## 4 Discussion
During this step, CO\({}_{2}\) is generated, which can lead to increased oxaloacetate production [45]. This could also explain why the MDH3 protein, which is responsible for the interconversion of malate and oxaloacetate, also shows significant differences. Glycine metabolism can contribute to the synthesis of protein [46]. This nitrogenous compound was only quantified in wine inoculated with strain N62. It can be formed from TCA [41], with one route of synthesis via the cleavage of threonine lactose, and another from glyoxylate by L-alanine glyoxylate aminotransferase [46]. In relation to this amino acid, the HEM1 protein, which catalyzes the synthesis of 5-aminolevulinate and CoA from succinyl-CoA and glycine, has been significantly quantified in this strain [47]. L-aspartic acid in G1 may be used as a nitrogen source. This nitrogenous compound is involved in the L-lysine synthesis pathway [33]. This last amino acid is usually one of the main sources of nitrogen at the beginning of fermentation [36], as it seems to occur  
Figure 7: The most relevant proteins and amino acids with significant differences involved in nitrogen metabolism at three bars of pressure. The color in the protein names represents a greater quantity in strain N62 (green) and G1 (red). PEP (phosphoenolpyruvate), Alfa-KG (Alfa-Ketoglutarate), OAA (oxaloacetate), and GABA (gamma-amino-n-butyric acid). in both strains, being formed from 2-oxoglutarate and acetyl-CoA. However, significant differences were found between the wines inoculated with both strains; N62 had a lower concentration, which could be due to the fact that in this strain, the proteins ACO1 and ACO2, which are important for TCA and could be involved in L-lysine synthesis [48],were positively regulated, whereas in G1, only one protein related to the L-lysine biosynthetic pathway, LYS9, was quantified with significant differences [49]. L-Glutamic acid is significantly consumed by G1; as shown in Figure 3a, it is part of TCA, and this compound could be used for arginine biosynthesis [42], producing succinic acid [50], which is related to a protein with significant differences with respect to TCA, in this strain. KGD1 (or OGDHC) is the rate-limiting enzyme within the TCA cycle, converting 2-oxoglutate to succinyl-CoA, reducing NAD\({}^{+}\) to NADH + H\({}^{+}\), and producing CO\({}_{2}\)[51]. OGDHC consists of three subunits: E1, 2-oxoglutate dehydrogenase (OGDH), the E2 subunit dihydroxyllysine succinyltransferase (DLST), and E3, D-2-hydroxyglutarate pyruvate transhydrogenase (DLD) [52]. In the wine inoculated with N62, a significantly higher concentration of glutamic acid was found. This compound is formed from L-glutamate, and GLT1 has been identified as the specific protein of this strain responsible for its synthesis [53]. In addition, this nitrogenous compound is involved in the production of ornithine, as occurring in this wine, and in the formation of gamma-aminobutyric acid [33]. Alternatively, by transaminating this acid with succinic acid, it can also produce aspar acid [38]. Another way to obtain a higher concentration of gamma-aminobutyric acid would be to convert the glutamate in wine when the yeast releases decarboxyllases [54]. Both possibilities would explain the concentration of this amino acid in this wine. L-arginine is one of the most important amino acids in wine [38; 55]. In strain N62, it is completely consumed, so it could be used as an important nitrogen source, which would explain why a large amount of ammonium chloride is used for the synthesis of other nitrogenous compounds. On the other hand, strain G1 synthesized L-arginine significantly. CPA1 and CPA2 have been quantified as strain G1-specific proteins involved in the synthesis pathway of this nitrogenous compound [56], which could explain the results obtained (Figures 2 and 7). In addition, L-ornithine can be converted to L-arginine [57], as may be the case in this strain, given the significant differences in these amino acids with respect to the base wine.