynthetic pathways.
Lipids play important role in the body than previously believed.
Lipids play the role of storage of energy
forming cell membranes
intracellular signalling
hormonal regulation
enzyme cofactors
maintenance of body temperature …
PLASMA MEMBRANES
All mammalian cells are enclosed by a plasma membrane which is composed of various lipids, proteins and carbohydrates.
Each layer of plasma membrane lipid bilayer is formed primarly by phospholipids. Additionally, cholesterol maintains membrane fluidity.
Membranes are generally %40-%50 protein. Protein and lipid
composition is unique for each membrane.
Fluid mosaic model
The assembly of proteins and lipids into a membrane creates a fluid mosaic model.
Proteins and lipids undergo two dimensional lateral diffusion in
membrane. Transverse diffusion is energetically unfavorable, exept flippase process.
Fluidty is affected:- long chain saturated fatty acids interact strogly and
reduce fluidity, - double bonds increase fluidity,-cholesterol prevents
movement of fatty acid chains and reduces fluidity,- fluidity increases
with temperature,
LIPID METABOLISM
is the synthesis and
degradation of lipids in cells
OVERVIEW
Exogenous (dietary) & Endogenous Lipids
Digestion and Transport of Dietary Lipids
Lipoproteins
Synhesis of Fatty Acids (FA), Triacylglycerols (TAG) and Phospholipids
Triacylglycerol Mobilization and Fatty Acid Oxidation
Cholesterol Metabolism
Ketone Bodies
Bile salts and Bile Acids
Dietary lipids (fats): Triacylglycerols (TAGs) are the major fat in the
human diet (90%). In addition to TAGs, free fatty acids, phospholipids
, cholesterol and cholesterol esters are present in the foods we eat.
Digestion of Triacylglycerols and the Other Fats
Lingual lipase/ gastric lipase (limited digestion)
Action of bile acid : In the intestine, the fats are emulsified by bile salts.
Action of pancreatic lipase: In the intestine pancreatic lipase-
together with colipase- hydrolyses fatty acids of all chain lengths from position 1 and 3 of the glycerol moiety of the TAG, producing free fatty acids and 2-monoacyl glycerol.
The pancreas also produces esterases that remove fatty acids from
compounds such as cholesterol esters and phospholipase A2 that
digests phospholipids to a free fatty acid and lysophospholipid.
ABSORPTION OF DIETARTY LIPIDS
In the small intestine, dietary fat emulsified by bile salts, which are synhesized in the liver and stored in the gall-bladder. The products of enzymatic digestion form micells with bile acids in the intestinal lumenThe micelles interact with with the enterocyte membrane and allow diffusion of the lipid-soluble components acros sthe membrane. The bile acids do not enter enterocyte , they remain in the intestinal lumen travel down and are reabsorbed sent back to liver by enterohepatic circulation. Short and medium chained fatty acids do not require bile salts for their absorption. Within the intestinal epithelial cells resynthesize TAG from free fatty acids and 2-monoacylglycerol and package with apolipoprotein , phospholips, and cholesterol esters into a water soluble lipoprotein particle called chylomicron (CM). The cylomicrons are secreted into the lymph and eventually into the circulation where they can distrubute dietary lipids to all tissues.
