Involvement of Ferredoxin in Desaturation of Lipid-Bound Oleate in Chloroplasts

Abstract

Direct addition of different free fatty acids together with appropriate cofactors to detergent-ruptured chloroplasts results in fatty acid labeling of monogalactosyl diacylglycerol. During subsequent incubation these lipid-linked fatty acids are desaturated, i.e. 18:1 to 18:2 and 18:3 and to a small extent also 16:0 to 16:3. The formation of 18:2 was also observed after incorporation of 18:1 into sulfolipid and phosphatidyl choline. Density gradient centrifugation separated a membrane fraction from detergentruptured chloroplasts which in the presence of appropriate cofactors incorporated 18:1 and 18:2 into the above-mentioned lipids. In the light, desaturation was dependent on added ferredoxin, whereas in the dark, in addition to ferredoxin NAD(P)H was also required. Preliminary evidence for the involvement of membranebound ferredoxin:NADP oxidoreductase (FNR) as a third component of desaturation in the dark was obtained by inhibitor studies including antibodies against FNR. Desaturation of lipid-bound 18:1 and 18:2 resembles stearoyl-ACP desaturation with respect to its requirement of reduced ferredoxin and oxygen.According to presently accepted schemes, photosynthetically active cells express different sets of several desaturases in two different compartments, i.e. in plastids and ER membranes (23). In the stroma of plastids stearoyl-ACP desaturase (18) introduces the first cisdouble bond into an octadecanoyl (18:0) thioester, whereas the subsequent desaturation occurs after incorporation of the resulting oleoyl (18:1) residues as oxygen esters into membrane lipids (23). The sequen-

In Situ Molecular Identification of the Plastid ω3 Fatty Acid Desaturase FAD7 from Soybean: Evidence of Thylakoid Membrane Localization

“…In that sense, fatty acid desaturases need electron donors for their function. In the plastids, reduced ferredoxin provides the electrons necessary for FAD6 and FAD7 activity (Schmidt and Heinz, 1990a). Alternatively, several electron paramagnetic resonance signals, corresponding to iron-sulfur clusters, semiquinones, or flavins were detected in envelope preparations and proposed as components of an alternative electron transport chain that would facilitate electrons to the desaturases in the envelope (Jäger-Vottero et al, 1997).…”

“…Heterologous expression studies in yeast (Saccharomyces cerevisiae) have proven successful for the analysis of posttranscriptional regulation and activity of endoplasmic reticulum v6 and v3 desaturases (FAD2 and FAD3;. However, these studies are not suited for the analysis of plastid desaturases as they require electron transport chains from the chloroplast (Schmidt and Heinz, 1990a).Relatively little is known about the subcellular localization of fatty acid desaturases in plants, largely due to the lack of monospecific antibodies against them. In the only previous study concerning this question, immunocytological evidence of the subcellular localization of FAD2 and FAD3, the v6 and v3 desaturases of the reticulum, respectively, was obtained by developing an epitope-tagging scheme that allowed the immunological detection of transiently expressed FAD2…”

“…Heterologous expression studies in yeast (Saccharomyces cerevisiae) have proven successful for the analysis of posttranscriptional regulation and activity of endoplasmic reticulum v6 and v3 desaturases (FAD2 and FAD3; . However, these studies are not suited for the analysis of plastid desaturases as they require electron transport chains from the chloroplast (Schmidt and Heinz, 1990a).…”

Acyl-lipid desaturases and their importance in the tolerance and acclimatization to cold of cyanobacteria

“…This last type of desaturase is the most efficient regulator of the extent of unsaturation of membrane lipids in response to Acyl [24,25]. The other, present in the chloroplasts of plant cells and in cyanobacterial cells, uses ferredoxin as the electron donor [21,26,27]. A unique characteristic of the acyllipid desaturases is that they recognize, by an unknown mechanism, exact positions within various carbon chains at which double bonds are to be specifically introduced.…”

Isolation and Characterization of Mutants Affected in Lipid Metabolism of Chlamydomonas Reinhardtii

“…The desaturation activity affected in h f 9 did not exhibit strict substrate-specificity for the polar head groups of chloroplast lipids. Chloroplast 0 6 desaturation in spinach was shown to require NADPH and ferredoxin as electron donors [32]. h f 9 may be damaged in the desaturase itself or an electron transport system for the desaturation.…”