Oxidation of Fatty Acids
The digestion of fats starts in the small intestine. Fats are emulsified by the bile salts and hydrolysed by the pancreatic lipases to form free fatty acids. These free fatty acids combine with glycerol (produced by the glycolytic process) to form triglycerides. They combine with proteins to form lipoproteins and enter into circulation to perform various biolological functions such as oxidation, storage and formation of new lipids. Thus the various fatty acids may exist in the free form as well as in the esterified form (Triglyceride) in blood.
Fatty acids are the immediate source for oxidation of fats in various tissues viz. liver, adipose tissue, muscles, heart, kidney, brain, lungs and testes.
Fatty acids are oxidised to CO2 and water with the liberation of large amount of energy. Oxidation is brought about in the mitochondria because all the enzymes required for oxidation are present in the mitochondria. Oxidation of fatty acids is of three types, based on the position of the carbon atom which gets oxidised (α, β and γ)
However b-oxidation of fatty acids is predominant and widely prevalent and it provides large amount of energy than a and g oxidation. b-oxidation of fatty acids can be conveniently studied under different stages as detailed below.
Fatty acids are relatively inert chemical molecules and hence they must be converted to an active intermediate for the initiation of b-oxidation. The activation of fatty acids takes place in the cytosol in the presence of ATP, coenzyme A and acyl CoA synthetase. The activated fatty acid then enters into mitochondria with the help of a carrier protein, carnitine in the presence of a enzyme carnitine acyl transferase.
Oxidation of acyl CoA (fatty acid) takes place through several steps leading to the formation of acetyl CoA (C2) and an acyl CoA having two carbon atoms less than
the original fatty acid with which the b-oxidation cycle originally started. Acyl CoA then enters into a similar oxidation cycle until all the carbon atoms are released as acetyl CoA. These reactions require cofactors like Flavin Adenine Dinucleotide (FAD) and Nicotinamide Adenine Dinucleotide (NAD+).