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 and oleic. 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. 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 are also known. Some of these VLCFAs are also of the odd-chain variety.
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. 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. 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 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. 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.
