Читать книгу Handbook of Enology: Volume 1 - Pascal Ribéreau-Gayon - Страница 51

The reducing power of NADH produced by glycolysis must be transferred to an electron acceptor in order to regenerate NAD⁺. In alcoholic fermentation, it is not pyruvate but rather acetaldehyde, its decarboxylation product, that serves as the terminal electron acceptor. With respect to glycolysis, alcoholic fermentation contains two additional enzymatic reactions.

The first decarboxylates pyruvic acid, catalyzed by pyruvate decarboxylase (PDC). The cofactor is thiamine pyrophosphate (TPP) (Figure 2.4). TPP and pyruvate form an intermediate compound. More precisely, the carbon atom located between the nitrogen and the sulfur of the thiazole ring of TPP is ionized. It forms a carbanion, which readily combines with the pyruvate carbonyl group. The second step reduces acetaldehyde into ethanol by NADH. This reaction is catalyzed by alcohol dehydrogenase, whose active site contains a Zn²⁺ ion.

Saccharomyces cerevisiae PDC comprises two isoenzymes: a major form, PDC1, representing 80% of the decarboxylase activity, and a minor form, PDC5, whose function remains uncertain.

From an energy viewpoint, glycolysis followed by alcoholic fermentation therefore supplies the yeast with two molecules of ATP per molecule of glucose degraded or 14.6 biologically usable kcal per mole of glucose fermented. From a thermodynamic viewpoint, the change in free energy during the degradation of a mole of glucose into ethanol and CO₂ is −40 kcal. The difference (25.4 kcal) is dissipated in the form of heat.

FIGURE 2.4 Structure of thiamine pyrophosphate (TPP).