Page 57 - Color Atlas of Biochemistry

P. 57

48 Biomolecules

Fatty acids and fats (see p. 390). As the organism is capable of
elongating fatty acids by adding C 2 units, but
is not able to introduce double bonds into the
A. Carboxylic acids
end sections of fatty acids (after C-9), arachi-
The naturally occurring fatty acids are carbox- donic acid has to be supplied with the diet.
ylic acids with unbranched hydrocarbon Linoleic and linolenic acid can be converted
chains of 4–24 carbon atoms. They are into arachidonic acid by elongation, and they
present in all organisms as components of can therefore replace arachidonic acid in the
fats and membrane lipids. In these com- diet.
pounds, they are esterified with alcohols
(glycerol, sphingosine, or cholesterol). How-
ever, fatty acids are also found in small B. Structure of fats
amounts in unesterified form. In this case, Fats are esters of the trivalent alcohol glycerol
they are known as free fatty acids (FFAs). As with three fatty acids. When a single fatty acid
free fatty acids have strongly amphipathic is esterified with glycerol, the product is re-
properties (see p. 28), they are usually present ferred to as a monoacylglycerol (fatty acid res-
in protein-bound forms. idue = acyl residue).
The table lists the full series of aliphatic Formally, esterification with additional
carboxylic acids that are found in plants and fatty acids leads to diacylglycerol and ulti-
animals. In higher plants and animals, un- mately to triacylglycerol, the actual fat (for-
branched, longchain fatty acids with either merly termed “triglyceride”). As triacylglycer-
16 or 18 carbon atoms are the most common— ols are uncharged, they are also referred to as
e. g., palmitic and stearic acid. The number of neutral fats. The carbon atoms of glycerol are
carbon atoms in the longer, natural fatty acids not usually equivalent in fats. They are distin-
is always even. This is because they are bio- guished by their “sn” number, where sn
synthesized from C 2 building blocks (see stands for “stereospecific numbering.”
p.168). The three acyl residues of a fat molecule
Some fatty acids contain one or more may differ in terms of their chain length and
isolated double bonds, and are therefore “un- thenumber ofdoublebonds they contain.
saturated.” Common unsaturated fatty acids This results in a large number of possible
include oleic acid and linoleic acid. Of the two combinations of individual fat molecules.
possible cis–trans isomers (see p. 8), usually When extracted from biological materials,
only the cis forms are found in natural lipids. fats always represent mixtures of very similar
Branched fatty acids only occur in bacteria. A compounds, which differ in their fatty acid
shorthand notation with several numbers is residues. A chiral center can arise at the mid-
used for precise characterization of the struc- dle C atom (sn -C-2) of a triacylglycerol if the
ture of fatty acids—e g., 18:2;9,12 for linoleic two external fatty acids are different. The
acid. The first figure stands for the number of monoacylglycerols and diacylglycerols shown
C atoms, while the second gives the number here are also chiral compounds. Nutritional
of double bonds. The positions of the double fats contain palmitic, stearic, oleic acid, and
bonds follow after the semicolon. As usual, linoleic acid particularly often. Unsaturated
numbering starts at the carbon with the high- fatty acids are usually found at the central
est oxidation state (i. e., the carboxyl group C atom of glycerol.
corresponds to C-1). Greek letters are also The length of the fatty acid residues and
commonly used (α =C-2; β =C-3; ω =the the number of their double bonds affect the
last carbon, ω-3 = the third last carbon). melting point of the fats. The shorter the fatty
Essential fatty acids are fatty acids that acid residues and themoredoublebonds they
have to be supplied in the diet. Without ex- contain, the lower their melting points.
ception, these are all polyunsaturated fatty
acids: the C 20 fatty acid arachidonic acid
(20:4;5,8,11,14) and the two C 18 acids linoleic
acid (18:2;9,12) and linolenic acid
(18:3;9,12,15). The animal organism requires
arachidonic acid to synthesize eicosanoids

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