Principles of Biochemistry

6.5.1 Metabolism of fat

Digestion of fat

Triacylglycerols constitute ~90% of dietary lipids and are the major form of metabolic energy storage material. They are broken down by co-ordinated actions of bile salt &  lipases, and the products are absorbed by the intestine and they are converted into lipoproteins and transported to liver and other tissues and stored as triglycerides.

When energy is needed the triglyceride is first hydrolyzed to fatty acids and glycerol mostly in adipose tissue by lipase.Bile salts of bile acids synethesized & secreted by liver cells in bile. In the intestinal inmer, their amphipathic moleculer concentrate on the surface of each fat droplet to form a surface monolayer. This lowers the surface tension of fat droplets & so the interjacial thension between fat droplet & the surrounding aqueous medium. This causestiny droplets to flow out from the surfaces of larger droplets, forming a stable fine emulsion in the aqueous medium (emulsifying action).By decimating each large fat droplet into many smaller dres,emulsification enhances the surface area of fat droplets & thereby increases lipase action on them. The free fatty acids are released into the plasma where they combine with serum albumin. Long chain fatty acids are oxidized in liver, heart, kidney, muscle,lung, testis, brain and adipose tissue. Glycerol is utilized by liver, kidney, intestine and lactating mammary gland where the activating enzyme glycerokinase is present abundantly.

Oxidation of fatty acids

The complete oxidation of fatty acid in the body ultimately leads to oxidation to CO₂ and H₂O, and the liberation of energy. The fatty acids of fat are broken down into two carbon units as acetyl CoA, CH₃COCoA, by an overall process called ß oxidation.

The ß-oxidation means the oxidations takes place in the ß carbon of the fatty acid with the removal of 2 carbon atoms at a time from the carboxyl end of the molecule. The fatty acids containing even number and odd number of carbon atoms as well as the unsaturated fatty acids are oxidized by ß-oxidation.