In the presence of excess sugar, yeast, especially S. cere — visiae, has a strong tendency to undergo alcoholic fermen­tation, even when oxygen is available in excess (Van Diken and Scheffers, 1986). In the biochemical pathway for
carbohydrate metabolism in most yeasts, two modes of disaccharide metabolism exist. While extracellular hydro­lysis of sucrose to glucose and fructose followed by trans­port of these monosaccharides into the cell is the most common method for sucrose metabolism in yeast, trans­port of disaccharides by proton—sugar symport followed by intracellular hydrolysis occurs in maltose and lactose metabolism (Weusthuis et al., 1994). However, hydrolysis of sucrose can occur either intracellularly or extracellu­larly in S. cerevisiae (Santos et al., 1982), followed by the phosphorylation of glucose to glucose-6-phosphate, which is subsequently catabolized to pyruvate via the Embden—Meyerhof—Parnas pathway (Figure 7.2). Although most of the synthesized pyruvate is decarboxy — lated to acetaldehyde (ethanal) by PDC followed by the reduction of acetaldehyde to ethanol by ADH, a small proportion of the pyruvate is converted to fusel alcohols such as isobutanol (Figures 7.1 and 7.2; Table 7.1).

KIV is an important precursor for valine biosynthesis, which is also shared by isobutanol production. KIV biosynthesis is initiated by the condensation of two pyruvate molecules to 2-acetolactate, which is catalyzed by acetolactate synthase (ILV2 + ILV6; Figure 7.2). Notably, ILV6 is the regulatory subunit of acetolactate synthase and an enhancer of ILV2 catalytic activity
(Chen et al., 2011). The 2-acetolactate is reduced to

2,3- dihydroxyisovalerate via catalysis by acetohydrox — yacid reductoisomerase (ILV5), the precursor for KIV biosynthesis (Figure 7.2; Velasco et al., 1993). Thus, KIV is produced through catalysis of 2,3-dihydroxyisovalerate by dihydroxyacid dehydratase (ILV3). Further, the bidirec­tional conversion between KIV and valine is catalyzed by aminotransferases (Bat1 and Bat2). While aminotransferase Bat1 is present in the mitochondrial matrix of S. cerevisiae, aminotransferase Bat2 is present in the cytosol (Kispal et al., 1996; Chen et al., 2011). Next, KIV is decarboxylated by PDC, a KDC, to isobutyraldehyde and subsequently, reduced to isobutanol by ADHs (Figure 7.2).