Generation of a stable, aminotyrosyl radical-induced α2β2 complex of Escherichia coli class Ia ribonucleotide reductase.

Ribonucleotide reductase (RNR) catalyzes the conversion of nucleoside diphosphates to deoxynucleoside diphosphates (dNDPs). The Escherichia coli class Ia RNR uses a mechanism of radical propagation by which a cysteine in the active site of the RNR large (α2) subunit is transiently oxidized by a stable tyrosyl radical (Y•) in the RNR small (β2) subunit over a 35-Å pathway of redox-active amino acids: Y122• ↔ [W48?] ↔ Y356 in β2 to Y731 ↔ Y730 ↔ C439 in α2. When 3-aminotyrosine (NH2Y) is incorporated in place of Y730, a long-lived NH2Y730• is generated in α2 in the presence of wild-type (wt)-β2, substrate, and effector. This radical intermediate is chemically and kinetically competent to generate dNDPs. Herein, evidence is presented that NH2Y730• induces formation of a kinetically stable α2β2 complex. Under conditions that generate NH2Y730•, binding between Y730NH2Y-α2 and wt-β2 is 25-fold tighter (Kd = 7 nM) than for wt-α2