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.

Functional Diversity of HemN-like Proteins

“…Because multiple reaction turnovers were observed in the absence of external reductant, it is likely that upon decarboxylation the electron could go back to the [4Fe-4S] cluster to regenerate its active +1 state . A similar redox-neutral reaction has also been observed for other radical SAM enzymes. − …”

Menaquinone Biosynthesis: New Strategies to Trap Radical Intermediates in the MqnE-Catalyzed Reaction

“…Aminofutalosine synthase (MqnE) is a radical SAM enzyme involved in the futalosine-dependent menaquinone biosynthesis and is the first of only four examples of radical SAM enzymes in which the 5′-deoxyadenosyl radical is utilized for C–X bond formation (X = C, S, As) rather than for H atom abstraction. − Mechanistic studies on MqnE have established the intermediacy of the captodative radical 3 as well as an aryl radical anion 7 (Figure A), , and the structure of MqnE complexed to 8 has recently been determined . The rearrangement reaction ( 3 – 5 ) can be blocked by replacing the bridging oxygen in the native substrate with a methylene group.…”

Insights into Arsenic Secondary Metabolism in Actinomycetes from the Structure and Biosynthesis of Bisenarsan

“…2 In fact, it was recently reported that the conversion of DMAs(III) to 5′-deoxy-5′dimethylarsinoyl-adenosine, a key intermediate of the arsenosugars, is catalyzed by the radical SAM enzyme ArsS, which is distributed in a wide range of bacterial phyla. 4,5 Arsenosugars and AsFAs are thought to be converted to As-containing complex lipids, which comprise the majority of organoarsenic natural products, or degraded to arsenobetaine (Figure S1). 2 On the other hand, the soil bacterium Burkholderia gladioli GSRB05 was reported to convert As(III) to the As-bearing amino acid arsinothricin in a different manner; a C−As bond is formed between inorganic As(III) and a 3-amino-3-carboxypropyl radical by the radical SAM enzyme ArsL, and subsequent As-methylation by the SAM methyltransferase ArsM and autoxidation complete arsinothricin biosynthesis (Figure S1).…”