The fatty acids in the body may arise from the diet or may be synthesized from carbohydrates as well.
Acetyl CoA is the building block for the fatty acid chain. Therefore, any substance which can yield acetyl CoA may serve as a source of fatty acids.
The acetate position of acetyl CoA supplies the carbon atoms of the fatty acid chain, and the chain is, in an overall sense, built up essentially by a process of successive additions of this two-carbon fragment to an existing chain.
This accounts for the fact that naturally occurring fatty acids have an even number of carbon atoms.
Several processes have been recognized for the synthesis of fatty acids. One process, which takes place in liver mitochondria is essentially the reverse, of the ^-oxidative pathway.
The principal exception is that saturation of the a-/3 double bond requires NADPH whereas desaturation requires FAD.
Most fatty acid synthesis takes place in the cell cytoplasm by a process which involves CO, fixation.
The process proceeds as follows: The acetyl CoA is converted to malonyl CoA by fixation of CO2, e.g., carboxylation.
The malonyl CoA then reacts with the Co-enzyme A derivative of a fatty acid to form the Co-enzyme A derivative of a new fatty acid containing two additional carbon atoms, e.g., the malonyl CoA reacts with acetyl CoA to form acetoacetyl Co-enzyme A. The entire reaction can be represented as follows:
The alcohol is dehydrated and the double bond is reduced to give butyryl CoA.
Butyryl CoA, thus formed, then reacts with another malonyl CoA and repeats the entire sequence of the reactions given above.
In this way, the fatty acids are synthesized, two carbons being added with each sequence of reactions, which are repeated, etc.