Review of Aroma Formation through Metabolic Pathways of Saccharomyces cerevisiae in Beverage Fermentations

The starting material (A) used in fermentation will affect glucose repression and nitrogen catabolite repression (B) and also derepression (C). Sugar concentration, the types of sugar, and the relative ratios of the different sugars will affect the length of catabolite repression and the flavor compounds produced during fermentation. (B) In high-sugar environments, glucose is preferentially taken up by hexose transporters (Hxt) and phosphorylated to glucose-6-phosphate in glycolysis (De Deken 1966). The rise in glucose-6-phosphate and ATP in the cell inactivates the Snf1 complex (Wilson et al. 1996). When Snf1 is inactive, Mip1 moves to the nucleus, recruits the repressors Tup1 and Ssn6, and the MAL genes involved in the uptake of alternative carbon sources are blocked (red proteins and metabolites) (De Vit et al. 1997). Extracellular glucose is sensed by a G protein–coupled receptor (Gpr1) and by Gpa2 (not shown), and a rise in glucose-6-phosphate increases Cyr1 activity (Rolland et al. 2000). The signal creates an increase in cAMP, which binds to Bcy1 (of the PKA complex), releasing the Tpk1 catalytic subunits (Toda et al. 1987). Tpk1 phosphorylates several proteins outside the nucleus, and these proteins block the transcription of the stress-responsive element (STRE) genes (e.g., Hsp12 and Hsp104) and enhances acetate ester synthesis (green proteins and metabolites) (Thevelein 1994). In the presence of preferred nitrogen sources, ammonium is taken up into the cell, and specific amino acids (glutamine and glutamate) are taken up preferentially by amino acid permeases. The TOR complex is activated, phosphorylating Gln3, which then recruits the repressor Ure2, effectively blocking amino acid uptake of poorer nitrogen sources (blue proteins and metabolites) (Bertram et al. 2000). TOR1 is linked to carbon metabolism, as it is involved in regulating glucose activation and glycolysis during fermentation (Hardwick et al. 1999) (dashed arrows). (C) In low-glucose environments, Snf1 and Mig1 are phosphorylated, initiating their translocation to the cytoplasm (De Vit et al. 1997). The proteins involved in the uptake of alternative carbon sources (i.e., maltose and maltotriose) are translated (red proteins and enzymes) (De Vit et al. 1997). Additionally, the PKA complex is not activated, allowing expression of the STRE genes (green proteins and enzymes). Low-nitrogen conditions do not trigger activation of the TOR complex, allowing Gln3 to activate multiple nitrogen permeases for nitrogen uptake, including the ammonium permeases Mep1, 2, and 3; Gap1; Agp1 (general amino acid permeases); and Put4 (proline-specific permease) (Beck and Hall 1999). Gln1p and Gdh2p are translated and involved in glutamate and α-ketoglutarate synthesis, respectively (blue proteins and enzymes) (Miller and Magasanik 1990, Filetici et al. 1996). The sugar transporters are represented by shades of red; darker red represents a higher sugar-uptake preference. PM: plasma membrane; NM: nuclear membrane.