New insights into the reduction of protein degradation in freshwater fish by proanthocyanidins: Inhibition mechanism and the conformational changes of endogenous cathepsin B.

Endogenous cathepsin B (CTSB) plays an important role in protein degradation, which accelerates the decline in the quality of grass carp muscle. Proanthocyanidins (PC) can protect fish texture by inhibiting protein degradation; however, the corresponding mechanism is still unclear. This study comprehensively explored the inhibitory effect of PC on the conformational changes of CTSB through multispectral and molecular simulation methods. PC inhibited CTSB activity through a reversible mixed-type inhibition mode with IC value of 134.56 ± 0.02 μmol/L. The binding of PC damaged the hydrogen bonding network structure of CTSB. The docking score was -7.1 kcal/mol, representing a high affinity between CTSB and PC. Molecular simulation found that PC maintained the stability of the CTSB-PC complex by interacting with several key residues (Trp221, Gly24 and Gly198) of CTSB. Therefore, this study can provide a theoretical basis for the application of polyphenols in inhibiting endogenous CTSB-induced fish muscle protein degradation.