Unlocking the Zn-enriching potential of industrial yeast strains—an experimental journey from metal analysis to proteomics

Nutritional supplements such as trace element-enriched yeasts are becoming increasingly popular to overcome the worldwide problem of zinc (Zn) deficiency. Unlike selenium-enriched yeast, which is already authorized in the European Union, Zn-enriched yeasts (ZnY) have not yet been approved for food purposes in the European Union, as their evaluation is still ongoing, demanding more comprehensive data regarding the Zn species present in ZnY. This study screens ten different industrial yeast strains regarding their Zn-enrichment quota, with further characterization of selected strains using spectroscopic and proteomic approaches. Microfermentation experiments on the industrial yeasts showed Zn levels spanning 0.06-51 pg/cell. Large-scale fermentation in bioreactors was carried out with two strains excelling in either biomass or Zn accumulation. A combination of inductively coupled plasma mass spectrometry (ICP-MS) and various spectroscopic methods confirmed the Zn enrichment, while suggesting that fractions of the Zn accumulated on the cell surface, with simultaneously high values of phosphorus being present. Speciation via X-ray absorption spectroscopy (XAS) analyses revealed that Zn species are transformed and Zn is coordinated to P-O-ligands and to amino acid ligands in both strains. Proteomic analysis showed that ZnY cells moved from a Zap1-governed Zn balance to an intracellular excess response, implying cellular Zn uptake. This study demonstrates that, in a Zn-excess medium, industrial yeast strains exhibit variability in Zn-accumulation capacity, cellular Zn-localization, and regulatory responses involving the expression of Zn-binding proteins. The presented findings contribute to optimizing industrial fermentation processes for producing Zn-rich yeast biomass and enhance the understanding of Zn regulation in yeast, aiding in the approval of Zn-enriched yeasts for supplements and novel food applications. KEY POINTS: • Zn enrichment in yeasts is strongly time and strain dependent • Zn proteome changes under Zn excess suggest that Zn is partly internalized in the yeast cells • Beside proteins, phosphorous compounds seem to be Zn-binding ligands in Zn-enriched yeast.