Department of Bioorganic Chemistry
Studies to elucidate the functional aspects of the desaturase-mediated
double bond formation in fatty acids
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Andreas Habel
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Fatty acids are carboxylic acids with long-chain hydrocarbon side groups
and play a fundamental role in many biological processes. Fatty acids are
rarely free in nature but, rather occur in esterified form as the major
component of lipids. Lipids/fatty acids are sources of energy (e.g.
b-oxidation) and are an integral part of cell membranes which are
indispensable for processing biological or biochemical information. In
higher plants and animals, the predominant fatty acid residues are those of C16
and C18 species (e.g. palmitic -, oleic -, stearic acids). Moreover,
fatty acids can be divided into two groups: the saturated fatty acids and
the unsaturated fatty acids which contain one or more carbon double bond in
cis-configuration. Interestingly, humans and other mammals
only have a
limited spectrum of the key enzymes (desaturases) that are required for the
formation of these particular double bonds in unsaturated fatty acids. Thus,
humans have to take up some fatty acids through their diet. Such essential
fatty acids, for example, are linoleic acid (C18:2); linolenic acid (C18:3),
arachidonic acid (C20:4) and are shown in Fig. 1.
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Fig 1: Some essential unsaturated fatty acids
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In contrast, insects and plants are able to synthesize a much larger
variety of unsaturated fatty acids and their derivatives. This may, in part,
be reflected by the fact that in insects and plants fatty acids are integral
components of pheromones, small chemical molecules essential for
communication processes among species or plants and insects.
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Desaturases: Molecular and Mechanistical Aspects
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Unsaturated fatty acids are produced by terminal desaturases that belong
to the class of nonheme-iron enzymes. Mammals have terminal desaturases of
broad chain-length specificities, for example, the delta9-, delta6-,
delta5- and delta4-fatty acetyl-CoA desaturases. Each
of these enzymes are part of an electron-transport system that contains two
other proteins, namely Cytochrome b5 and NADH-Cytochrome b5 reductase (Fig.
2).
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Fig 2: electron transfer in the desaturation of fatty acids
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This protein complex is located at the endoplasmatic reticulum. There is
mounting evidence that all desaturase-mediated double bond formation occur
through the same principle, summarized in Fig. 3.
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Fig 3: removal of hydrogen
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As part of this project we will analyze the mechanistical and stereochemical aspects of the double bond formation in fatty acids using
uncommon and novel desaturases from plants or insects. As model systems we
choose, desaturase enzymes from the pheromone biosynthesis
pathway of Antherea polyphemus (delta6- and delta11-desaturases)
and the delta6-desaturase from the sphingolipid pathway in plants
(Fig. 4)
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Fig 4: (a) pheromones of
Antherea polyphemus; (b)
sphingolipid pathway in plants
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The main goal of our study is to elucidate whether the 1,2-dehydration
reaction occurs through a syn or anti elimination reaction
(whether the two hydrogen atoms are either eliminated from the same side of
the molecule or from different sides). Moreover, we will analyze the
mechanism of the desaturase reaction, via kinetic isotope
effects.
To study these details of the desaturase-mediated double bond formation
in fatty acids we need to synthesize various enantiomeric pure
isotope-labeled fatty acids or their derivatives. These substrates will be
converted in a desaturase reaction. Subsequently, the formed products will
be isolated and analyzed by GC-MS-spectroscopy. With this
approach we hope to get a better insight in the mechanistical and
stereochemical aspects of the desaturase reaction.
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