Inhibitory mechanism of a substrate-type angiotensin I-converting enzyme inhibitory peptide

The inhibitory mechanism during the reaction between peptides from Phascolosoma esculenta and angiotensin I converting enzyme (ACE) was studied. Thermodynamics features of the peptide-ACE binding reaction measured by isothermal titration calorimetry (ITC) assay, combining with the decrease of ACE inhibition within 24 h implied that Gly-Asn-Gly-Ser-Gly-Tyr-Val-Ser-Arg decomposed during the reaction and played as a substrate-type inhibitor. The hydrolysate Gly-Asn-Gly-Ser-Gly-Tyr-Val (GNGSGYV) without ACE inhibitory activity was identified by UPLC & Q-TOF MS, while the hydrolysate Ser-Arg (SR) showed competitive inhibition of ACE with IC 50 value of 790 μM. However, GNGSGYV and SR have synergistic effect on ACE and result in an ACE inhibitory IC 50 value of 170 μM. The synergistic mechanism illustrated by two-step molecular docking discovered that SR firstly attacked the catalytic Zn of ACE and formed coordinate bond, and then GNGSGYV bound with the arginine of SR and residues along the channel of ACE active site by hydrogen bonds to block the substrate from entering. The circular dichroism spectra also verified that SR and GNGSGYV significantly changed the secondary structure of ACE.