Читать книгу Handbook of Enology: Volume 1 - Pascal Ribéreau-Gayon - Страница 62

The ammonium ion and amino acids found in grape must supply the yeast with nitrogen. The yeast can also synthesize most of the amino acids necessary for constructing its proteins. It fixes an ammonium ion on a carbon skeleton derived from the metabolism of sugars. The yeast uses the same reaction pathways as all organisms. Glutamate and glutamine play an important role in this process (Cooper, 1982; Magasanik, 1992).

FIGURE 2.20 Incorporation of the ammonium ion in α‐ketoglutarate catalyzed by NADP glutamate dehydrogenase (NADP‐GDH).

NADP⁺ glutamate dehydrogenase (NADP⁺‐GDH), a product of the GDH1 gene, produces glutamate (Figure 2.20) from an ammonium ion and an α‐ketoglutarate molecule. The latter is an intermediate product of the citric acid cycle. The yeast also possesses an NAD⁺ glutamate dehydrogenase (NAD⁺‐GDH), produced by the GDH2 gene. This dehydrogenase is involved in the oxidative catabolism of glutamate. It produces the inverse reaction of the previous one, liberating the ammonium ion used in the synthesis of glutamine. NADP‐GDH activity is at its maximum when the yeast is cultivated on a medium containing exclusively ammonium as its source of nitrogen. The NAD‐GDH activity, however, is at its highest level when the principal source of nitrogen is glutamate. Glutamine synthetase (GS) produces glutamine from glutamate and ammonium. This amidationrequires the hydrolysis of an ATP molecule (Figure 2.21).

Through transamination reactions, glutamate then serves as an amino group donor in the biosynthesis of different amino acids. Pyridoxal phosphate (PLP) is the transaminase cofactor (Figure 2.22); it is derived from pyridoxine (vitamin B₆).

The carbon skeleton of amino acids originates from intermediates of glycolysis (pyruvate, 3‐phosphoglycerate, and phosphoenolpyruvate), the citric acid cycle (α‐ketoglutarate and oxaloacetate), or the pentose phosphate cycle (ribose 5‐phosphate and erythrose 4‐phosphate). Some of these reactions are very simple, such as the formation of aspartate or alanine by transamination of glutamate into oxaloacetate or pyruvate:

Other biosynthetic pathways are more complex, but still occur in yeasts as in the rest of the living world. The amino acids can be classified into six biosynthetic families, depending on their nature and their carbon precursor (Figure 2.23):

1 In addition to glutamate and glutamine, proline and arginine are formed from α‐ketoglutarate.

2 Asparagine, methionine, lysine, threonine, and isoleucine are derived from aspartate, which comes from oxaloacetate. ATP can activate methionine to formS‐adenosylmethionine, which can be demethylated to form S‐adenosylhomocysteine, the hydrolysis of which liberates adenine to produce homocysteine.FIGURE 2.21 Amidation of glutamate into glutamine by glutamine synthetase (GS).FIGURE 2.22 Pyridoxal phosphate (PLP) and pyridoxamine phosphate (PMP).FIGURE 2.23 General biosynthesis pathways of amino acids.

3 Pyruvate is the starting point for the synthesis of alanine, valine, and leucine.

4 3‐Phosphoglycerate leads to the formation of serine and glycine. The condensation of homocysteine and serine produces cystathionine, a precursor of cysteine.

5 The imidazole ring of histidine is formed from ribose 5‐phosphate and adenine coming from ATP.

6 The amino acids possessing an aromatic ring (tyrosine, phenylalanine, and tryptophan) are derived from erythrose 4‐phosphate and phosphoenolpyruvate. These two compounds are intermediates of the pentose cycle and glycolysis, respectively. Their condensation forms shikimate. The condensation of this compound with another molecule of phosphoenolpyruvate produces chorismate, aprecursor of aromatic amino acids.