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Escherichia coli class Ia ribonucleotide reductase (RNR) is composed of two subunits that form an active α2β2 complex. The nucleoside diphosphate substrates (NDP) are reduced in α2, 35 Å from the essential diferric-tyrosyl radical (Y 122  • ) cofactor in β2. The Y 122  • -mediated oxidation of C 439 in α2 occurs by a pathway (Y 122 ⇆ [W 48 ] ⇆ Y 356 in β2 to Y 731 ⇆ Y 730 ⇆ C 439 in α2) across the α/β interface. The absence of an α2β2 structure precludes insight into the location of Y 356 and Y 731 at the subunit interface. The proximity in the primary sequence of the conserved E 350 to Y 356 in β2 suggested its importance in catalysis and/or conformational gating. To study its function, pH-rate profiles of wild-type β2/α2 and mutants in which 3,5-difluorotyrosine (F 2 Y) replaces residue 356, 731, or both are reported in the presence of E 350 or E 350 X (X = A, D, or Q) mutants. With E 350 , activity is maintained at the pH extremes, suggesting that protonated and deprotonated states of F 2 Y 356 and F 2 Y 731 are active and that radical transport (RT) can occur across the interface by proton-coupled electron transfer at low pH or electron transfer at high pH. With E 350 X mutants, all RNRs were inactive, suggesting that E 350 could be a proton acceptor during oxidation of the interface Ys. To determine if E 350 plays a role in conformational gating, the strong oxidants, NO 2 Y 122  • -β2 and 2,3,5-F 3 Y 122  • -β2, were reacted with α2, CDP, and ATP in E 350 and E 350 X backgrounds and the reactions were monitored for pathway radicals by rapid freeze-quench electron paramagnetic resonance spectroscopy. Pathway radicals are generated only when E 350 is present, supporting its essential role in gating the conformational change(s) that initiates RT and masking its role as a proton acceptor.

biochemistry

Glutamate 350 Plays an Essential Role in Conformational Gating of Long-Range Radical Transport in <i>Escherichia coli</i> Class Ia Ribonucleotide Reductase

Glutamate 350 Plays an Essential Role in Conformational Gating of Long-Range Radical Transport in Escherichia coli Class Ia Ribonucleotide Reductase