Allosteric Inhibition of XXXX Ribonucleotide Reductase by @entity17699 Entails the Stabilization of a Hexamer.

@entity1 @entity17699 @entity24547 @entity855 @entity4334 @entity2151
UNASSIGNED: @entity2151 reductases (RNRs) are responsible for all de novo biosynthesis of DNA precursors in nature by catalyzing the conversion of @entity24547 to deoxyribonucleotides. Because of its essential role in cell division, @entity1 RNR is a target for a number of anticancer drugs in clinical use. Like other class Ia RNRs, @entity1 RNR requires both a radical-generation subunit (b) and @entity4334 -binding subunit (a) for activity. Because of their complex dependence on allosteric effectors, however, the active and inactive quaternary forms of many class Ia RNRs have remained in question. Here, we present an X-ray crystal structure of the @entity1 a subunit in the presence of inhibiting levels of @entity17699 , depicting a ring-shaped hexamer (a6) where the active sites line the inner hole. Surprisingly, our small-angle X-ray scattering (SAXS) results indicate that @entity1 a forms a similar hexamer in the presence of @entity855 , an activating effector. In both cases, a6 is assembled from dimers (a2) without a previously proposed tetramer intermediate (a4). However, we show with SAXS and electron microscopy that at millimolar @entity855 , the @entity855 -induced a6 can further interconvert with higher-order filaments. Differences in the @entity17699 - and @entity855 -induced a6 were further examined by SAXS in the presence of the b subunit and by activity assays as a function of @entity855 or @entity17699 . Together, these results suggest that @entity17699 -induced a6 is more stable than the @entity855 -induced a6 and that stabilization of this ring-shaped configuration provides a mechanism to prevent access of the b subunit to the active site of a.
