A non-structural pure enzyme protein forms an LCST type of stimuli-responsive and reversible hydrogel with novel structure and catalytic activity

Hydrogels have a wide range of applications such as in biomedicine, cosmetics and soft electronics. Compared to polymer hydrogels based on covalent bonding, protein hydrogels offer distinct advantages owing to their biocompatibility and better access to molecular engineering. However, pure and natural protein hydrogels have been seldom reported except for structural proteins like collagen and silk fibrin. Here, we report the unusual ability and mechanism of a unique natural enzyme, lipoate-protein ligase A (LplA) of E. coli to self-assemble into a stimuli-responsive and reversible hydrogel of the low critical solution temperature (LCST) type. This is the first globular and catalytic protein found to form a hydrogel in response to temperature, pH and the presence of ions. Protein structure based analysis reveals the key residues responsible for the gel formation and mutational studies confirms the essential roles of hydrogen bonding between the C-terminal domains and electrostatic interactions in the N-terminal domains. Characterization of phase transitions of wild type LplA and its mutants using small angle X-ray scattering (SAXS) yields details of the gelation process from initial dimer formation over a pre-gel-state to full network development. Further electron microscopic analyses and modeling of SAXS data suggest an unusual interlinked ladder-like structure of the macroscopic crosslinking network with dimers as ladder steps. The unique features of this first reported protein hydrogel may open up hitherto inaccessible applications, especially those taking advantage of the inherent catalytic activity of LplA.