title: Cation mediation of radical transfer between Trp48 and Tyr356 during O2 activation by protein R2 of Escherichia coli ribonucleotide reductase: relevance to R1-R2 radical transfer in nucleotide reduction?
abstract: A tryptophan 48 cation radical (W48(+)(*)) forms concomitantly with the Fe(2)(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 O(2) 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 [Mg(2+)] 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.
authors.affiliation: Department of Biochemistry and Molecular Biology, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
authors.forename: Lana
authors.initials: L
authors.lastname: Saleh
authors.forename: J Martin
authors.initials: JM
authors.lastname: Bollinger
mesh_terms: D000595:Amino Acid Sequence; D004926:Escherichia coli / Q000201:enzymology*; D005609:Free Radicals / Q000737:chemistry; D007700:Kinetics; D015636:Magnesium Chloride / Q000494:pharmacology*; D012264:Ribonucleotide Reductases / Q000378:metabolism*; D014364:Tryptophan / Q000737:chemistry*; D014443:Tyrosine / Q000737:chemistry*
chemical_list: D005609:Free Radicals; D015636:Magnesium Chloride; D014443:Tyrosine; D014364:Tryptophan; D012264:Ribonucleotide Reductases; C108427:ribonucleotide reductase R2 subunit