Odd-chain fatty acid
Odd-chain fatty acids are those that contain an odd number of carbon atoms in the structure. Oxidation of odd-carbon fatty acids requires three additional enzymes. The first is Propionyl-Coa Cabroxylase. This enzyme is responsible for carboxylating the α-carbon of a Propionyl-CoA to produce D-methylmalonyl-CoA. 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. Scientifically, this is a fairly recently discovered enzyme, it was researched during the late 1900's and one of the first few publications of the enzyme was published in 1961. Although not much was known, researchers concluded that there was indeed an racemic reaction prior to reaching succinyl-CoA.  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.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. With these three reactions completed to success, the Fatty Acid is allowed to continue through normal β-Oxidation rounds.
Almost all animal fatty acid synthesis is done by assembling two 2C acetyl-CoA molecules together. 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 synthesis of long-chain fatty acids with an odd number of carbon atoms, which 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. Odd-chain and branched-chain fatty acids, which form propionyl-CoA, can serve as minor precursors for gluconeogenesis.
- 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.
- 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.
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