Computational Design of a Highly Stable Dicopper Catechol Oxidase

Type 3 (T3) Cu proteins play essential roles in binding and activating molecular oxygen (O₂) and are prevalent across all domains of life. Despite sharing the same coordination motif, T3 Cu proteins display divergent functions: hemocyanin transports O₂, while tyrosinase catalyzes the hydroxylation of monophenols and the subsequent oxidation of diphenols and catechol oxidase oxidizes only diphenols. Here, we report the design and characterization of a di-Cu protein (Cu-HC4) inspired by the active sites of natural T3 Cu proteins to investigate the structural features that facilitate catalytic oxidase activity. Cu-HC4 is roughly 1/5th the size of the commercially available mushroom tyrosinase and shares only around 20% sequence identity with the T3 Cu protein templates. Notably, Cu-HC4 displays high thermostability and exhibits diphenol oxidation activity at ambient and elevated temperatures (≥60 °C). Cu-HC4 also initiates the formation of melanin polymers, mimicking melanin biosynthesis of natural tyrosinases. Mechanistic investigations demonstrate that Cu-HC4 utilizes both Cu centers cooperatively for diphenol oxidation and requires O₂ for catalysis like natural Cu oxidases but follows a distinct catalytic pathway compared to those enzymes. Cryo-EM characterization of a tetrameric form of HC4 reveals slight deviations in the relative positions of the active site His residues that may account for differences in reactivity between Cu-HC4 and natural T3 Cu enzymes.