Odd-chain fatty acid

Odd-chain fatty acids are those fatty acids that contain an odd number of carbon atoms. Most fatty acids are even chain, e.g. stearic (C16) and oleic (C18). So in addition to being classified according to their saturation of unsaturation, fatty acids are also classified according to the odd vs. even numbers of constituent carbon atoms. In terms of physical properties, odd and even fatty acids are similar, generally being colorless, soluble in alcohols, and often somewhat oily.[1] On a molecular level, the odd-chain fatty acids are biosynthesized and metabolized slightly differently from the even-chained relatives. In addition to the usual C12-C22 long chain fatty acids, some very long chain fatty acids (VLCFAs) are also known. Some of these VLCFAs are also of the odd-chain variety.[2]

Heptadecanoic acid with its seventeen carbon atoms is an important odd-chain fatty acid.

Biosynthesis
Nervonic acid is a monounsaturated odd-chain fatty acid, which serves as a biomarker for certain human disorders.

The most common OCFA are the saturated C15 and C17 derivatives, respectively pentadecanoic acid and heptadecanoic acid.[3] The synthesis of even-chained fatty acid synthesis is done by assembling acetyl-CoA precursors. Because the segments are each two carbons in length the resulting fatty acid has an even number of carbon atoms in it. However, propionyl-CoA instead of acetyl-CoA is used as the primer for the biosynthesis of long-chain fatty acids with an odd number of carbon atoms.[4]

Metabolism

Oxidation of odd-carbon fatty acids requires three additional enzymes. The first is propionyl-Coa carboxylase. This enzyme is responsible for carboxylating the α-carbon of a Propionyl-CoA to produce D-methylmalonyl-CoA.[5] After this, methylmalonyl-CoA epimerase carries out an isomerization reaction. Specifically, the D-isomer produced by the carboxylase reaction is transformed into the L-isomer of Methylmalonyl-CoA. This is a recently discovered enzyme, it was researched during the late 1900s and the first publication was in 1961. Researchers concluded that there was indeed an racemic reaction prior to reaching succinyl-CoA. [6] To reach Succinyl-CoA, the L-Isomer of the if Methylmalonyl-Coa is used as a substrate by Methylmalonyl-CoA mutase. This reaction is essentially a switch in positions between the carboxylated α-Carbon and the β-Carbon. For this reaction to take place, the enzyme works with a cofactor known as a Vitamin B12, allowing the mechanism to take place through a free radical mechanism[7] With these three reactions completed to success, the fatty acid is allowed to continue through normal β-Oxidation rounds.

Occurrence

OCFAs are found particularly in ruminant fat and milk (e.g. heptadecanoic acid). Some plant-based fatty acids, also have an odd number of carbon atoms, and Phytanic fatty acid absorbed from the plant chlorophyll has multiple methyl branch points. As a result, it breaks down into three odd-numbered 3C Propionyl segments as well as three even-numbered 2C Acetyl segments and one even numbered 4C Isobutynoyl segment. In humans, in sharp contrast to butyrate and octanoate, the odd-chain SCFA, propionate, has no inhibitory effect on glycolysis and does not stimulate ketogenesis.[8] Odd-chain and branched-chain fatty acids, which form propionyl-CoA, can serve as minor precursors for gluconeogenesis.[9][4]

References
  1. Smith, S. (1994). "The Animal Fatty Acid Synthase: One Gene, One Polypeptide, Seven Enzymes". The FASEB Journal. 8 (15): 1248–1259. doi:10.1096/fasebj.8.15.8001737. PMID 8001737. S2CID 22853095.
  2. Řezanka, Tomáš; Sigler, Karel (2009). "Odd-Numbered Very-Long-Chain Fatty Acids from the Microbial, Animal and Plant Kingdoms". Progress in Lipid Research. 48 (3–4): 206–238. doi:10.1016/j.plipres.2009.03.003. PMID 19336244.
  3. Pfeuffer, Maria; Jaudszus, Anke (2016). "Pentadecanoic and Heptadecanoic Acids: Multifaceted Odd-Chain Fatty Acids". Advances in Nutrition: An International Review Journal. 7 (4): 730–734. doi:10.3945/an.115.011387. PMC 4942867. PMID 27422507.
  4. Rodwell VW. Harper's Illustrated Biochemistry (31st ed.). McGraw-Hill.
  5. Wongkittichote P, Ah Mew N, Chapman KA (December 2017). "Propionyl-CoA carboxylase - A review". Molecular Genetics and Metabolism. 122 (4): 145–152. doi:10.1016/j.ymgme.2017.10.002. PMC 5725275. PMID 29033250.
  6. Mazumder R, Sasakawa T, Kaziro Y, Ochoa S (August 1961). "A new enzyme in the conversion of propionyl coenzyme A to succinyl coenzyme A". The Journal of Biological Chemistry. 236 (8): PC53-5. PMID 13768681.
  7. .Mancia F, Evans PR (June 1998). "Conformational changes on substrate binding to methylmalonyl CoA mutase and new insights into the free radical mechanism". Structure. London, England. 6 (6): 711–20. doi:10.1016/S0969-2126(98)00073-2. PMID 9655823.
  8. Morand C, Besson C, Demigne C, Remesy C (March 1994). "Importance of the modulation of glycolysis in the control of lactate metabolism by fatty acids in isolated hepatocytes from fed rats". Archives of Biochemistry and Biophysics. 309 (2): 254–60. doi:10.1006/abbi.1994.1110. PMID 8135535.
  9. Baynes J, Dominiczak M. Medical Biochemistry (4th ed.). Elsevier.
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