Selective incorporation of 5‐hydroxytryptophan blocks long range electron transfer in oxalate decarboxylase

Oxalate decarboxylase from Bacillus subtilis is a binuclear Mn‐dependent acid stress response enzyme that converts the mono‐anion of oxalic acid into formate and carbon dioxide in a redox neutral unimolecular disproportionation reaction. A π‐stacked tryptophan dimer, W96 and W274, at the interface between two monomer subunits facilitates long‐range electron transfer between the two Mn ions and plays an important role in the catalytic mechanism. Substitution of W96 with the unnatural amino acid 5‐hydroxytryptophan leads to a persistent EPR signal which can be traced back to the neutral radical of 5‐hydroxytryptophan with its hydroxyl proton removed. 5‐Hydroxytryptophan acts as a hole sink preventing the formation of Mn(III) at the N‐terminal active site and strongly suppresses enzymatic activity. The lower boundary of the standard reduction potential for the active site Mn(II)/Mn(III) couple can therefore be estimated as 740 mV against the normal hydrogen electrode at pH 4, the pH of maximum catalytic efficiency. Our results support the catalytic importance of long‐range electron transfer in oxalate decarboxylase while at the same time highlighting the utility of unnatural amino acid incorporation and specifically the use of 5‐hydroxytryptophan as an energetic sink for hole hopping to probe electron transfer in redox proteins.