Bimetallic Mn, Fe, Co, and Ni Sites in a Four-Helix Bundle Protein: Metal Binding, Structure, and Peroxide Activation.

Bimetallic active sites in enzymes catalyze small-molecule conversions that are among the top 10 challenges in chemistry. As different metal cofactors are typically incorporated in varying protein scaffolds, it is demanding to disentangle the individual contributions of the metal and the protein matrix to the activity. Here, we compared the structure, properties, and hydrogen peroxide reactivity of four homobimetallic cofactors (Mn(II)2, Fe(II)2, Co(II)2, Ni(II)2) that were reconstituted into a four-helix bundle protein. Reconstituted proteins were studied in solution and in crystals. All metals bind with high affinity and yield similar cofactor structures. Cofactor variants react with H2O2 but differ in their turnover rates, accumulated oxidation states, and trapped peroxide-bound intermediates. Varying the metal composition thus creates opportunities to tune the reactivity of the bimetallic cofactor and to study and functionalize reactive species.