Aminofutalosine Synthase: Evidence for Captodative and Aryl Radical Intermediates Using β-Scission and S_RN1 Trapping Reactions

Abstract

Aminofutalosine synthase (MqnE) is a radical SAM enzyme involved in the menaquinone biosynthetic pathway. In this communication, we propose a novel mechanism for this reaction involving the addition of the adenosyl radical to the substrate double bond to form a captodative radical followed by rearrangement and decarboxylation to form an aryl radical anion which is then oxidized by the [4Fe-4S] cluster. Consistent with this proposal, we describe the trapping of the captodative radical and the aryl radical anion using radical triggered C-Br fragmentation reactions. We also describe the trapping of the captodative radical by replacing the vinylic carboxylic acid with an amide.

C–C bond forming radical SAM enzymes involved in the construction of carbon skeletons of cofactors and natural products

“…Intriguingly, halogenation of the substrate in the MqnE reaction resulted in loss of the specificity of the radical attack. 25 In the case of MoaA, the enzyme may activate the acceptor substrate using its auxiliary cluster (see Section 3.1). While further mechanistic investigations are needed to test such hypotheses, it is possible that acceptor activation plays a key role in the enzyme catalyzed radical reactions.…”

“…29. 25 Use of 3′-Br/Cl–3-[(1-carboxyvinyl)-oxy]benzoic acid ( 63 and 64 ) resulted in the formation of the compound 66 as the major product (Fig. 29a) due to fast homolytic cleavage of the carbon–halogen bond β to the radical center ( k C–Br = 3 × 10 8 s −1 , and k C–Cl = 4 × 10 6 s −1 ).…”

“…The same reactions also produced a minor product 68 as a result of loss in regiospecificity of the radical attack. 25 Since no rearrangement products were observed, the rate constant for the rearrangement reaction was estimated as slower than 2 × 10 5 s −1 25. …”

“…29b). 25 In synthetic chemical reactions, aryl halide radical anions structurally similar to 70 are known to undergo dehalogenation through elimination of a bromide anion to yield a neutral aryl radical, similar to 72 , followed by radical quenching via H-atom transfer. Therefore, the formation of 73 in the MqnE reaction with 69 was rationalized to be the result of formation of the aryl radical anion intermediate 70 followed by bromide elimination and radical quenching by an H-atom donor (X–H, Fig.…”

“…30 was proposed, 25 where MqnE reductively cleaves SAM and generate 5′-dA • which attacks the C-3′ of 3-[(1-carboxyvinyl)-oxy]benzoic acid ( 58 ). The resulting C-2′ radical 74 undergoes rearrangement via the spiroepoxide 75 and generates a radical cation intermediate.…”