## 3 Results
### Molecular Docking Study  
A docking simulation was performed to investigate the interactions between ACE inhibitory peptides (ligands) and ACE molecules (receptors) [7; 41]. Calculations were performed using the CDocker energy value (known as interaction energies), the binding sites, and the information on formed interaction bonds. Lisinopril was selected to be docked first to dACE (PDB code: 1O86), before the processing of molecular docking of ACE inhibitory peptides to validate the precision of this current model. A significant positional similitude of lisinopril and the one obtained from the lisinopril-tACE complex (1O86.pdb) proved the accuracy of this model (Figure A6). The docking simulation of human tACE (PDB code: 1O86) with potential peptides (WLQL and LPSW) was performed using Discovery Studio Visualized 3.0 software (Accelrys Software, Cambridge, UK). The main active sites of ACE conformation were named pocket S1 (Ala354, Glu384, and Tyr523 residues), S2 (Gln281, Tyr520, Lys511, His513, and His353 residues), and S1' (Glu162 residue) [26; 42; 43]. Alternatively, Zn (II), which plays a critical role in the ACE activity, binds with ACE residues His383, His387, and Glu411 through coordination bonds to form the more stable zinc-binding motif HEXMH (tetrahedral structure) [26; 44]. The stability of the zinc-binding motif HEXXH is a key factor in the binding affinity between ACE and inhibitors [26; 44]. Moreover, the Arg522 residue that works as a notable ligand to the second Cl\({}^{-}\) of the ACE, the binding site for lisinopril, and the substrate of ACE, may enhance ACE inhibitory activity [26; 44]. These molecules can interact with the enzyme through hydrogen bonding (H-bond), hydrophobic interactions, electrostatic forces, and van der Waals forces. Among them, H-bonds are considered as contributing elements to the structural stability of the ligand-receptor complex [8; 45]. Two tetrapeptides WLQL and LPSW were selected for the docking simulation with tACE based on their high ACE inhibitory activity compared to the remaining peptides. Small fragments (WL and QL from WLQL; LP and SW from LPSW) obtained from pre-incubation with ACE were also used for molecular docking to clearly understand their role in enhancing the ACE inhibitory activity. As shown in Figure 4, all the obtained peptides could interact with the ACE receptor through the S1, and S2 pockets, and the other amino acid residues. Theoretically, the lower binding free energy of the peptide-ACE complexes can be interpreted as a thermodynamic property, resulting in a more stable complex, meaning a higher inhibitory effect of the peptides. According to their CDocker energies, the ACE inhibitory activity of the following peptides decreased: WLQL, QL, SW, WL, LPSW, and LP (Table 3). Via in vitro assay, the peptide WLQL was proven to possess a higher ACE inhibitory effect with an IC\({}_{50}\) value of 16.87 uM, as compared to LPSW with an IC\({}_{50}\) value of 20.80 uM. The CDocker energy of WLQL was \(-\)80.4321 kJ/mol. The tetrapeptide WLQL formed five H-bonds with Arg522 (2.0 A; 2.0 A; 2.2 A) (important ligand to the second Cl\({}^{-}\) of the ACE) and Glu384 (2.2 A; 2.4 A), and four \(\pi\)-\(\pi\) bonds with Tyr523 (5 A; 6.3 A), and His383 (4 A; 4.3 A), which was bound to a zinc ion. The amino group of tryptophan in WLQL also formed four H-bonds with Tyr523 (5 A; 6.3 A) and His383 (4 A; 4.3 A) residues, which were  
The CDOcker energy of WL was 58.8494 kJ/mol. The dipeptide WL interacted with the ACE receptor via two H-bonds, including Glu384 (3.3 A) at the S1 pocket and another residue, such as Asn70 (2.0 A). Moreover, no \(\pi\)-\(\pi\) bonds were recognized in this case. The binding to the active site of ACE may allow the peptide WL to more easily react with the ACE, leading to an enhanced ACE inhibitory effect (Figure 4B; Tables 3 and 5). As shown in Figure 4C and Table 3, the dipeptide QL bound to the second binding site instead of the active sites of the ACE receptor.