Acetylation: The Key to Increased Xanthophyll Titers in Yarrowia lipolytica

Strains of the lipid-accumulating yeast Yarrowia lipolytica have been generated for the production of β-carotene, canthaxanthin, lycopene, astaxanthin, or zeaxanthin via the introduction of heterologous biosynthetic activities, enhancement of the native isoprenoid pathway, and classical breeding.

For lycopene, β-carotene, and canthaxanthin, continued strain improvements have resulted in strains with increasing titers in lab-scale fermentors.  However, for the hydroxylated xanthophylls astaxanthin and zeaxanthin, a maximum threshold was observed.  Strains producing these xanthophylls exhibit a loss of viability over the course of a fermentation and production plateaus. This toxicity is presumably due to the polar nature of these molecules, limiting their solubility in the Y. lipolytica lipid body.

As some natural producers accumulate astaxanthin or zeaxanthin as fatty acyl esters, we searched for enzyme candidates that would esterify these xanthophylls, and thereby mitigate toxicity by enhancing lipid solubility. A Saccharomyces acetyltransferase was found that quantitatively converted the xanthophyll pool to the corresponding diacetate ester. Here, we evaluate xanthophyll producing strains of Y. lipolytica expressing this enzyme and describe the resulting effects on cell morphology, cell viability, carotenoid production kinetics, and product profile in lab-scale fermentors.