Читать книгу Handbook of Enology: Volume 1 - Pascal Ribéreau-Gayon - Страница 64

The ammonium ion is essential for the synthesis of amino acids necessary for building proteins, but yeasts cannot always find sufficient quantities in their environment. Fortunately, they can obtain ammonium from available amino acids through various reactions.

FIGURE 2.24 Active amino acid transfer mechanisms in the yeast plasma membrane. P, protein playing the role of an amino acid “symporter.”

FIGURE 2.25 Oxidative deamination of an amino acid, catalyzed by a transaminase and glutamate dehydrogenase.

The most common pathway is the transfer of an α‐amino group, originating from one of many different amino acids, onto α‐ketoglutaric acid to form glutamate. Aminotransferases or transaminases catalyze this reaction, whose prosthetic group is PLP. Glutamate is then deaminated by an oxidative pathway to form NH₄⁺ (Figure 2.25). These two reactions can be summarized as follows:

During transamination, PLP is temporarily transformed into pyridoxamine phosphate (PMP). The PLP aldehyde group is bound to a lysine residue ε‐amino group on the active site of the aminotransferase to form an intermediate (E‐PLP) (Figure 2.26). The α‐amino group of the amino acid that is the transamination substrate displaces the lysine ε‐amino group bound to PLP. The cleavage of this intermediate liberates PMP and the keto acid corresponding to the amino acid substrate. PMP can in turn react with another keto acid to furnish a second amino acid and regenerate PLP.

The partial reactions can be written in the following manner:

the balance for which is:

FIGURE 2.26 Mode of action of pyridoxal phosphate (PLP) in transamination reactions. Formation of intermediates between PLP and aminotransferase or the amino acid substrate.

Some amino acids, such as serine and threonine, possess a hydroxyl group on their β carbon. They can be directly deaminated by dehydration. A dehydratase catalyzes this reaction, producing the corresponding keto acid and ammonium (Figure 2.27).