**Author details**

230 Dehydrogenases

engineering approaches.

have complex medium requirements and they are generally unable to grow and produce organic acids at the low pH values. These restrictions provide strong incentives to integrate and optimize succinate production pathways in other microorganisms via metabolic

The popularity of *S. cerevisiae* in basic and applied research is undoubtedly influenced by its classification as GRAS (generally regarded as safe) by the U.S. Food and Drug Administration (FDA). Baker's yeast *S. cerevisiae* was the first eukaryotic organism in which complete genomic sequence was determined. Several databases such as the *Saccharomyces*  Genome Database (http://www.yeastgenome.org/) and the Comprehensive Yeast Genome Database (http://mips.gsf.de/genre/proj/yeast/) contain an enormous amount of information concerning *S. cerevisiae* genes, open reading frames, and gene products. The yeast *S. cerevisiae* became a well established eukaryotic model organism to study fundamental biological processes such as aging, mRNA transport, the cell cycle, and many more. *Saccharomyces cerevisiae* grows well in a simple chemically defined medium, under acidic conditions, even at pH values equal 3. At such low pH values, many weak acids, including succinate, occur predominantly in their undissociated form. This is advantageous for industrial production, as it reduces the need for titration with alkali and allows for direct recovery of undissociated acids. Consequently, there is no need for large quantities of acidifying agents, and the formation of salt byproducts (e.g. gypsum) is strongly reduced. In addition, *S. cerevisiae* robust tolerance in acidic conditions represents a major advantage

in that it lowers the risk of contamination in industrial fermentation [44, 49].

The yeast-based fermentation process, which operates at a much lower pH than competing processes, allows succinic acid to be produced with a significantly higher energy efficiency compared to the traditional method. This compound is not accumulated intracellularly. It is also one of the first bio-based processes that sequesters carbon dioxide in the production process [47, 53]. This makes the yeast *Saccharomyces cerevisiae* a suitable and promising

The metabolic engineering strategy was used for the oxidative production of succinic acid by deletion SDH1, SDH2 genes in the genome. Arikawa et al. [55] reported an increased succinic acid productivity in sake yeast strains with deletions of TCA cycle genes. In comparison to the wild-type, succinate levels were increased up to2.7 fold in a strain with simultaneous disruption of a subunit of succinate dehydrogenase (SDH1) and fumarase (FUM1) under aerobic conditions. The single deletion of gene SDH1 led to a1.6-fold increase of succinic acid production. In another study on sake yeast strains, the deletion of genes encoding for succinate dehydrogenase subunits (SDH1, SDH2, SDH3, and SDH4) also resulted inincreased succinate productivity in aerobic conditions. Raab et al. [48] reported the construction of yeast strains for the biotechnological production of succinic acid. The genes *SDH1, SDH2, IDH1* and *IDP1*, which encode mitochondrial enzymes were deleted with the aim to disrupt succinate and isocitrate dehydrogenase activity to redirect the carbon flux and to allow succinate to accumulate as an end-product. This study showed that the yeast *S. cerevisiae* is capable of synthesizing significant amounts of succinic acid, which is exported quantitatively into the culture broth and not being accumulated intracellularly.

candidate for the biotechnological production of succinic acid on an industrial scale.

Dorota Kregiel *Institute of Fermentation Technology and Microbiology, Technical University of Lodz, Poland* 
