Hydrogen Bond Network between Amino Acid Radical Intermediates on the Proton-Coupled Electron Transfer Pathway of E. coli α2 Ribonucleotide Reductase

[Image: see text] Ribonucleotide reductases (RNRs) catalyze the conversion of ribonucleotides to deoxyribonucleotides in all organisms. In all Class Ia RNRs, initiation of nucleotide diphosphate (NDP) reduction requires a reversible oxidation over 35 Å by a tyrosyl radical (Y(122)•, Escherichia coli) in subunit β of a cysteine (C(439)) in the active site of subunit α. This radical transfer (RT) occurs by a specific pathway involving redox active tyrosines (Y(122) ⇆ Y(356) in β to Y(731) ⇆ Y(730) ⇆ C(439) in α); each oxidation necessitates loss of a proton coupled to loss of an electron (PCET). To study these steps, 3-aminotyrosine was site-specifically incorporated in place of Y(356)-β, Y(731)- and Y(730)-α, and each protein was incubated with the appropriate second subunit β(α), CDP and effector ATP to trap an amino tyrosyl radical (NH(2)Y•) in the active α2β2 complex. High-frequency (263 GHz) pulse electron paramagnetic resonance (EPR) of the NH(2)Y•s reported the g(x) values with unprecedented resolution and revealed strong electrostatic effects caused by the protein environment. (2)H electron–nuclear double resonance (ENDOR) spectroscopy accompanied by quantum chemical calculations provided spectroscopic evidence for hydrogen bond interactions at the radical sites, i.e., two exchangeable H bonds to NH(2)Y(730)•, one to NH(2)Y(731)• and none to NH(2)Y(356)•. Similar experiments with double mutants α-NH(2)Y(730)/C(439)A and α-NH(2)Y(731)/Y(730)F allowed assignment of the H bonding partner(s) to a pathway residue(s) providing direct evidence for colinear PCET within α. The implications of these observations for the PCET process within α and at the interface are discussed.