Читать книгу Handbook of Enology: Volume 1 - Pascal Ribéreau-Gayon - Страница 63

The penetration of ammonium and amino acids into the yeast cell activates numerous membrane protein transporters or permeases (Section 1.3.2). Saccharomyces cerevisiae has at least two specific ammonium ion transporters (Dubois and Grenson, 1979). Their activity is inhibited by several amino acids, in a noncompetitive manner.

Two distinct categories of transporters ensure amino acid transport:

1 A general amino acid permease (GAP) transports all of the amino acids. The ammonium ion inhibits and represses GAP. GAP therefore appears to be active during winemaking only when the must no longer contains ammonium, i.e. after the end of the cell growth phase. It acts as a “nitrogen scavenger” toward amino acids (Cartwright et al., 1989).

2 Saccharomyces cerevisiae also has many specific GAPs (at least 11). Each one ensures the transport of one or more amino acids.

The transport of nitrogen sources is subject to complex regulations depending on the nitrogen content of the medium, by means of a system called nitrogen catabolite repression (NCR) (Beltran et al., 2004).

Toward the end of fermentation, yeasts excrete significant but variable amounts of different amino acids. During fermentation, yeasts assimilate between 1 and 2 g/l of amino acids. Finally, at the end of alcoholic fermentation, a few hundred milligrams of amino acids per liter remain; proline generally represents half of this amount.

Contrary to must hexoses that penetrate the cell by facilitated diffusion, ammonium and amino acids require active transport. Their concentration in the cell is generally higher than in the external medium. The permeases involved couple the transport of an amino acid molecule (or ammonium ion) with the transport of a hydrogen ion. The hydrogen ion moves in the direction of the concentration gradient: the concentration of protons in the must is higher than in the cytoplasm. The amino acid and the proton are linked to the same transport protein and penetrate the cell simultaneously. This concerted transport of two substances in the same direction is called symport. Obviously, the proton that penetrates the cell must then be exported to avoid acidification of the cytoplasm. This movement is made against the concentration gradient and requires energy. The membrane ATPase ensures the excretion of the hydrogen ion across the plasma membrane, acting as a proton pump (Figure 2.24).

Ethanol strongly limits amino acid transport. In an alcohol medium, it modifies the composition and the properties of the phospholipids of the plasma membrane. The membrane becomes more permeable to H⁺ ions in the medium, and these ions massively penetrate the interior of the cell by simple diffusion. The membrane ATPase must increase its operation to control the intracellular pH. As soon as this task monopolizes the ATPase, the symport of the amino acids no longer functions. In other words, at the beginning of fermentation, and for as long as the ethanol concentration in the must is low, yeasts can rapidly assimilate amino acids and concentrate them in the vacuoles for later use, according to their biosynthesis needs.