Odd-chain fatty acids are those

fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ...

s 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. 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.

Biosynthesis

The most common OCFA are the saturated C15 and C17 derivatives, respectively

pentadecanoic acid Pentadecylic acid, also known as pentadecanoic acid or C15:0 is an odd-chain saturated fatty acid. It is a colorless solid. A laboratory preparation involves permanganate oxidation of 1-hexadecene 1-Hexadecene, also known as 1-cetene, is a l ...

and

heptadecanoic acid Margaric acid, or heptadecanoic acid, is a saturated fatty acid. Its molecular formula is CH3(CH2)15CO2H. Classified as an odd-chain fatty acid, it occurs as a trace component of the fat and milkfat of ruminants. Salts and esters of margaric acid ...

. The synthesis of even-chained

synthesis is done by assembling

acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized fo ...

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.

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. 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. 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.

Occurrence

OCFAs are found particularly in

ruminant Ruminants ( suborder Ruminantia) are hoofed herbivorous grazing or browsing mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. Th ...

fat and

milk Milk is a white liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals (including breastfed human infants) before they are able to digest solid food. Immune factors and immune-modulati ...

(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 Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to ...

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.{{cite book , title = Medical Biochemistry , edition = 4th , first1 = John , last1 = Baynes , first2 = Marek , last2 = Dominiczak , name-list-style = vanc , publisher = Elsevier

References

Fatty acids