## Results and Discussion
### Structure-Activity Relationship of MMP-4, MMP-7, and MMP-12  
The structure-activity relationship of the functional molecule will provide the basis for predicting the activity of new compounds, but so far it is still difficult to illuminate the relationship between structural characteristics and antioxidant activity of peptides [5; 46]. In general, molecular size, hydrophobicity, and amino acid composition and sequence are regarded as key factors for the activity of antioxidant peptides [47; 48].  
Low-MW antioxidant peptides have a highly pronounced ability to hold back the oxidative stress because they can easily interact with target radicals to terminate the free radical chain reactions [5; 49; 50]. In addition, [PERSON] et al. reported that the antioxidant activities of protein hydrolysates with average MW (AMW) from 0.64 kDa to 257.19 kDa were negatively related to the logarithm of their AMW [31]. The MWs of MMP-4, MMP-7, and MMP-12 were 763.82, 660.75, and 739.75 Da, which help them easily interacting with ROS to prevent oxidative stress.  
The hydrophobic and aromatic amino acids are believed to play an important role in the antioxidant activity of peptides [5; 51]. The abundance of hydrophobic amino acids, such as Pro, Met, Leu, Tyr, Phe and Val in the peptide sequences seems to enhance the antioxidant activities by increasing peptide solubility in lipids, and the interaction with radical species [52; 53; 54]. [PERSON] et al. reported that hydrophobic amino acid residues (Leu and Ile) in AEDKLIQ played an important role on the hydrophobic targets [55]. Met residue acted as a reactive site in the PMRGGGGYHY, WMFDW, WMGPY, and EMGPA through formatting a sulfoxide structure to scavene free radicals [18; 56]. Pro residue can help antioxidant peptides, such as LDEPDPL, WMGPY and EMGPA, scavenge ROS because of the pyrroline ring [18; 57]. Furthermore, aromatic groups of Trp, Phe, and Tyr residues can scavenge free radicals by providing protons [55; 56; 57; 58]. Therefore, hydrophobic/aromatic amino acid residues in the sequences of EDIVCW (Ile, Val, Cys and Trp), MEPVW (Met, Pro, Val and Trp), and YWDAW (Tyr, Trp, Ala and Trp) play an important role for their antioxidant activity.  
Polar amino acid residues, especially acidic amino acid residues, play a crucial part in the HOC scavenging and metal ion chelating capacities of antioxidant peptides [57; 59]. The same result was reported by [PERSON] et al. that Glu, Asp and Lys within the sequence of AEDKLIQ were conducive to the radical scavenging potential and metal-chelating activity [55]. In addition, the antioxidant function of Asp seemed to have nothing to do with where it appears in the sequence of peptides [55]. Therefore, acid amino acids, including the Glu and Asp residues in DIVCW, and Glu residue in MEPVW, should be very helpful in improving their antioxidant ability.  
Interestingly, recent reports showed that melatonin could protect human erythrocytes from H\({}_{2}\)O\({}_{2}\)-induced oxidative damage on anion exchange capability through Band 3 in the absence of the endogenous erythrocytes antioxidant system [60]. Therefore, we will also look at whether three isolated peptides (MMP-4, MMP-7, and MMP-12) have similar functions to melatonin.