Fat desaturation is essentially the opposite of hydrogenation. It is the reaction of a fatty acid molecule in which two hydrogen atoms are lost at the appropriate point in the chain (at the site of a single carbon bond) and a double bond is formed between adjacent carbons.
This process is done with appropriate enzymes (so-called desaturases), which are divided into:
- delta (e.g., Δ5desaturase) – the double bond is formed at a particular position, counting from the (carboxyl) start of the chain
- omega (for example ω3desaturase) – the double bond is formed at a particular position, counting from the ω-end of the chain
Desaturation often occurs e.g. within the LC-PUFA synthesis from SC-PUFA. However, for the chain elongation itself, the corresponding elongase (the enzyme which elongates) is responsible. The elongase inserts two carbon atoms to the beginning of the chain, leaving the remainder (especially the “ω-tail” with the complete double bond structure) intact. E.g. EPA can be altered to DPA (docosa-22 vs. eicosa-20) by elongation. Subsequently, using Δ4desaturase, a new double bond (a sixth one – hexa) can be formed at the 4th position to form DHA.
Since mammals do not have either Δ12desaturase or Δ15desaturase, they can’t make double-bonded carbon atoms at the 12th and 15th carbon (ie, 3 and 6 carbons from the end in case of the 18-carbon MUFA), and thus create the essential fatty acids ALA and LA.