8-6-desaturase

A second class of herbicides primarily affects ( -carotene desaturase. These herbicides are apparent feedback inhibitors of PD as well. This class of compounds includes dihydropyrones like LS 80707 [90936-96-2] (56) and 6-methylpyridines (57,58). The third class consists of the ben2oylcyclohexane-diones, eg, 2-(4-chloro-2-nitroben2oyl)-5,5-dimethyl-cyclohexane-I,3-dione. This class of atypical bleaching herbicides induces phytoene accumulation when appHed either pre- or post-emergence. However, it does not inhibit phytoene desaturase activity in vitro (59). Amitrole also has been considered a bleaching herbicide, though its main mode of action is inhibition of amino acid synthesis. 

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. 

FIGURE 25.14 The conversion of stearoyl-CoA to oleoyl-CoA in eukaryotes is catalyzed by stearoyl-CoA desaturase in a reaction sequence that also involves cytochrome -65 and cytochrome -65 reductase. Two electrons are passed from NADH through the chain of reactions as shown, and two electrons are also derived from the fatty acyl substrate. 

Carefully study the reaction mechanism for the stearoyl-CoA desaturase in Figure 25.14, and account for all of the electrons flowing through the reactions shown. Also account for all of the hydrogen and oxygen atoms involved in this reaction, and convince yourself that the stoichiometry is correct as shown. 

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. 

In Synechococcus sp. strain PCC 7002, the temperature-regulated mRNA accumulation of the three desaturase genes, desA (A12 desaturase), desB (co3... 

MONOUNSATURATED FATTY ACIDS ARE SYNTHESIZED BY A A DESATURASE SYSTEM... 

SYNTHESIS OF POLYUNSATURATED FATTY ACIDS INVOLVES DESATURASE ELONGASE ENZYME SYSTEMS... 

Additional double bonds introduced into existing mo-nounsamrated fatty acids are always separated from each other by a methylene group (methylene interrupted) except in bacteria. Since animals have a desaturase, they... 

Figure 23-3. Biosynthesis of the co9, co6,and co3 families of polyunsaturated fatty acids. Each step is catalyzed by the microsomal chain elongation or desaturase system 1,elongase 2,A desaturase 3,A desaturase 4,A desaturase. ( .Inhibition.)...

Figure 23-4. Conversion of linoleate to arachido-nate. Cats cannot carry out this conversion owing to absence of A desaturase and must obtain arachidonate in their diet.

Biosynthesis of unsatutated long-chain fatty acids is achieved by desaturase and elongase enzymes, which introduce double bonds and lengthen existing acyl chains, respectively. 

Tocher DR, Leaver MJ, Hodgson PA Recent advances in the biochemistry and molecular biology of fatty acyl desaturases. Prog Lipid Res 1998 37 73. 

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... 

Clearly, the control of gene expression at the transcriptional level is a key regulatory mechanism controlling carotenogenesis in vivo. However, post-transcriptional regulation of carotenoid biosynthesis enzymes has been found in chromoplasts of the daffodil. The enzymes phytoene synthase (PSY) and phytoene desaturase (PDS) are inactive in the soluble fraction of the plastid, but are active when membrane-bound (Al-Babili et al, 1996 Schledz et al, 1996). The presence of inactive proteins indicates that a post-translational regulation mechanism is present and is linked to the redox state of the membrane-bound electron acceptors. In addition, substrate specificity of the P- and e-lycopene cyclases may control the proportions of the p, P and P, e carotenoids in plants (Cunningham et al, 1996). 

The carotenoid pathway may also be regulated by feedback inhibition from the end products. Inhibition of lycopene cyclisation in leaves of tomato causes increase in the expression of Pds and Psy-1 (Giuliano et al, 1993 Corona et al, 1996). This hypothesis is supported by other studies using carotenoid biosynthesis inhibitors where treated photosynthetic tissues accumulated higher concentrations of carotenoids than untreated tissues (reviewed by Bramley, 1993). The mechanism of this regulation is unknown. A contrary view, however, comes from studies on the phytoene-accumulating immutans mutant of Arabidopsis, where there is no feedback inhibition of phytoene desaturase gene expression (Wetzel and Rodermel, 1998). 

AL-BABiLi s, VON LiNTiG J, HAUBRUCK H and BEYER p (1996) A novel, soluble form of phytoene desaturase from Narcissus pseudonarcissus chromoplasts is Hsp70-complexed and competent for flavinylation, membrane association and enzymatic activation . Plant J,9, 601-12. 

BARTLEY G E, SCOLNIK p A and BEYER p (1999) Two Arabidopsis thaliana carotene desaturases, ph)4oene desaturase and zeta-carotene desaturase, expressed in EicAencA/a coli, catalyze a poly-cis pathway to yield pro-lycopene , Eur JBiochem, 259, 396-403. 

FRASER P D, MISAWA N, LINDEN H, SHIGEYUKI Y, KOBAYASHI K and SANDMANN G (1992) Expression mE. coli, purification and reactivation of a recombinant Erwinia uredovora phytoene desaturase ,Chem, 267, 19891-5. 

LINDEN H, MISAWA N, SAITO T and SANDMANN G (1994) A uovel caroteuoid biosynthesis gene coding for -carotene desaturase functional expression, sequence and phylogenetic origin . Plant Mol Biol, 24, 369-79. 

WETZEL c M and RODERMEL s R (1998) Regulation of phytoene desaturase expression is independent of leaf pigment content in Arabidopsis thaliana , Plant Mol Biol, 37, 1045-53. 

Besides the capacity of CRTI to introduce all four double bonds in the conversion of phytoene to lycopene, the enzyme produces different geometric isomers than does PDS/ZDS (see graphic, side-by-side comparison in Fraser and Bramley ). CRTI produces all-trans isomers. Studies that have examined the function of the paired plant desaturases acting together, from Arabidopsis, and from maize and from... 

Over-expression of bacterial phytoene synthase led to only modest increases in pigment accumulation (except in the case of chloroplast-contaiifing tissues). Attention turned to CrtI, one gene that might control flux through the entire four desaturation steps from phytoene to lycopene (discussed in Section 5.3.2.4). Only a modest increase in carotenoid content in tomatoes and a variety of changes in carotenoid composition including more P-carotene, accompanied by an overall decrease in total carotenoid content (no lycopene increase), resulted when CrtI was over-expressed under control of CaMV 35S. Apparently, the bacterial desaturase... 

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