Characterization and Mechanistic Study of the Radical SAM Enzyme ArsS Involved in Arsenosugar Biosynthesis

Abstract

Arsenosugars are a group of arsenic‐containing ribosides that are found predominantly in marine algae but also in terrestrial organisms. It has been proposed that arsenosugar biosynthesis involves a key intermediate 5′‐deoxy‐5′‐dimethylarsinoyl‐adenosine (DDMAA), but how DDMAA is produced remains elusive. Now, we report characterization of ArsS as a DDMAA synthase, which catalyzes a radical S‐adenosylmethionine (SAM)‐mediated alkylation (adenosylation) of dimethylarsenite (DMAsIII) to produce DDMAA. This radical‐mediated reaction is redox neutral, and multiple turnover can be achieved without external reductant. Phylogenomic and biochemical analyses revealed that DDMAA synthases are widespread in distinct bacterial phyla with similar catalytic efficiencies; these enzymes likely originated from cyanobacteria. This study reveals a key step in arsenosugar biosynthesis and also a new paradigm in radical SAM chemistry, highlighting the catalytic diversity of this superfamily of enzymes.

Radical SAM‐Dependent Demetallation of Heme

“…[ 1‐6 ] In most cases, dAdo radical abstracts a hydrogen atom from the substrate to initiate highly diverse reactions. [ 1‐6 ] The dAdo radical can also add to sp 2 carbons [ 7‐10 ] or heteroatoms [ 11‐14 ] to result in an adenosylation reaction. Although in general, radical SAM enzymes have evolved dedicated microenvironments to finely control the reactivity of intermediates to achieve a specific catalytic outcome, promiscuous activities leading to off‐pathway shunt products have been observed for several enzymes, [ 15‐20 ] highlighting the remarkable catalytic plasticity of this superfamily of enzymes.…”

Consecutive Methylation Catalyzed by TsrM, an Atypical Class B Radical SAM Methylase

“…In most cases, dAdo radical abstracts a hydrogen atom from the substrate to produce 5ˊ‐deoxyadenosine (dAdoH) and a substrate radical, thereby initiating highly diverse reactions. [ 2‐3 ] The dAdo radical can also add to sp 2 carbons [ 4‐8 ] or heteroatoms [ 9‐12 ] to result in adenosylation reactions. Although in general radical SAM enzymes have evolved dedicated microenvironments to tune the reactivity of intermediates to achieve a specific catalytic outcome, promiscuous reactions have been observed in various systems, [ 13‐25 ] highlighting the remarkable catalytic plasticity of this superfamily of enzymes.…”

The Promiscuous Activity of the Radical SAM Enzyme NosL toward Two Unnatural Substrates

“…[ 15‐20 ] In a few cases, dAdo radical can directly add to a sp 2 carbon or a heteroatom to result in an adenosylation reaction. [ 21‐25 ] In the case of NosL, the dAdo radical abstracts a hydrogen atom from the L ‐Trp amino group to produce a nitrogen‐ centered tryptophanyl radical 3 . [ 26‐28 ] The Cα—COO – bond is then cleaved to release a carboxyl radical •CO 2 – 5 , which adds to the indole C2 of indole‐3‐ethanimine ( 4 ) to produce 6 , a stable radical that can be well characterized by electron paramagnetic resonance (EPR) spectroscopy.…”