Uno Ferro, a de novo designed protein, binds transition metals with high affinity and stabilizes semiquinone radical anion.

Metalloenzymes often utilize radicals in order to facilitate chemical reactions. Recently, DeGrado and coworkers have discovered that model proteins can efficiently stabilize semiquinone radical anion produced by oxidation of 3,5-di-tert-butylcatechol (DTBC) in the presence of two zinc ions. Here, we show that the number and the nature of metal ions have relatively minor effect on semiquinone stabilization in model proteins, with a single metal ion being sufficient for radical stabilization. The radical is stabilized by both metal ion, hydrophobic sequestration, and interactions with the hydrophilic residues in the protein interior resulting in a remarkable, nearly 500 mV change in the redox potential of the SQ•¯/catechol couple as compared to bulk aqueous solution. Moreover, we have created 4G-UFsc, a single metal ion-binding protein with pM affinity for zinc that is higher than any other reported model systems and is on par with many natural zinc-containing proteins. We expect that the robust and easy-to-modify DFsc/UFsc family of proteins will become a versatile tool for mechanistic model studies of metalloenzymes.