How ATP and dATP act as molecular switches to regulate enzymatic activity in the prototypic bacterial class Ia ribonucleotide reductase

Class Ia ribonucleotide reductases (RNRs) are subject to allosteric regulation to maintain the appropriate deoxyribonucleotide levels for accurate DNA biosynthesis and repair. RNR activity requires a precise alignment of its 2 and {beta}2 subunits such that a catalytically-essential radical species is transferred from {beta}2 to 2. In E. coli, when too many deoxyribonucleotides are produced, dATP binding to RNR generates an inactive 4{beta}4 state in which {beta}2 and 2 are separated, preventing radical transfer. ATP binding breaks the -{beta} interface, freeing {beta}2 and restoring activity. Here we investigate the molecular basis for allosteric activity regulation in the prototypic E. coli class Ia RNR. Through the determination of six crystal structures we are able to establish how dATP binding creates a binding pocket for {beta} on that traps {beta}2 in the inactive 4{beta}4 state. These structural snapshots also reveal the numerous ATP-induced conformational rearrangements that are responsible for freeing {beta}2. We further discover, and validate through binding and mutagenesis studies, a previously unknown nucleotide binding site on the subunit that is crucial for the ability of ATP to dismantle the inactive 4{beta}4 state. These findings have implications for the design of allosteric inhibitors for bacterial RNRs.