A 13CO2 Enrichment Experiment to Study the Synthesis Pathways of Polyunsaturated Fatty Acids of the Haptophyte Tisochrysis lutea
Résumé
The production of polyunsaturated fatty acids (PUFA) in Tisochrysis lutea was studied using
the gradual incorporation of a 13C-enriched isotopic marker, 13CO2, for 24 h during the exponential
growth of the algae. The 13C enrichment of eleven fatty acids was followed to understand the
synthetic pathways the most likely to form the essential polyunsaturated fatty acids 20:5n-3 (EPA)
and 22:6n-3 (DHA) in T. lutea. The fatty acids 16:0, 18:1n-9 + 18:3n-3, 18:2n-6, and 22:5n-6 were the
most enriched in 13C. On the contrary, 18:4n-3 and 18:5n-3 were the least enriched in 13C after long
chain polyunsaturated fatty acids such as 20:5n-3 or 22:5n-3. The algae appeared to use different routes
in parallel to form its polyunsaturated fatty acids. The use of the PKS pathway was hypothesized
for polyunsaturated fatty acids with n-6 configuration (such as 22:5n-6) but might also exist for n-3
PUFA (especially 20:5n-3). With regard to the conventional n-3 PUFA pathway, ∆6 desaturation of
18:3n-3 appeared to be the most limiting step for T. lutea, “stopping” at the synthesis of 18:4n-3 and
18:5n-3. These two fatty acids were hypothesized to not undergo any further reaction of elongation
and desaturation after being formed and were therefore considered “end-products”. To circumvent
this limiting synthetic route, Tisochrysis lutea seemed to have developed an alternative route via ∆8
desaturation to produce longer chain fatty acids such as 20:5n-3 and 22:5n-3. 22:6n-3 presented a lower
enrichment and appeared to be produced by a combination of different pathways: the conventional
n-3 PUFA pathway by desaturation of 22:5n-3, the alternative route of ω-3 desaturase using 22:5n-6
as precursor, and possibly the PKS pathway. In this study, PKS synthesis looked particularly effective
for producing long chain polyunsaturated fatty acids. The rate of enrichment of these compounds
hypothetically synthesized by PKS is remarkably fast, making undetectable the 13C incorporation
into their precursors. Finally, we identified a protein cluster gathering PKS sequences of proteins that
are hypothesized allowing n-3 PUFA synthesis
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