Identification of the Aldo-Keto Reductase Responsible for <span style="font-variant: small-caps">d</span>-Galacturonic Acid Conversion to <span style="font-variant: small-caps">l</span>-Galactonate in <i>Saccharomyces cerevisiae</i>

Abstract

d-galacturonic acid (d-GalUA) is the main constituent of pectin, a complex polysaccharide abundant in several agro-industrial by-products such as sugar beet pulp or citrus peel. During several attempts to valorise d-GalUA by engineering the popular cell factory Saccharomyces cerevisiae, it became obvious that d-GalUA is, to a certain degree, converted to l-galactonate (l-GalA) by an endogenous enzymatic activity. The goal of the current work was to clarify the identity of the responsible enzyme(s). A protein homology search identified three NADPH-dependent unspecific aldo-keto reductases in baker’s yeast (encoded by GCY1, YPR1 and GRE3) that show sequence similarities to known d-GalUA reductases from filamentous fungi. Characterization of the respective deletion mutants and an in vitro enzyme assay with a Gcy1 overproducing strain verified that Gcy1 is mainly responsible for the detectable reduction of d-GalUA to l-GalA.

Keywords

• d-galacturonic acid%22%2C%22default_operator%22%3A%22AND%22%7D%7D%7D"> <span style="font-variant: small-caps">d</span>-galacturonic acid
• l-galactonate%22%2C%22default_operator%22%3A%22AND%22%7D%7D%7D"> <span style="font-variant: small-caps">l</span>-galactonate
• <i>Saccharomyces cerevisiae</i>
• aldo-keto reductase
• Gcy1