Identification of two novel microalgal enzymes involved in the conversion of the ω3-fatty acid, eicosapentaenoic acid, into docosahexaenoic acid

摘要

pMarine microalgae such as iPavlova/i and iIsochrysis/i produce abundant amounts of the ω3-PUFAs (polyunsaturated fatty acids), EPA (eicosapentaenoic acid, 20:5in/i–3) and DHA (docosahexaenoic acid, 22:6in/i–3). The pathway leading to the conversion of EPA into DHA in these lower eukaryotes is not well established although it is predicted to involve an elongation step, catalysed by an elongating enzyme complex, leading to the conversion of EPA into ω3-DPA (ω–3-docosapentaenoic acid, 22:5in/i–3); followed by a desaturation step, catalysed by a Δ4-desaturase, which results in the conversion of DPA into DHA. To date, the enzymes involved in the elongation of EPA have not been identified from any lower eukaryote. In the present study, we describe the identification of microalgal genes involved in the two-step conversion of EPA into DHA. By expressed sequence tag analysis, a gene (ipavELO/i) encoding a novel elongase was identified from iPavlova/i, which catalysed the conversion of EPA into ω3-DPA in yeast. Unlike any previously identified elongase from higher or lower eukaryotes, this enzyme displayed unique substrate specificity for both in/i–6 and in/i–3 Csub20/sub-PUFA substrates, with no activity towards any Csub18/sub- or Csub22/sub-PUFA substrates. In addition, a novel Δ4-desaturase gene (iIgD4/i) was isolated from iIsochrysis/i, which was capable of converting ω3-DPA into DHA, as well as adrenic acid (22:4in/i–6) into ω6-DPA. Yeast co-expression studies, with ipavELO/i and iIgD4/i, revealed that these genes were capable of functioning together to carry out the two-step conversion of EPA into DHA./p