Biology Reference
In-Depth Information
intracellular changes via downstream effectors. Prenylated proteins are produced by
post-translational modifications where two types of isoprenoids are covalently attached
to cysteine residues at or near the terminal carboxyl group (Chapter 6). The two isopre-
noids are farnesyl (15 carbons, 3 isoprene units) or geranylgeranyl (20 carbons, 4 isoprene
units).
Unusual Fatty Acids
The number of frequently occurring fatty acids is
10 in plants and ~20 in animals.
However, there is a tireless search for other rare and unusual fatty acids in various hidden
corners of the biosphere. Unlike mammals, plant and bacterial fatty acids often have odd
numbered carbon chains. Bacteria commonly have methyl branches or even other functional
groups including cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl rings or epoxy or
hydroxyl groups in their fatty acid chains. At one extreme is the most unsaturated fatty
acid found in nature, 28:8 isolated from a dinoflagellate. There have now been thousands
of strange fatty acids identified that are currently considered to be a curiosity in search of
a function, and so will not be discussed in this topic.
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SUMMARY
Lipids are insoluble in water and have large regions of their surface composed of hydro-
carbons with very few polar groups. Their aversion to water is responsible for the hydro-
phobic effect that drives membrane stability. Membrane lipids are 'amphipathic', with
a polar end anchoring the lipid to the aqueous interface and a long hydrophobic segment
that forms the oily membrane interior. In human cells there are over 1,000 major lipid species
and countless minor lipids. There have been several attempts to classify lipids, the most
recent being a lipidomics-based system by Fahy et al. from 2005. The most important lipid
component of membranes is fatty acids. A fatty acid is a mono-carboxylic acid with a long
(normally 14
24 carbons), un-branched, hydrophobic tail which may be either saturated
or unsaturated (usually 0
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6 cis non-conjugated double bonds). Fatty acids are the basic
building blocks of complex membrane lipids and so are largely responsible for membrane
structure, fluidity, and function.
Although fatty acids comprise the bulk of the membrane hydrophobic interior, they rarely
exist in free, non-esterified form. Chapter 5 discusses complex, polar membrane lipids where
fatty acids are normally found esterified to phospholipids, sphingolipids or some membrane
proteins. A major non-esterified class of membrane lipid, sterols (particularly cholesterol), is
also discussed.
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References
[1] Dowhan W. Molecular basis for membrane phospholipid diversity: Why are there so many lipids? Ann Rev
Biochem 1997;66:199
232.
[2] Nicolaides N, Santos EC. The di- and triesters of the lipids of steer and human meibomian glands. Lipids
1985;20:454
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67.
[3] Borchman D, Foulks GN, Yappert MC, Tang D, Ho DV. Spectroscopic evaluation of human tear lipids. Chem
Phys Lipids 2007;147:87
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102.
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