## 3 Results
### Proteomic Analysis  
A total of 1994 proteins were identified in all tea samples during fermentation by _A. niger_ PW-2 (Table S4). The changes in the abundance of all proteins are illustrated by a heatmap displayed in Figure S1. The GO classes that were highly represented included the cellular process and metabolic process of the biological process; the cellular anatomical entity of the cellular component; and the catalytic activity and binding of the molecular function (Figure S2A and Table S5). The KEGG pathways that were enriched included \"carbohydrate metabolism\" (329), \"amino acid metabolism\" (261), \"energy metabolism\" (127), \"lipid metabolism\" (97), \"metabolism of other amino acids\" (95), \"metabolism of cofactors and vitamins\" (89), \"nucleotide metabolism\" (59), \"glycan biosynthesis and metabolism\" (26) and \"metabolism of terpenoids and polyketides\" (17) (Figure S2B and Table S5). The CAZYmes that are responsible for the synthesis of all carbohydrates are glycosyltransferases (GTs), and those involved in the degradation include carbohydrate esterases (CEs), glycoside hydrolases (GHs), auxiliary activities (AAs) and polysaccharide (PLs); carbohydrate binding modules (CBMs) are responsible for recognition. The CAZYmes identified in this study could be divided into GHs (103), AAs (23), CBMs (15), CEs (25), GTS (14) and PLs (6) (Table S5).  
The proposed metabolic pathway of tea phenols is shown in Figure 5A, and the abundances of the related proteins are shown as heatmaps in Figure 5B,C. The heatmap of GHs (Figure 5B and Table S6) showed that the abundance of most GHs increased during the whole fermentation period and was the highest at the late fermentation stage (Days 18-21). Seven proteins in the tannase and feruloyl esterase family (TNS) were identified by manual checking (Figure 5B and Table S7). Most of these proteins had higher levels at the late fermentation stage (Days 15-21). Tea phenols including catechins, gallates, gallic acids and catechol were oxidized to quinones by catechol oxidase (COD), peroxidase (POD) or lactose (LCS) during fermentation. As shown in Figure 5A, as the fermentation degree was increased, more quinones were converted to theaflavins, thearubigins and theabrownins by COD, POD and LCS [23]. The levels of most CODs obviously increased on Day 3 of fermentation and then decreased. The levels of some proteins in the COD and POD groups also increased during the mid-fermentation stage (Days 3-18), while those of other proteins in the COD, LCS and POD groups reached their maximum levels at the end of fermentation (Day 21) (Figure 5B and Table S7).  
To better understand the dynamic changes in metabolites during fermentation by _A. niger_ PW-2, the proposed metabolic pathway analyzed based on the KEGG pathway was annotated with the determined metabolites and enzymes (Figure 6A). The changes in the contents of metabolites including amino acids, differential volatile compounds and alkaloids are also included in Figure 6A. The profiles of enzymes involved in amino acid synthesis and degradation are shown by a heatmap in Figure 6B,C, respectively, and the detailed information is shown in Table S7.  
Figure 5: The hypothetical metabolic pathway of tea phenols (**A**), and the abundance heatmap of related enzymes: heatmap of GHs (**B**), and heatmap of TNS, COD, LCS and POD (**C**). GHs, glycoside hydrolases; TNS, tannase; COD, catechol oxidase; LCS, lactose; POD, peroxidase.  
Figure 6: The map of the metabolic pathway based on differential metabolites (**A**), and the heatmap analysis of enzymes related to the biosynthesis and metabolism of metabolites (**B**,**C**). GABA, \(\gamma\)-aminobutyric acid.