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.
Highlights
The sugar acid D-galacturonic acid (D-GalUA) is, apart from D-glucose and L-arabinose, an abundant monomer in pectins
Several agro-industrial waste streams such as sugar beet pulp or citrus peels are rich in pectins [2]
The valorisation of pectin-rich biomass residues has to include concepts for the conversion of D-GalUA into valuable products since the sugar acid makes up approximately 70% of pectins [1,5,6]
Summary
The sugar acid D-galacturonic acid (D-GalUA) is, apart from D-glucose and L-arabinose, an abundant monomer in pectins. It 914 has been an interesting auxiliary result of the study conducted by Perpelea et al [29] that the reference strain. It became obvious that this conversion depended on the presence of the co-substrate glycerol This result matched reports of other authors who previously showed that (i) some D-GalUA can be taken up by one or more endogenous uptake mechanism(s) in S. cerevisiae [27,30] and (ii) one or more endogenous reductase(s) in S. cerevisiae can convert D-GalUA to L-GalA. The goal of the current study was to identify the enzyme(s) that is/are responsible for the observed conversion of D-GalUA to L-GalA in baker’s yeast
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