Energy dissipation of osteopontin at a HAp mineral interface: Implications for bone biomechanics

Osteopontin (OPN) is a one of the most abundant non-collagenous proteins in the bone's organic matrix. OPN is responsible for mediating bonding at mineral interfaces in the extrafibrillar space and recent evidence shows that it is a major contributor to bone's fracture resistance. While several experimental studies have identified an important role for calcium ions in mediating energy dissipation in OPN protein networks, the underlying molecular mechanisms remain largely unknown. In the current study, the role of calcium ions on energy dissipation at OPN interface with hydroxyapatite (HAp) as the main bone mineral was investigated. For the first time, the three-dimensional structure of OPN proteins were predicted, and it was found that calcium ions greatly influenced the final protein configuration and energy dissipation performance. Under small deformation, the compact cOPN structure, resulting from calcium ions presence, facilitated greater energy dissipation through sacrificial bond breaking and mechanisms mediated by the surface-bound calcium. At larger deformation, the compact structure also enabled cOPN to dissipate higher energy. Moreover, it was found that phosphorylation of OPN played an important role in energy dissipation. While previous studies have shown that OPN dissipated energy by forming aggregate networks, this study also showed that network formation is not necessary and that individual OPN proteins can dissipate large amounts of energy at HAp interfaces.