Thiol-Rich fp-6 Controls the Tautomer Equilibrium of Oxidized Dopa in Interfacial Mussel Foot Proteins.

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 (fp-6) and Dopa-incorporated foot protein type 3 fast variant (fp-3F), which were produced in bacterial cells. The redox chemistry of Dopa in fp-3F and the role of fp-6 were observed using a UV-vis spectrophotometer and a surface forces apparatus (SFA). We discovered that fp-6 shifts the tautomer equilibrium to ΔDopa as a preferred tautomer for oxidized Dopa in fp-3F and makes fp-3F better on underwater surface adhesion.