Fat catabolism: generation of energy by fatty acid oxidation
Fat (triacylglycerol) and Fatty Acids: 90% of dietary lipids are tryacylglycerol, a Michael Cooper Has cut
hydrophobic, neutral molecule made from reaction of OH group of glycerol and - his calorie intake to
COO- group of fatty acids. nearly half of the daily
Fatty acids are made up of a long hydrophobic hydrocarbon chain (highly
reduced) and a carboxylic acid polar group. Different kinds of fatty acids play He does not have any fat
very important structural (as major component of membrane structures) and storage, thus he faces
functional role. the following problems;
In this part of discussion we will mainly focus of the digestion, transport and 1. Looks sick: bad
catabolism of triglycerides. Although other lipids like cholesterol, sphingolipids complexion or look
are important too but they will not be covered in this course.
2. He feels terribly cold
Sources of Fat available for oxidation: 1. Dietary fat, 2. Excess dietary fat stored even at 20 oC: no
in adipose tissue and 3. Excess carbohydrate calories that are converted to FAT insulation.
in liver and transported to adipose tissues.
3. Cannot afford to
Function: starve even for a few
days; no fat store to
-Stored fat acts as major energy source when there is no carbohydrate available,
-it also serves as source for supply of fatty acids required for important cellular
-provide insulation and complexion to body
Different kinds of
We will focus on
Cholesterol plays an
important role in
Tri-acylglycerols (fat) molecules are highly hydrophobic. After ingestion they are Some important players involved in lipid digestion and transport:
present in the form of oil droplets. In order to get digested and absorbed, they
Bile Acids: These are amphiphathic (with polar and non-polar structures)
need to be emulsified (partially solubilized.
detergent like molecules synthesized in liver. They help solubilize or emulsify
Liver produces bile acids which are collected in gall bladder and released into the triglycerides (fat) in the small intestine.
intestine, where it emulsifies fat.
Pancreatic Lipases: These are pancreatic enzymes that catalyze the
Bile acids are amphipathic, detergent like molecules capable of binding to hydrolysis of fat to release fatty acids and glycerol in intestine.
hydrophobic as well as hydrophilic molecules.
Bile acid and Fatty acid binding proteins: They facilitate absorption of
lipids in intestine.
Intestinal Fatty acid binding proteins (I-FABP): these proteins are present
inside the intestinal cells, they bind to fatty acids and protect cells from the
detergent like behavior of fatty acids.
Chylomicrons: These are lipoprotein granules containing specific
lipoproteins, dietary cholesterol, phospholipids and triacylglycerol. These
transport granules transport cholesterol and lipids from intestine to adipose
tissues and liver via blood.
Lipoprotein lipase: These enzymes are present in the capillaries of the
peripheral tissues. They digest triglycerides into Fatty acid and glycerol.
Triacylglycerol lipase: Present in the adipocytes, and activated by protein
kinase to produce fatty acids.
Chylomicrons transport cholesterol and lipids from intestine to adipose
tissues and liver via blood.
Very low density lipoproteins (VLDL): are synthesized in liver and transport
endogenous triacylglycerol and cholesterol from liver to adipose and muscle
tissue. Capilary lipoprotein lipases degrade VLDL and fatty acid and glycerol
are delivered to adipocytes or muscle.
After giving up their triacylglycerols, the VLDL remnants appear in the blood
as Intermediate density lipoproteins (IDL) and then as low density
IDL and LDLs are taken back by liver by specific receptor mediated
High density lipoproteins (HDL) are assembled from the degradation
products of other lipoproteins. HDLs mop up excess cholesterol from other
tissues and deliver it to liver.
Utilization of stored fat
•Low blood glucose and energy
need trigger the secretion of
epinephrine and glucagon
•Activation of adnylate cyclase
•Production of cAMP
•Activation of cAMP-dependent
•Phosphorylation and activation
of triacylglycerol lipase
•Release of fatty acids in blood
and binding of fatty acids to
•Transport of FA through serum
albumin to the muscle tissues
•B-oxidation of FA to produce
Catabolism of glycerol after
After the triacylglycerol (fat) is
digested by lipase, it releases
fatty acids, and glycerol.
Glycerol can be converted to
glycerol 3 phosphate and then
to dihydroxy acetone
DHAP is converted to
Glyceraldehyde 3 P by triose
phosphate isomerase (reaction
# 5) which enters the GAPDH
reaction (reaction # 6)of