## 3 Results and Discussion
### Cellular Amino Acid Metabolic Process
#### 3.3.2 Cellular Amino Acid Metabolic Process at the End of the Second Fermentation
Proteins are shown as nodes, and the existence of interactions between them are represented by lines (the connection between nodes). Line thickness indicates the strength of the different interactions. Nodes with the same color represent specific clusters: sulfur amino acid metabolic process (red nodes), branched-chain amino acid metabolic process (light blue nodes), glutamine metabolic process (light green nodes), glycine metabolic process (yellow nodes), glutamate metabolic process (light purple nodes), serine metabolic process (dark green nodes), arginine metabolic process (light blue nodes), and asparagine metabolic process (orange nodes). the second fermentation is stressed, viability is low, and its nitrogen metabolism is strong affected, stopping the biosynthesis of other nitrogenous cellular constituents as well as other compounds formed from its carbon skeleton [10]. The number of identified proteins was higher in the P29 strain compared to the G1 strain, which offered the possibility of establishing a greater number of interactions between these proteins. In both strains, the metabolism of sulfur amino acids, methionine, and cysteine obtained a greater number of proteins involved in these processes: 5 proteins in the P29 strain and 9 proteins in the G1 strain. In strain P29, glutamate metabolism in the middle of the second fermentation and arginine and sulfur amino acids metabolism at the end of the second fermentation stood out; while in the G1 strain, glycine and tryptophan metabolism stood out in the middle of the second fermentation and glutamate metabolism at the end of the second fermentation. These results represent a first approach in the search for a greater and broader knowledge of the metabolism of yeast during the production of Spanish sparkling wine (cava). However, more research would be necessary, along with a metabolomic study where amino acids are quantified and an assay of enzymatic activities is performed, to reach more solid conclusions. The confirmation of these results by additional tests based on genomic, transcriptomic, and metabolomic activity assays could help achieve a better understanding of the metabolism of amino acids and, therefore, of yeast during the second fermentation in the production of sparkling wines. [PERSON]-[PERSON] conceptualized the research, performed experiments, provided resources, and supervised the work; [PERSON] conceptualized the research, supervised the work, and provided resources; [PERSON]-[PERSON] performed experiments; [PERSON]-[PERSON] participated in the investigation; [PERSON] conceptualized the research, and reviewed and edited the manuscript; [PERSON]-[PERSON] performed data analysis and wrote the original draft. All authors have read and agreed to the published version of the manuscript. This research was granted funding from the \"Consejeria de Economia, Conocimiento, Empresas y Universidad\", Programa Operativo FEDER de Andalucia 2014-2020 ([PERSON], [PERSON]), Reference 1380480-R.  
Not applicable. Kind help of the staff at the Central Research Support Service (SCAI) of the University of Cordoba with the protein analyses is gratefully acknowledged. The authors declare no conflict of interest.