Evidence for an Organometallic Species Formed in the ArsL Reaction

The radical S -adenosylmethionine (SAM) superfamily comprises over 800,000 unique sequences of enzymes that catalyze more than 100 distinct reactions. Canonical radical SAM (RS) enzymes are composed of a full or partial triose phosphate isomerase fold and contain a highly conserved CX ₃ CX ₂ C motif. The cysteines in the conserved motif ligate an [Fe ₄ S ₄ ] cluster used in the reductive cleavage of SAM to yield methionine and a 5′-deoxyadenosyl 5′-radical (5′-dA•). The 5′-dA• is typically used to initiate catalysis by abstracting a hydrogen atom from a bound substrate, yielding 5′-deoxyadenosine and a substrate radical. ArsL, a recently characterized RS enzyme involved in the biosynthesis of the antibiotic arsinothricin (AST), catalyzes a reaction that deviates from the canonical RS reaction, forming methylthioadenosine and a 3-amino-3-carboxypropyl radical (ACP•). The ACP• is used to construct a carbon-arsenic bond to form the organoarsenic compound hydroxyarsinothricin (AST-OH), the penultimate enzymatic step in forming AST. While investigating how ArsL suppresses the formation of the 5′-dA• in favor of the ACP•, we discovered that ArsL forms a unique organometallic species containing a bond between the gamma carbon of ACP and the unique iron of the [Fe ₄ S ₄ ] cluster. We propose that ArsL uses this novel species to generate a sufficiently electrophilic carbon that can be attacked by arsenous acid, thereby forming the carbon-arsenic bond.