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Odd-chain fatty acid

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Odd-chain fatty acids are those fatty acids that contain an odd number of carbon atoms. In addition to being classified according to their saturation or unsaturation, fatty acids are also classified according to their odd or even numbers of constituent carbon atoms. With respect to natural abundance, most fatty acids are even chain, e.g. palmitic (C16) and stearic (C18). In terms of physical properties, odd and even fatty acids are similar, generally being colorless, soluble in alcohols, and often somewhat oily.[1] 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]

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

Metabolism

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Biosynthesis

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The most common OCFA are the saturated C15 and C17 derivatives, respectively pentadecylic acid and margaric acid.[3] Even-chained fatty acids are synthesized 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]

Degradation

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Beta oxidation of odd-numbered fatty acids

Compared to the oxidation of even-numbered fatty acids, the oxidation of odd-chain fatty acids produces propionyl-CoA in addition to acetyl-CoA, which means that the oxidation 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 a racemic reaction prior to reaching succinyl-CoA.[6] Finally, methylmalonyl-CoA mutase, a vitamin B12-dependent enzyme, converts L-methylmalonyl-CoA into succinyl-CoA using a free radical mechanism. Succinyl-CoA is an intermediate of the TCA cycle and can readily enter there.[7]

Examples

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Lipid

number

Name Salt/Ester Name Formula Mass

(g/mol)

Diagram
Common Systematic Common Systematic Molecular Structural
C3:0 Propionic acid Propanoic acid Propionate Propanoate C3H6O2 CH3CH2COOH 74.08
 
C15:0 Pentadecylic acid Pentadecanoic acid Pentadecanoate Pentadecanoate C15H30O2 CH3(CH2)13CO2H 242.40
 
C17:0 Margaric acid Heptadecanoic acid Margarate Heptadecanoate C17H34O2 CH3(CH2)15CO2H 270.45
 
C17:1 Heptadecenoic acid Cis-10-heptadecenoic acid Heptadecenoate Cis-10-heptadecenoat C17H32O2 CH₃-(CH₂)₇-CH=CH-(CH₂)₇-COOH 268.4

Occurrence

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OCFAs are found particularly in ruminant fat and milk (e.g. pentadecylic 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, propionic acid is produced by intestinal bacteria in the gut.[8] 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.[9] Odd-chain and branched-chain fatty acids, which form propionyl-CoA, can serve as minor precursors for gluconeogenesis.[10][4]

References

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  1. ^ Smith S (December 1994). "The animal fatty acid synthase: one gene, one polypeptide, seven enzymes". FASEB Journal. 8 (15): 1248–1259. doi:10.1096/fasebj.8.15.8001737. PMID 8001737. S2CID 22853095.
  2. ^ Rezanka T, Sigler K (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 M, Jaudszus A (July 2016). "Pentadecanoic and Heptadecanoic Acids: Multifaceted Odd-Chain Fatty Acids". Advances in Nutrition. 7 (4): 730–734. doi:10.3945/an.115.011387. PMC 4942867. PMID 27422507.
  4. ^ a b 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–PC55. doi:10.1016/S0021-9258(18)64092-X. 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. 6 (6): 711–720. doi:10.1016/S0969-2126(98)00073-2. PMID 9655823.
  8. ^ Macfabe DF (2012-08-24). "Short-chain fatty acid fermentation products of the gut microbiome: implications in autism spectrum disorders". Microbial Ecology in Health and Disease. 23 (0). doi:10.3402/mehd.v23i0.19260. PMC 3747729. PMID 23990817.
  9. ^ 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): 254–260. doi:10.1006/abbi.1994.1110. PMID 8135535.
  10. ^ Baynes J, Dominiczak M. Medical Biochemistry (4th ed.). Elsevier.
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