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Alpha Oxidation


Alpha Oxidation

  • Alpha – oxidation is defined as the oxidation of fatty acid (methyl group at beta carbon) with the removal of one carbon unit adjacent to the α-carbon from the carboxylic end.
  • The carbon unit is removed in the form of CO2.
  • Alpha oxidation occurs in those fatty acids that have a methyl group (-CH3) at the beta-carbon, which blocks beta oxidation.
  • There is no production of ATP.

Alpha Oxidation



Location

Peroxisomes are the cellular sites for α-oxidation.

  • Substrate: Phytanic acid, which is present in milk or derived from phytol present in chlorophyll and animal fat.
  • Phytanic acid is a 20-carbon, branched-chain fatty acid.

The Pathway

  • Branched chain fatty acids are oxidized at the α-carbon (mainly in brain and other nervous tissue), and the carboxyl carbon is released as CO2. Branches can interfere with the normal β-oxidation pathway, most often at the acyl-CoA dehydrogenase step.
  • The fatty acid is thus degraded by one carbon initially, and then two carbons at a time. Both acetyl-CoA and propionyl-CoA are products if the branches are methyl groups.

Steps of alpha oxidation

  1. Activation of phytanic acid
  2. Hydroxylation
  3. Removal of formyl CoA (CO2)
  4. Oxidation of Pristanal
  5. Beta-oxidation of pristanic acid

Reactions Involved

Reactions Involved phytanic acid




  1. Phytanic acid is first attached to CoA to form phytanoyl-CoA.
  2. Phytanoyl-CoA is oxidized by phytanoyl-CoA dioxygenase, in a process using Fe2+and O2, to yield 2-hydroxyphytanoyl-CoA.
  3. 2-hydroxyphytanoyl-CoA is cleaved by 2-hydroxyphytanoyl-CoA lyase in a TPP-dependent reaction to form pristanal and formyl-CoA (in turn later broken down into formate and eventually CO2).
  4. Pristanal is oxidized by aldehyde dehydrogenase to form pristanic acid (which can then undergo beta-oxidation).
  5. Propionyl-CoA is released as a result of beta oxidation when the beta carbon is substituted.

Significance of alpha oxidation

  • α- oxidation is important in the catabolism of branched-chain fatty acids.
  • Oxidation of methylated fatty acid.
  • Production of cerebronic acid which synthesizes cerebroside and sulfatides.
  • Production of odd chain fatty acids.
  • The reaction is also a route for the synthesis of hydroxy fatty acids. The α-hydroxy fatty acid can be further oxidized and decarboxylated to a fatty acid one carbon shorter than the original. Thus, if an odd-chain-length compound is used initially, an even-chain-length acid is produced that can be further oxidized by β-oxidation.

Associated Diseases

  • Refsum’s disease is the only defect in peroxisomal α-oxidation currently known.
  • The Refsum’s disease is an autosomal recessive disorder and is the deficiency of phytanoyl-CoA hydroxylase.
  • The clinical characteristics of Refsum’s disease include peripheral neuropathy and ataxia, retinitis pigmentosa, and abnormalities of skin and bones.

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.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/alpha-oxidation
  3. https://www.slideshare.net/AnupShamsherBudhatho/fatty-acid-oxidation-beta-alpha-omega-and-peroxisomal
  4. https://en.wikipedia.org/wiki/Alpha_oxidation
  5. Rodwell, V. W., Botham, K. M., Kennelly, P. J., Weil, P. A., & Bender, D. A. (2015). Harper’s illustrated biochemistry (30th ed.). New York, N.Y.: McGraw-Hill Education LLC.

Alpha Oxidation

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