Modulation of Y356 photooxidation in E. coli class Ia ribonucleotide reductase by Y731 across the α2:β2 interface

Substrate turnover in class Ia ribonucleotide reductase (RNR) requires reversible radical transport across two subunits over 35 Å, which occurs by a multistep proton-coupled electron-transfer mechanism. Using a photooxidant-labeled β2 subunit of Escherichia coli class Ia RNR, we demonstrate photoinitiated oxidation of a tyrosine in an α2:β2 complex, which results in substrate turnover. Using site-directed mutations of the redox-active tyrosines at the subunit interface, Y356F(β) and Y731F(α), this oxidation is identified to be localized on Y356. The rate of Y356 oxidation depends on the presence of Y731 across the interface. This observation supports the proposal that unidirectional PCET across the Y356(β)-Y731(α)-Y730(α) triad is crucial to radical transport in RNR.