Biology Reference
In-Depth Information
C. LIPID CLASSIFICATION SYSTEMS
With so many different lipids, it is not surprising that several different classification systems
have been proposed. However, any classification system is artificial, and so has both advan-
tages and disadvantages. It does not matter what the system is, some lipids will not fit cleanly
into a single category, and so may be placed into several categories, or may even fall entirely
between categories. One good example of this conundrum is classifying the membrane lipid
sphingomyelin (SM, Chapter 5). SM contains sphingosine and so can be classified as a sphin-
golipid, but it also contains phosphate, making it a phospholipid. So, where does SM belong?
Each year countless numbers of previously unknown lipids are isolated and described and so
any classification system must be flexible and expandable to accommodate the newcomers. It
has become evident that a comprehensive nomenclature system is desperately needed before
the task becomes too immense for the scientific community to tackle.
One important theme that runs through all of the proposed membrane lipid classification
systems is the fundamental importance of fatty acids. The simplest division of lipids begins
with the process of hydrolysis (discussed in Chapter 5). If the hydrolyzed lipid releases a free
fatty acid it is deemed 'complex'. If it does not, it is 'simple'. Ironically, while esterified
fatty acids, found in complex lipids, are an essential component of membrane structure,
un-esterified free fatty acids are at most a minor component of membranes and in fact are
harmful to membrane stability (Chapter 5). The two basic lipid types, simple and complex,
are then further subdivided. Two classification systems are shown in
Tables 4.1 and 4.2
.
The earlier system published by Hauser and Poupart in 1991
[4]
is presented in
Table 4.1
.
TABLE 4.1
The 1991 Classification System of Hauser and Poupart
[4]
.
1.
Nonhydrolyzable (Nonsaponifiable) Lipids
Hydrocarbons
Simple alkanes
Terpenes (isoprenoid compounds)
Substituted hydrocarbons
Long-chain alcohols
Long-chain fatty acids
Detergents
Steroids
Vitamins
2.
Simple esters
Acylglycerols
Cholesterol esters
Waxes
3.
Complex lipids
Glycerophospholipids
Sphingolipids
4.
Glycolipids
Glycoglycerolipids
Glycosphingolipids
Cerebrosides
Gangliosides
Lipopolysaccharides
