Molecular Dynamics Investigation into pH Dependent Metal Binding of the Intrinsically Disordered Worm Jaw Protein, Nvjp-1.

Sclerotization of the jaw is mediated by metal binding to the histidine-rich jaw protein, Nvjp-1. Previous studies showed that the mechanical properties of Nvjp-1 hydrogels could be modulated with zinc binding as well as the associated anion. Here, we show that the mechanical properties of Nvjp-1 hydrogels can be modulated by pH and that zinc binding to Nvjp-1 is stable at both acidic and alkaline pH conditions. To probe the mechanism of Zn binding to Nvjp-1 at different pH conditions, we utilized all atom molecular dynamics simulations employing a polarizable force field. At low pH conditions, polar residues predominantly interacted with Zn, with at most two residues interacting with a given zinc ion. Surprisingly, little to no Zn binding was observed with the abundant Nvjp-1 acidic residues, which form salt-bridges with the protonated histidines to effectively block their binding to Zn ions. As the pH was shifted to alkaline conditions, Zn binding residues reconfigured to form additional coordination bonds with histidine, resulting in a reduction in the radius of gyration that correlated with hydrogel sclerotization. Furthermore, acetate ions were shown to facilitate the capture of zinc ions through association with protonated histidines at low pH, freeing acidic residues to interact with Zn ions and increasing the number of Zn ions that diffuse into the Nvjp-1 interior. Thus, these studies provide valuable molecular insights into how amino acid residues in Nvjp-1 manage metal salt binding and coordination in hydrogels as a function of the pH and ionic environments.