Metal Chelation Dynamically Regulates the Mechanical Properties of Engineered Protein Hydrogels

Engineering protein hydrogels with dynamically tunable mechanical and physical properties is of great interest due to their potential applications in biomedical engineering and mechanobiology. In our recent work, we engineered a novel dynamic protein hydrogel using a redox responsive, mutually exclusive protein (MEP)-based folding switch as the building block. By modulating the redox potential, the MEP-based folding switch can switch its conformation between two distinct states, leading to a significant change of the proteins' effective contour length of the polypeptide chain and an effective change of the cross-linking density of the hydrogel network (Kong, N. et al. Adv. Funct. Mater. 2014, 24, 7310). Building upon this work, here we report an engineered metal-chelation based method to dynamically regulate mechanical and physical properties of MEP-based protein hydrogels. We engineered a bihistidine metal binding motif in the host domain of the MEP. The binding of bivalent ions (such as Ni²⁺) enhances the thermodynamic stability of the host domain and results in the shift of the conformational equilibrium between the two mutually exclusive conformations of the MEP. Thus, the bihistidine mutant can serve as a metal ion responsive-folding switch to regulate the conformational equilibrium of the MEP. Using this bihistidine mutant of MEP as building blocks, we engineered chemically cross-linked protein hydrogels. We found that the mechanical and physical properties (including Young's modulus, resilience, and swelling degree) of this hydrogel can be regulated by metal chelation in a continuous and reversible fashion. This dynamic change is due to the metal chelation-induced shift of the conformational equilibrium of the MEP and consequently the effective cross-linking density of the hydrogel. Our results demonstrate a general strategy to engineer MEP-based dynamic protein hydrogels that may find applications in mechanobiology and tissue engineering.