Читать книгу Handbook of Enology: Volume 1 - Pascal Ribéreau-Gayon - Страница 48

The synthesis of living material is endergonic, requiring the consumption of energy. Chlorophyllous plants, called phototrophs, collect solar energy. Some bacteria obtain energy from the oxidation of minerals: they are chemolithotrophs. Like animals and most bacteria, fungi, including yeast, are chemoorganotrophs: they draw their necessary energy from the degradation of organic nutrients, which are their true “fuel.”

In a growing organism, energy produced by degradation reactions (catabolism) is transferred to the chain of synthesis reactions (anabolism). Conforming to the laws of thermodynamics, energy furnished by the degradation of a substrate is only partially converted into work; this is called free energy (the rest is dissipated in the form of heat). Part of this free energy can be used for transport, movement, or synthesis. In most cases, the free energy transporter particular to biological systems is adenosine triphosphate (ATP). This molecule is rich in energy because its triphosphate unit contains two phosphoanhydride bonds (Figure 2.1). The hydrolysis of ATP into adenosine diphosphate (ADP) results in the liberation of a large quantity of free energy (7.3 kcal/mol). Biosynthesis and the active transport of metabolites make use of this free energy.

In this reaction, ∆G°′ is the change in free energy. ATP is thus considered to be “the universal currency of free energy in biological systems” (Stryer, 1992). In reality, microorganism growth or, in this case, yeast growth is directly related to the quantity of ATP furnished by the metabolic pathways used for degrading a substrate. It is very indirectly related to the quantity of substrate degraded.

FIGURE 2.1 Structure of adenosine triphosphate (ATP).

In the living cell, there are two processes that produce ATP: substrate‐level phosphorylation and oxidative phosphorylation. Both of these pathways exist in wine yeasts.

Substrate‐level phosphorylation can be either aerobic or anaerobic. During oxidation by electron loss, a phosphate ester bond is formed. It is a high‐energy bond between the oxidized carbon of the substrate and a molecule of inorganic phosphate. This bond is then transferred to the ADP by transphosphorylation, thus forming ATP. This process takes place during glycolysis (Section 2.2.1).

Oxidative phosphorylation is an aerobic process. The production of ATP is linked to the transport of electrons to an oxygen molecule by the cytochrome respiratory chain. This oxygen molecule is the final acceptor of the electrons. These reactions occur in the mitochondria.

This chapter describes the principal biochemical reactions occurring during grape must fermentation by wine yeasts. It covers sugar metabolism, i.e. the biochemistry of alcoholic fermentation and nitrogen metabolism.