11-desaturation

Historically, many attempts have been made to systematize the arrangement of fatty acids in the glyceride molecule. The even (34), random (35), restricted random (36), and 1,3-random (37) hypotheses were developed to explain the methods nature utilized to arrange fatty acids in fats. Invariably, exceptions to these theories were encountered. Plants and animals were found to biosynthesize fats and oils very differently. This realization has led to closer examination of biosynthetic pathways, such as chain elongation and desaturation, in individual genera and species. 

This impressive reaction is catalyzed by stearoyl-CoA desaturase, a 53-kD enzyme containing a nonheme iron center. NADH and oxygen (Og) are required, as are two other proteins cytochrome 65 reductase (a 43-kD flavo-protein) and cytochrome 65 (16.7 kD). All three proteins are associated with the endoplasmic reticulum membrane. Cytochrome reductase transfers a pair of electrons from NADH through FAD to cytochrome (Figure 25.14). Oxidation of reduced cytochrome be, is coupled to reduction of nonheme Fe to Fe in the desaturase. The Fe accepts a pair of electrons (one at a time in a cycle) from cytochrome b and creates a cis double bond at the 9,10-posi-tion of the stearoyl-CoA substrate. Og is the terminal electron acceptor in this fatty acyl desaturation cycle. Note that two water molecules are made, which means that four electrons are transferred overall. Two of these come through the reaction sequence from NADH, and two come from the fatty acyl substrate that is being dehydrogenated. 

Organisms differ with respect to formation, processing, and utilization of polyunsaturated fatty acids. E. coli, for example, does not have any polyunsaturated fatty acids. Eukaryotes do synthesize a variety of polyunsaturated fatty acids, certain organisms more than others. For example, plants manufacture double bonds between the A and the methyl end of the chain, but mammals cannot. Plants readily desaturate oleic acid at the 12-position (to give linoleic acid) or at both the 12- and 15-positions (producing linolenic acid). Mammals require polyunsaturated fatty acids, but must acquire them in their diet. As such, they are referred to as essential fatty acids. On the other hand, mammals can introduce double bonds between the double bond at the 8- or 9-posi-tion and the carboxyl group. Enzyme complexes in the endoplasmic reticulum desaturate the 5-position, provided a double bond exists at the 8-position, and form a double bond at the 6-position if one already exists at the 9-position. Thus, oleate can be unsaturated at the 6,7-position to give an 18 2 d5-A ,A fatty acid. 

Mammals can add additional double bonds to unsaturated fatty acids in their diets. Their ability to make arachidonic acid from linoleic acid is one example (Figure 25.15). This fatty acid is the precursor for prostaglandins and other biologically active derivatives such as leukotrienes. Synthesis involves formation of a linoleoyl ester of CoA from dietary linoleic acid, followed by introduction of a double bond at the 6-position. The triply unsaturated product is then elongated (by malonyl-CoA with a decarboxylation step) to yield a 20-carbon fatty acid with double bonds at the 8-, 11-, and 14-positions. A second desaturation reaction at the 5-position followed by an acyl-CoA synthetase reaction (Chapter 24) liberates the product, a 20-carbon fatty acid with double bonds at the 5-, 8-, IT, and ITpositions. 

Although there are many potential places around the steroid nucleus where desaturation may occur, only a few are of commercial value. The most important is the introduction of a second double bond in ring A already containing a double bond at position 4. Thus ... 

The desaturation takes place by the stereospecific removal of hydrogens from C-l and C-2 (in fact it is the la and the 20 hydrogens that are removed). 

The asymmetrical anhydrobase (20) constitutes the first step of the formation of trimethine thiazolocyanine when a 2-methylthiazolium salt reacts either with a benzothiazolium or its opened form [which is bis-o-(formylmethylamino)(diphenyl disulfide] (Scheme 26). In a second step, 20 is protonated by a second molecule of 2-methylthiazolium. It results in cleavage of the benzothiazoline ring, which gives 21 together with the formation of the monomeric anhydrobase (22). Cleavage of the C-S bond of 20 can be explained by the important electronic desaturation of the C atom observed in NMR spectrum and the great polarizability of the C-S bond in this type of ring (48). 

Initial features are mostly pulselessness, difficulty in ventilation, desaturation, and a decreased end-tidal CO2. Cutaneous symptoms are observed in 66-70% of patients in case of IgE-mediated reactions but in more than 90% in non-IgE-mediated reactions. On the contrary, cardiovascular collapse and bronchospasm are more frequent in IgE-dependent reactions (table 2). Severe anaphylaxis may be a primary cardiac arrest [9]. 

The fluidity of the cellular membrane presents a limiting factor at low temperatures. After a decrease in temperature, membranes are too rigid and must therefore be desaturated. Normally, membranes are in a liquid crystalline form... 

Cyanobacteria, prokaryotic algae that perform oxygenic photosynthesis, respond to a decrease in ambient growth temperature by desaturating the fatty acids of membrane lipids to compensate for the decrease in the molecular motion of the membrane lipids at low temperatures. During low-temperature acclimation of cyanobacterial cells, the desaturation of fatty acids occurs without de novo synthesis of fatty acids [110, 111]. All known cyanobacterial desaturases are intrinsic membrane proteins that act on acyl-Hpid substrates. 

A shift in temperature from 38 to 22 °C leads to desaturation of fatty acids in Anabaena variabilis [110], resulting in control of the fluidity of the plasma membrane. Mutants have been isolated in Synechocystis PCC 6803 that were defective in desaturation of fatty acids, and the growth rate of one of these mutants was much lower than that of the wild-type at 22 °C [112]. It turned out that the mutant strain had a mutation in the gene desA, and when the wild-type allele was introduced into the chilling-sensitive cyanobacterium Anacystis nidulans, it resulted in increasing the tolerance of that strain to low temperature [113]. These experiments nicely demonstrate the existence of a mechanism of adaptation to low temperature in a chilling-tolerant cyanobacterium. 

The desaturation of l5-cis phytoene into lycopene occurs in four stepwise dehydrogenations, yielding phytofluene, ( -carotene, neurosporene and lycopene... 

Isomerisation of 15-c/5-phytoene to the all-/ra x configuration must occur during the desaturation steps, since most desaturated carotenes are in the all-trans form. The CRTI type desaturases appear to be able to carry out this isomerisation themselves (Fraser et al, 1992 Bartley etal, 1999), but mutants of PDS/ZDS-type organisms accumulate cis isomers of unsaturated carotenes, suggesting the presence of a separate isomerase (Clough and Pattenden, 1983 Ernst and Sandmann, 1988). Three recent publications have reported the cloning of a carotene isomerase (CrtlSO) from tomato (Isaacson et al, 2002), Arabidopsis (Park et al, 2002) and Synechocystis 6803 (Breitenbach... 

ALBRECHT M, KLEIN A, HUGUENEY p, SANDMANN G and KUNTZ M (1995) Molecular cloning and functional expression in E. coli of a novel plant enzyme mediating -carotene desaturation , FEES Lett, 372, 199-202. 

MAYER M p, NiEVELSTEiN V and BEYER p (1992) Purification and characterisation of aNADPH-dependent oxidorednctase from chloroplasts of Narcissus-a redox mediator possibly involved in carotene desaturation . Plant Physiol Biochem, 30, 389-98. 

MORSTADT L, GRABER P, DE PASCALIS L, KLEINIG H, SPETH V and BEYER P (2002) Chemiosmotic ATP synthesis in photo synthetically inactive chromoplasts from Narcissus pseudonarcissus L. linked to a redox pathway potentially also involved in carotene desaturation , Planta, 215, 132-40. 

Desaturation and Isomerization to Coeored Carotenoids Biosynthesis of Lycopene... 

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