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Browsing by Author "Qiu, Xiao"

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    Biosynthetic mechanisms of omega-3 polyunsaturated fatty acids in microalgae
    (Journal of Biological Chemistry, 2024-08) Sun, Kaiwen; Meesapyodsuk, Dauenpen; Qiu, Xiao
    Marine microalgae are the primary producers of u3 poly- unsaturated fatty acids (PUFAs), such as octadecapentaenoic acid (OPA, 18:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) for food chains. However, the biosynthetic mechanisms of these PUFAs in the algae remain elusive. To study how these fatty acids are synthesized in microalgae, a series of radio- labeled precursors were used to trace the biosynthetic process of PUFAs in Emiliania huxleyi. Feeding the alga with 14C- labeled acetic acid in a time course showed that OPA was solely found in glycoglycerolipids such as monogalactosyldiacylgly- cerol (MGDG) and digalactosyldiacylglycerol (DGDG) synthe- sized plastidically by sequential desaturations while DHA was exclusively found in phospholipids synthesized extraplastidi- cally. Feeding the alga with 14C-labeled a-linolenic acid (ALA), linoleic acid (LA), and oleic acid (OA) showed that DHA was synthesized extraplastidically from fed ALA and LA, but not from OA, implying that the aerobic pathway of DHA biosyn- thesis is incomplete with missing a D12 desaturation step. The in vitro enzymatic assays with 14C-labeled malonyl-CoA showed that DHA was synthesized from acetic acid by a PUFA synthase. These results provide the first and conclusive biochemistry evidence that OPA is synthesized by a plastidic aerobic pathway through sequential desaturations with the last step of D3 desaturation, while DHA is synthesized by an extraplastidic anaerobic pathway catalyzed by a PUFA synthase in the microalga.