Radical SAM enzyme spore photoproduct lyase: Properties of the Ω organometallic intermediate and identification of stable protein radicals formed during substrate-free turnover

Spore photoproduct lyase is a radical S-adenosyl-Lmethionine (SAM) enzyme with the unusual property that addition of SAM to the [4Fe-4S]1+ enzyme absent substrate results in rapid electron transfer to SAM with accompanying homolytic S-C5′ bond cleavage. Herein, we demonstrate that this unusual reaction forms the organometallic intermediate Ω in which the unique Fe atom of the [4Fe-4S] cluster is bound to C5′ of the 5′-deoxyadenosyl radical (5′- dAdo•). During catalysis, homolytic cleavage of the Fe-C5′ bond liberates 5′-dAdo• for reaction with substrate, but here, we use Ω formation without substrate to determine the thermal stability of Ω. The reaction of Geobacillus thermodenitrif icans SPL (GtSPL) with SAM forms Ω within ∼15 ms after mixing. By monitoring the decay of Ω through rapid freeze-quench trapping at progressively longer times we find an ambient temperature decay time of the Ω Fe-C5′ bond of τ ≈ 5-6 s, likely shortened by enzymatic activation as is the case with the Co-C5′ bond of B12. We have further used hand quenching at times up to 10 min, and thus with multiple SAM turnovers, to probe the fate of the 5′-dAdo• radical liberated by Ω. In the absence of substrate, Ω undergoes low-probability conversion to a stable protein radical. The WT enzyme with valine at residue 172 accumulates a Valo; mutation of Val172 to isoleucine or cysteine results in accumulation of an Ile• or Cys• radical, respectively. The structures of the radical in WT, V172I, and V172C variants have been established by detailed EPR/DFT analyses.