Use of 3-aminotyrosine to examine the pathway dependence of radical propagation in Escherichia coli ribonucleotide reductase.

Escherichia coli ribonucleotide reductase (RNR), an alpha2beta2 complex, catalyzes the conversion of nucleoside 5'-diphosphate substrates (S) to 2'-deoxynucleoside 5'-diphosphates. alpha2 houses the active site for nucleotide reduction and the binding sites for allosteric effectors (E). beta2 contains the essential diferric tyrosyl radical (Y(122)(*)) cofactor which, in the presence of S and E, oxidizes C(439) in alpha to a thiyl radical, C(439)(*), to initiate nucleotide reduction. This oxidation occurs over 35 A and is proposed to involve a specific pathway: Y(122)(*) --> W(48) --> Y(356) in beta2 to Y(731) --> Y(730) --> C(439) in alpha2. 3-Aminotyrosine (NH(2)Y) has been site-specifically incorporated at residues 730 and 731, and formation of the aminotyrosyl radical (NH(2)Y(*)) has been examined by stopped-flow (SF) UV-vis and EPR spectroscopies. To examine the pathway dependence of radical propagation, the double mutant complexes Y(356)F-beta2:Y(731)NH(2)Y-alpha2, Y(356)F-beta2:Y(730)NH(2)Y-alpha2, and wt-beta2:Y(731)F/Y(730)NH(2)Y-alpha2, in which the nonoxidizable F acts as a pathway block, were studied by SF and EPR spectroscopies. In all cases, no NH(2)Y(*) was detected. To study off-pathway oxidation, Y(413), located 5 A from Y(730) and Y(731) but not implicated in long-range oxidation, was examined. Evidence for NH(2)Y(413)(*) was sought in three complexes: wt-beta2:Y(413)NH(2)Y-alpha2 (a), wt-beta2:Y(731)F/Y(413)NH(2)Y-alpha2 (b), and Y(356)F-beta2:Y(413)NH(2)Y-alpha2 (c). With (a), NH(2)Y(*) was formed with a rate constant that was 25-30% and an amplitude that was 25% of that observed for its formation at residues 731 and 730. With (b), the rate constant for NH(2)Y(*) formation was 0.2-0.3% of that observed at 731 and 730, and with (c), no NH(2)Y(*) was observed. These studies suggest the evolution of an optimized pathway of conserved Ys in the oxidation of C(439).

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