EPR Characterization of the BlsE Substrate Radical Offers Insight into the Determinants of Reaction Outcome that Distinguish Radical SAM Dioldehydratases from Dehydrogenases.

A small but growing set of radical SAM (-adenosyl-l-methionine) enzymes catalyze the radical mediated dehydration or dehydrogenation of 1,2-diol substrates. In some cases, these activities can be interchanged via minor structural perturbations to the reacting components raising questions regarding the relative importance of hyperconjugation, proton circulation and leaving group stability in determining the reaction outcome. The present work describes trapping and electron paramagnetic resonance (EPR) characterization of an α-hydroxyalkyl radical intermediate during dehydration and dehydrogenation of cytosylglucuronic acid and its derivatives catalyzed by the radical SAM enzyme BlsE and its Glu189Ala mutant from the blasticidin S biosynthetic pathway. The substrate radical is found to have a dihedral angle between the electron spin carrier p-orbital and the C-O bond to be cleaved that appears to be sufficient to support elimination despite lying outside the strictly periplanar region. A more significant contributor to the gating of dehydration activity, however, appears to be establishment of a proper hydrogen bonding configuration in order to stabilize the accumulation of negative charge on the eliminated hydroxyl group.