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Mussels are marine organisms that are capable of constructing an underwater adhesion between their bodies and rigid structures. It is well known that mussels achieve underwater adhesion through the presence of mussel adhesive proteins (MAPs) that contain high levels of 3,4-dihydroxyphenylalanine (DOPA). Although the extraordinary underwater adhesive properties of mussels are attributed to DOPA, its capacity to play a dual role in surface adhesion and internal cohesion is inherently limited. However, mussels employ a combination of chemical moieties, not just DOPA, along with anatomical components, such as plaque and byssus, in underwater adhesion. This also involves junction proteins that connect the plaque and byssus. In this study, a novel hybrid MAP was bioengineered via the fusion of the plaque protein (foot protein type 1) and the histidine-rich domain of the junction protein (foot protein type 4). To achieve direct adhesion underwater, the adhesive should maintain surface adhesion without disintegrating. Notably, the histidine-Zn-coordinated hybrid MAP hydrogel maintained a high surface adhesion ability even after cross-linking because of the preservation of its unoxidized and non-cross-linked DOPA moieties. The formulated adhesive hydrogel system based on the bioengineered hybrid MAP exhibited self-healing properties, owing to the reversible metal coordination bonds. The developed adhesive hydrogel exhibits outstanding levels of bulk adhesion in underwater environments, highlighting its potential as an effective adhesive biomaterial. Therefore, the introduction of histidine-rich domains into MAPs may be applied in various studies to formulate mussel-inspired adhesives with self-healing properties and to fully utilize the adhesive ability of DOPA.

3,4-Dihydroxyphenylalanine (Dopa) is a versatile molecule that enables marine mussels to achieve successful underwater adhesion. However, due to its complicated redox chemistry and vulnerability to oxidation, controlling surface adhesion and cohesion has been a challenging issue to overcome. Foot protein type 6 (fp-6), a thiol-rich interfacial mussel adhesive protein, has been reported as a proteinaceous antioxidant for mussels that helps Dopa maintain surface adhesion ability. In this study, we focused on the role of fp-6 in oxidized Dopa. The effect on the tautomer equilibrium of oxidized Dopa was investigated using recombinant fp-6 (rfp-6) and Dopa-incorporated foot protein type 3 fast variant (drfp-3F), which were produced in bacterial cells. The redox chemistry of Dopa in drfp-3F and the role of rfp-6 were observed using a UV–vis spectrophotometer and a surface forces apparatus (SFA). We discovered that rfp-6 shifts the tautomer equilibrium to ΔDopa as a preferred tautomer for oxidized Dopa in drfp-3F and makes drfp-3F better on underwater surface adhesion.