The Enzyme Database


Enzyme Nomenclature News

April 2020

On the nomenclature of fatty acids

[Prepared by Ron Caspi]

The term "fatty acid" was originally coined to describe aliphatic monocarboxylic acids derived from or contained in esterified form in an animal or vegetable fat, oil or wax. However, over the years the term has been expanded to refer to additional, shorter monocarboxylic acids with an aliphatic tail such as propanoic and butanoic acids (but not the shorter formic and acetic acids).

The length of a fatty acid is determined by the length of the longest carbon chain, including the carbon of the carboxy group. Common natural fatty acids usually have an even number of carbons in the longest chain and can be either saturated or unsaturated. In many organisms they are often modified by branching, hydroxylation, methylation, epoxidation and other types of modifications.

Inside living cells fatty acids are rarely found in free form. They are usually bound to coenzyme A or acyl-carrier proteins, or form part of triglycerides, phospholipids, lipopolysaccharides, and cholesterol esters.

Many enzymes that act on fatty acids, as well as the corresponding fatty acyl-CoAs, alcohols, and aldehydes, recognise substrates with a limited range of chain lengths. To classify these enzymes, it is helpful to divide the substrates into smaller groups based on their chain length.

The following subgroups have been defined:

Short-chain fatty acids (SCFA) have 3–5 carbons in the longest chain.

Medium-chain fatty acids (MCFA) have 6–12 carbons in the longest chain.

Long-chain fatty acids (LCFA) have 13–22 carbons in the longest chain.

Very-long-chain fatty acids (VLCFA) have 23–27 carbons in the longest chain.

Ultra-long-chain fatty acids (ULCFA) have more than 27 carbons in the longest chain.

The same terminology applies to fatty acyl-CoAs, fatty acyl-[acyl-carrier proteins], fatty alcohols, and fatty aldehydes.

When used in the accepted names of enzymes, these terms refer to the chain-length range towards which the enzyme is most active. However, it should be noted that in many cases the enzyme may have some activity towards substrates above or below that range.