Omega oxidation (ω-oxidation) of fatty acid

• Omega oxidation (ω-oxidation) is a process of fatty acid metabolism in some species of animals.
• It is an alternative pathway to beta oxidation that, instead of involving the β carbon, involves the oxidation of the ω carbon (the carbon most distant from the carboxyl group of the fatty acid).
• The ω (omega)-carbon (the methyl carbon) of fatty acids is oxidized to a carboxyl group in the endoplasmic reticulum.
• The process is normally a minor catabolic pathway for medium-chain fatty acids (10-12 carbon atoms), but becomes more important when β oxidation is defective (because of mutation or a carnitine deficiency, for example).

Location of Omega oxidation

In vertebrates, the enzymes for ω oxidation are located in the smooth ER of liver and kidney cells, instead of in the mitochondria as with β-oxidation.

• Substrate: Medium to Long chain fatty acids (Fatty acid with 10-12 carbon atoms).
• End Product: Dicarboxylic acids are produced.

Steps in ω-oxidation of Fatty acids

• The first step introduces a hydroxyl group onto the ω-carbon.
  • The oxygen for this group comes from molecular oxygen (O₂) in a complex reaction that involves cytochrome P450 and the electron donor NADPH.
  • Reactions of this type are catalyzed by mixed function oxidases.
• Two more enzymes now act on the ω carbon:
  • Alcohol dehydrogenase oxidizes the hydroxyl group to an aldehyde, and
  • Aldehyde dehydrogenase oxidizes the aldehyde group to a carboxylic acid, producing a fatty acid with a carboxyl group at each end.
• At this point, either end can be attached to coenzyme A, or the molecule can enter the mitochondrion and undergo β-oxidation by the normal route.
• In each pass through the oxidation pathway, the “double-ended” fatty acid yields dicarboxylic acids such as succinic acid, which can enter the citric acid cycle, and adipic acid.

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Significance of Omega oxidation

• It is a subsidiary pathway for β-oxidation of fatty acids when β-oxidation is blocked.
• It is observed that ω- and (ω-1)-oxidation of fatty acids are related to energy metabolism in some laboratory animals such as musk shrews and Mongolian gerbils.
• Studies confirm that ω- and (ω-1)-oxidation of fatty acids play crucial roles in the production of insect pheromones of honeybees and in the formation of biopolyesters of higher plants.
• Many studies also have demonstrated that the ω-oxidation serves to provide succinyl-CoA for the citric acid cycle and for gluconeogenesis under conditions of starvation and diabetes.

References

1. Smith, C. M., Marks, A. D., Lieberman, M. A., Marks, D. B., & Marks, D. B. (2005). Marks’ basic medical biochemistry: A clinical approach. Philadelphia: Lippincott Williams & Wilkins.
2. https://www.slideshare.net/AnupShamsherBudhatho/fatty-acid-oxidation-beta-alpha-omega-and-peroxisomal
3. MIURA Y. The biological significance of ω-oxidation of fatty acids. Yamakawa T, ed. Proceedings of the Japan Academy Series B, Physical and Biological Sciences. 2013;89(8):370-382. doi:10.2183/pjab.89.370.
4. https://en.wikipedia.org/wiki/Omega_oxidation