Towards valorization of pectin-rich agro-industrial residues: Engineering of Saccharomyces cerevisiae for co-fermentation of d-galacturonic acid and glycerol

Identification of the Aldo-Keto Reductase Responsible for d-Galacturonic Acid Conversion to l-Galactonate in Saccharomyces cerevisiae

“…delivers the proof of concept for a co-fermentation of the two 'respiratory' carbon sources to ethanol and demonstrates a fast and complete consumption of D-GalUA in crude sugar beet pulp hydrolysate, at least under aerobic conditions [29].…”

“…Protein concentration in crude cell extracts was determined by the BCA assay [42] using a kit from Thermo Scientific. As mentioned in the introduction, a glycerol-utilizing derivative of the well-characterized S. cerevisiae strain CEN.PK consumed some D-GalUA when added to synthetic glycerol medium [29]. In the respective strain referred to here as CEN.PK DHA [34], the endogenous so-called L-glycerol-3-phosphate (L-G3P) pathway for glycerol catabolism (reviewed in Klein et al [43]) was replaced by the NAD-dependent dihydroxyacetone (DHA) pathway [44].…”

“…However, the achieved rates of D-GalUA consumption were far from economically viable even under aerobic conditions and in the presence of a cosubstrate. Recently, our laboratory significantly improved the situation by constructing a S. cerevisiae strain able to consume D-GalUA with a specific rate of 0.23 g g CDW −1 h −1 (CDW = cell dry weight) [29]. A challenging aspect when considering homo-ethanol fermentation from pectin hydrolysates is the electron balance since D-GalUA is more oxidized than neutral sugars.…”

“…In this context, we have considered glycerol an ideal co-substrate as a donor of electrons and embarked on the challenge to establish co-fermentation of D-GalUA and glycerol. In our study [29], a fungal D-GalUA pathway was inserted in a strain of the yeast S. cerevisiae previously equipped with an efficient NAD-dependent glycerol catabolic pathway. The study delivers the proof of concept for a co-fermentation of the two 'respiratory' carbon sources to ethanol and demonstrates a fast and complete consumption of D-GalUA in crude sugar beet pulp hydrolysate, at least under aerobic conditions [29].…”

“…It has been an interesting auxiliary result of the study conducted by Perpelea et al [29] that the reference strain (without any heterologous D-GalUA transporter or catabolic enzyme) also consumed a certain amount of D-GalUA. It also became obvious that this conversion depended on the presence of the co-substrate glycerol.…”