Lipids: Definitions, Classification and Nomenclature


Definitions

Although lipid analyst tend to have a firm understanding of what is meant by the term "lipid", there is no widely-accepted definition. The historical origins of the term ‘lipid’ make interesting reading, and should assist with a definition [1]. The term has evolved via ‘lipine’, ‘lipin’, ‘lipoid’ and ‘lipide’ and was originally used in a more restricted sense. Thus, Bloor [2] in 1920 classified lipoids into three groups, simple lipoids (fats and waxes), compound lipoids (phospholipoids and glycolipoids) and derived lipoids (fatty acids, alcohols and sterols). This classification appeals to me and appears to have been accepted until the 1950s. Only later and rather arbitrarily were compounds such as carotenoids, terpenes and steroidal hormones included in the term.

General text books usually describe lipids in woolly terms as a group of naturally occurring compounds, which have in common a ready solubility in such organic solvents as hydrocarbons, chloroform, benzene, ethers and alcohols. They include a diverse range of compounds, like fatty acids and their derivatives, carotenoids, terpenes, steroids and bile acids. It should be apparent that many of these compounds have little by way of structure or function to relate them. In fact, a definition of this kind is positively misleading, since many of the substances that are now widely regarded as lipids may be almost as soluble in water as in organic solvents.

There is no definition of lipid that has been accepted by any international body that recommends standards or comments on nomenclature issues. My preferred definition was first put forward in 1987 [3]. I recommend it until something better is proposed. It is -

Lipids are fatty acids and their derivatives, and substances related biosynthetically or functionally to these compounds.

A further useful guide to lipid nomenclature and structures is a paper by Fahy and colleagues [4], which is available from the Journal of Lipid Research as a free download here.... It is complemented by a website –  Lipid Maps.

I applaud their attempt to define the term 'lipid', though their definition still seems too broad for me , i.e.

Hydrophobic or amphipathic small molecules that may originate entirely or in part by carbanion-based condensations of thioesters (fatty acids, polyketides, etc.) and/or by carbocation-based condensations of isoprene units (prenols, sterols, etc.).

This definition appears to suggest that almost any organic compound not a carbohydrate or a protein is a lipid. It may make sense to a biochemist, but what are physical chemists, food scientists and so forth to make of it? Also, what constitutes 'small' in molecular terms?


Classification

cartoon scientistIn addition, Fahy and colleagues made a very interesting attempt to classify lipid structures in a manner that facilitates cataloguing by computer. I expressed a number of reservations when this paper was first published and subsequently, mainly because the document seemed to be heavily weighted towards animal lipids, while many unique and important plant lipids had been overlooked. This fault has been largely remedied in a second publication [5], where the authors generously admit their earlier bias and introduce new sub-classifications to encompass the plant glycosyldiacylglycerols including a key lipid in photosynthesis - sulfoquinovosyldiacylglycerols (although lower in their hierarchy than the glycerophospholipids). Sterols are now categorized according to their chemical structures rather than to their biological origins. If this is the agreed philosophy, why are the glycerophospholipids not included with the other glycerolipids?

That said, my own preference would be to elevate more of the sub-classes to a higher level for functional reasons. For example, the glycosyldiacylglycerols have similar physical properties and functions in plant membranes as the glycerophospholipids, and they should be classified in the same rank. I would have three analogous classes for sphingolipids, with perhaps a fourth for gangliosides. For example, to be consistent, phospho-sphingolipids and glyco-sphingolipids should be treated separately by analogy with the glycerolipid classifications. Similarly, in the fatty acyl group, I would elevate eicosanoids/docosanoids to a higher rank for functional reasons, and it appears to me anomalous to leave fatty alcohols and related wax constituents in this group. Another group in a classification limbo are the phosphoglycolipids (as defined here).

The general aims of the two papers are admirable, and hopefully what I consider the faults will be cured in time by dialogue. Lipid Maps is of course a private consortium, not an international standards body, although I expect that their proposals will be given serious consideration by IUPAC-IUB and other international bodies in due course.

I would also argue that a subdivision of glycerolipids into two broad classes according to polarity or complexity is so convenient for analysts, biochemists and physical chemists that it should be given greater weight in any classification system, i.e. those defined in the first edition of Lipid Analysis [6].

‘Simple lipids’ are those that yield on hydrolysis at most two types of primary products per mole; ‘complex lipids’ yield three or more primary hydrolysis products per mole.

This definition has echoes of Bloor’s "simple and compound lipoids". In practice, it is often necessary to subdivide the main groups further. For example, the complex lipids for many purposes are best considered in terms of either the glycerophospholipids (or simply as phospholipids), which contain a polar phosphorus moiety and a glycerol backbone, or the glycolipids, which contain a polar carbohydrate moiety.


Nomenclature

The subject of lipid nomenclature as opposed to classification is dealt with at many points in this website, but especially in relation to individual lipid classes as listed here. For definitive detailed accounts, we can serve you best by pointing to the following documents. All are available to all in the form of PDF files as free downloads from the journals specified below.

Alternatively, the information is available from websites containing the IUPAC guide to the nomenclature of glycerolipids and fatty acids or to that for glyco-/sphingolipids. However, I have found discrepancies between the published and website versions for the former, especially in the shorthand nomenclature for fatty acids. The print versions are correct.

The 'Lipid Glossary 2' by F.D. Gunstone and B. Herslöf (The Oily Press Ltd, 2000) is an invaluable source of information and is available from the publisher in the form of a PDF file as a free download (776 kb).


References

  1. Gidez, L.I. The lore of lipids. J. Lipid Res., 25, 1430-1436 (1984).
  2. Bloor, W.R. Outline of a classification of the lipids. Proc. Soc. Exp. Biol. Med., 17, 138-140 (1920).
  3. Christie, W.W. High-performance liquid chromatography and Lipids: A Practical Guide (Pergamon Press, Oxford) (1987).
  4. Fahy, E. and 17 others. A comprehensive classification system for lipids. J. Lipid Res., 46, 839-862 (2005) (DOI: 10.1002/ejlt.200405001) - reprinted in Eur. J. Lipid Sci. Technol., 107, 337-364 (2005).
  5. Fahy, E. and 9 others. Update of the LIPID MAPS comprehensive classification system for lipids. J. Lipid Res., 50, S9-S14 (2009) (DOI: 10.1194/jlr.R800095-JLR200).
  6. Christie, W.W. Lipid Analysis (1st edition) (Pergamon Press, Oxford)(1973).

W.W. Christie

James Hutton Institute (and Mylnefield Lipid Analysis), Invergowrie, Dundee (DD2 5DA), Scotland.

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