## 3 Results and Discussion
### Cellular Amino Acid Metabolic Process
#### 3.3.2 Cellular Amino Acid Metabolic Process at the End of the Second Fermentation  
At the end of the second fermentation, 43 specific proteins were identified in the P29 strain and 9 specific proteins in the G1 strain. These proteins were represented as nodes in the P29 strain (Figure 3A) and G1 strain (Figure 3B). A total of 234 interactions (number of edges) were observed, with a _p_-value of PPI enrichment <1.0 \(\times\) 10\({}^{-16}\) in the P29 strain, while in the G1 strain, 4 interactions were obtained, with a _p_-value of PPI enrichment <0.00047; such enrichment indicates that the proteins are at least partially biologically connected as a group. In view of the results obtained, the P29 strain increased the number of proteins related to the cellular metabolism of amino acids at the end of the second fermentation, unlike the G1 strain. In the P29 strain, the strongest interactions were observed between proteins belonging to the arginine metabolic process, the sulfur amino acid metabolic process, and the branched-chain amino acid metabolic process; those that obtained a greater number of proteins involved in their respective processes being the metabolism of arginine and sulfur amino acids (cysteine and methionine), 8 proteins in each process. Sulfur in methionine amino acid can be incorporated into cysteine. Furthermore, methionine has been shown to impact oxidative stress resistance and has the potential to be catabolized into \(\alpha\)-ketoglutate, which can directly enter the central carbon metabolism, providing additional support for why the demand for cell uptake was high. Glutamate, the most abundant supplemented amino acid, showed the highest uptake yield, and although no directly linked catabolic enzymes were significantly up-allocated, glutamate may be utilized metabolically for other transamination reactions because it is the major amino acid for this role [36]. On the other hand, Bna1p and Mmf1p did not interact with any protein. Bna1p is required for the _de novo_ biosynthesis of NAD from tryptophan via kynureine, and Mmf1p is required for the transamination of isoleucine. For the branched-chain amino acids (BCAA)isoleucine, leucine, and valine and the aromatic amino acids (AAA) phenylalanine, tryptophan, and tyrosine, catabolism, via the Ehrlich pathway, is a way for the cell to salvage nitrogen through transamination reactions [37]. In contrast, in the G1 strain, this strong interaction was between proteins involved with the glutamate metabolic process. However, Adh5p and Gfa1p did not interact. Adh5p is an alcohol dehydrogenase isoenzyme V and is involved in ethanol production, while Gfa1p converts fructose-6-phosphate to glucosamine-6-phosphate. This seems to indicate that the yeast is trying to maintain its cell growth and be able to cope with stressful conditions such as overpressure of CO2. Despite having a different number of proteins, both strains presented amino acids responsible for a high rate of cell growth [20], such as asparagine (P29 strain) and glutamate (G1 strain). Therefore, maintenance of cell viability should be expected in both strains during the second fermentation. The metabolism of these nitrogen compounds depends on the yeast strain, its physiological state, and the physicochemical properties of the wine and fermentation conditions [38]. This difference obtained with respect to the cellular metabolism of amino acids at the end of the second fermentation between both strains could be because they are different strains of yeast: a typical one for the elaboration of this type of wine (P29 strain) and a flor yeast (G1 strain) subjected to unusual CO2 overpressure conditions for this type of strain. This suggests that the G1 strain at the end of  
Figure 3: High confidence protein–protein interaction network map built using STRING v11 and based on the proteins of cellular amino acid metabolic process detected in _S. cerevisiae_ P29 (**A**) and G1 (**B**) at the end of the second fermentation.