Charge and Conformation in Adsorption of Osteopontin to Hydroxyapatite

Objectives: Interactions between extracellular matrix proteins and crystals play important roles in biomineralization. For example, the phosphoprotein osteopontin (OPN) is known to inhibit the formation of hydroxyapatite (HA) crystals. In this study, we have used computational techniques to study the roles of charge and conformation in the interaction between OPN and HA. Methods: Order/disorder analysis of rat OPN was performed using Predictor of Naturally Disordered Regions (www.pondr.com). A PONDR score >0.5 indicates a disordered sequence. Molecular dynamics (MD) was used to simulate the adsorption of OPN to HA. The rat OPN sequence was divided into 16-amino-acid blocks starting at the N-terminus. The {100} face of HA was simulated using coordinates derived from dental enamel. Simulations were run for 5 nsec using GROMACS as previously described (Grohe et al, JACS 129, 14946; 2007). Distances between peptide centre of mass and the surface layer of crystal atoms were then calculated. Results: PONDR analysis of rat OPN indicated that most of the sequence is highly disordered, with only 3 ordered regions: 1-31, 132-144 and 265-275 (Figure). MD analysis showed that those OPN sequences with net charge ≤-7 resulted in closest apposition to the {100} face of HA. Peptides adsorbing least well to the {100} face corresponded to sequences 17-32 (net charge -4), 113-128 (-2), 129-144 (+2) and 177-192 (0) (Figure). Thus, sequences predicted to interact best with the {100} face are characterized by high negative charge and low order. Conclusion: These studies suggest that the regions of the OPN molecule most capable of adsorbing to the {100} face of HA, which is quite basic, are both highly acidic and highly disordered. Lack of order may allow crystal-binding sequences of mineral-modulating proteins to maximize contacts between acidic side-chains and Ca2+ ions of the crystal face. Supported by the Canadian Institutes of Health Research.