Yeasts are ubiquitous unicellular fungi widespread in natural environments colonizing from terrestrial, to aerial to aquatic environments, where the successful colonization is intimately related to their physiological adaptability to a highly variable environment. The metabolic pathways of the central carbon metabolism are basically identical between different yeast species, suggesting that these microorganisms might constitute a metabolic homogenous group. Nonetheless, the mechanisms for nutrient uptake, the number of different isoenzymes and most importantly the regulation of fermentation and respiration differ substantially (Flores et al. 2000) and make yeasts a highly heterogeneous and complex metabolic group. In yeasts, like other heterotrophic organisms, the energy and carbon metabolism are intimately interconnected, ie, anabolism is coupled with catabolism. ATP is provided by the oxidation of organic molecules that also act as carbon sources for biosynthesis, and ultimately it is used as energetic currency for all kinds of cellular work. In the natural environment yeast species have a broad set of carbon sources (eg, polyols, alcohols, organic acids and amino acids) that can support their growth but preferentially they metabolize sugars. The information related to the metabolism of different carbon sources is huge, the most widely studied being sugars such as hexoses (glucose, fructose, galactose or mannose) and disaccharides (maltose or sucrose) as well as compounds with two carbons (ethanol or acetate). The metabolic networks employed for the metabolism of hexoses and disaccharides share the same pathways (most metabolic building blocks are derived from intermediaries of glycolysis, the tricarboxylic acid cycle (TCA), and the pentose phosphate pathway) and differ only in the initial basic steps of metabolism. However, significant changes could be observed when the metabolism of sugars is compared with that of the