Fig. 5

Degradation performance of TNC. a Degradation ratio of TNC in cathepsin K for 0–4^th week and 4^th–8^th week (n = 4), whiskers extending to 1.5×IQR boundaries. b Representative SEM images of TNC degraded in cathepsin K for 4 weeks. The yellow triangle represents the new-formed mineral crystals. Scale bar: 100 μm. The image in the yellow square was further manifested. c The calcium release property of TNC before and after 4-week degradation (n = 4). d The increased enzymatic activity of cathepsin K under different concentration of calcium ions. e The position of residues P88, Q92 and T101 in the structure of cathepsin K. The solvent accessible surface area of residues P88 (f), Q92 (g) and T101 (h) in cathepsin K, before and after its binding with calcium ion. i The root-mean-square fluctuation change of cathepsin K structure after combined with calcium ion. j Cathepsin K activity influenced by TNC before and after 4-week degradation (n = 4). k Schematic illustrating the interaction between TNC degradation and cathepsin K. **P < 0.01; ***P < 0.001