Monovalent Cation Activation of the Radical SAM Enzyme Pyruvate Formate-Lyase Activating Enzyme

Pyruvate formate-lyase activating enzyme (PFL-AE) is a radical S-adenosyl-L-methionine (SAM) enzyme that installs a catalytically essential glycyl radical on pyruvate formatelyase. We show that PFL-AE binds a catalytically essential, monovalent cation at its active site, yet another parallel with B-12 enzymes, and we characterize this cation site by a combination of structural, biochemical, and spectroscopic approaches. Refinement of the PFL-AE crystal structure reveals Na+ as the most likely ion present in the solved structures, and pulsed electron nuclear double resonance (END OR) demonstrates that the same cation site is occupied by Na-23 in the solution state of the as isolated enzyme. A SAM carboxylate-oxygen is an M+ ligand, and EPR and circular dichroism spectroscopies reveal that both the site occupancy and the identity of the cation perturb the electronic properties of the SAM-chelated iron sulfur cluster. ENDOR studies of the PFL-AE/[C-13-methyl]-SAM complex show that the target sulfonium positioning varies with the cation, while the observation of an isotropic hyperfine coupling to the cation by ENDOR measurements establishes its intimate, SAM-mediated interaction with the cluster. This monovalent cation site controls enzyme activity: (i) PFL-AE in the absence of any simple monovalent cations has little no activity; and (ii) among monocations, going down Group 1 of the periodic table from Li+ to Cs+, PFL-AE activity sharply maximizes at K+, with NH4+ closely matching the efficacy of K+. PFL-AE is thus a type I M+ activated enzyme whose M+-controls reactivity by interactions with the cosubstrate, SAM, which is bound to the catalytic iron-sulfur cluster.