A tryptophan 48 cation radical (W48+⊙) forms concomitantly with the Fe2(III/IV) cluster, X, during activation of oxygen for tyrosyl radical (Y122⊙) production in the R2 subunit of class I ribonucleotide reductase (RNR) from Escherichia coli. W48+⊙ is also likely to be an intermediate in the long-range radical transfer between R2 and its partner subunit, R1, during nucleotide reduction by the RNR holoenzyme. The kinetics of decay of W48+⊙ and formation of tyrosyl radicals during O2 activation (in the absence of R1) in wild-type (wt) R2 and in variants with either Y122, Y356 (the residue thought to propagate the radical from W48+⊙ into R1 during turnover), or both replaced by phenylalanine (F) have revealed that the presence of divalent cations at concentrations similar to the [Mg2+] employed in the standard RNR assay (15 mM) mediates a rapid radical-transfer equilibrium between W48 and Y356, Cation-mediated propagation of the radical from W48 to Y356 gives rise to a fast phase of Y⊙ production that is essentially coincident with W48+⊙ formation and creates an efficient pathway for decay of W48+⊙. Possible mechanisms of this cation mediation and its potential relevance to intersubunit radical transfer during nucleotide reduction are considered.
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