Dataset Information

Reversible Formation of Alkyl Radicals at [Fe4S4] Clusters and Its Implications for Selectivity in Radical SAM Enzymes.

ABSTRACT: All kingdoms of life use the transient 5'-deoxyadenosyl radical (5'-dAdo•) to initiate a wide range of difficult chemical reactions. Because of its high reactivity, the 5'-dAdo• must be generated in a controlled manner to abstract a specific H atom and avoid unproductive reactions. In radical S-adenosylmethionine (SAM) enzymes, the 5'-dAdo• is formed upon reduction of SAM by an [Fe4S4] cluster. An organometallic precursor featuring an Fe-C bond between the [Fe4S4] cluster and the 5'-dAdo group was recently characterized and shown to be competent for substrate radical generation, presumably via Fe-C bond homolysis. Such reactivity is without precedent for Fe-S clusters. Here, we show that synthetic [Fe4S4]-alkyl clusters undergo Fe-C bond homolysis when the alkylated Fe site has a suitable coordination number, thereby providing support for the intermediacy of organometallic species in radical SAM enzymes. Addition of pyridine donors to [(IMes)3Fe4S4-R]+ clusters (R = alkyl or benzyl; IMes = 1,3-dimesitylimidazol-2-ylidene) generates R•, ultimately forming R-R coupled hydrocarbons. This process is facile at room temperature and allows for the generation of highly reactive radicals including primary carbon radicals. Mechanistic studies, including use of the 5-hexenyl radical clock, demonstrate that Fe-C bond homolysis occurs reversibly. Using these experimental insights and kinetic simulations, we evaluate the circumstances in which an organometallic intermediate can direct the 5'-dAdo• toward productive H-atom abstraction. Our findings demonstrate that reversible homolysis of even weak M-C bonds is a feasible protective mechanism for the 5'-dAdo• that can allow selective X-H bond activation in both radical SAM and adenosylcobalamin enzymes.

SUBMITTER: Brown AC

PROVIDER: S-EPMC7447116 | biostudies-literature | 2020 Aug

REPOSITORIES: biostudies-literature

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Publications

Reversible Formation of Alkyl Radicals at [Fe4S4] Clusters and Its Implications for Selectivity in Radical SAM Enzymes.

Brown Alexandra C AC Suess Daniel L M DLM

Journal of the American Chemical Society 20200806 33

All kingdoms of life use the transient 5'-deoxyadenosyl radical (5'-dAdo•) to initiate a wide range of difficult chemical reactions. Because of its high reactivity, the 5'-dAdo• must be generated in a controlled manner to abstract a specific H atom and avoid unproductive reactions. In radical S-adenosylmethionine (SAM) enzymes, the 5'-dAdo• is formed upon reduction of SAM by an [Fe4S4] cluster. An organometallic precursor featuring an Fe-C bond between the [Fe4< ...[more]

PMID: 32696642

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Project description:A [4Fe-4S](+) cluster reduces a bound S-adenosylmethionine (SAM) molecule, cleaving it into methionine and a 5'-deoxyadenosyl radical (5'-dA(•)). This step initiates the varied chemistry catalyzed by each of the so-called radical SAM enzymes. The strongly oxidizing 5'-dA(•) is quenched by abstracting a H-atom from a target species. In some cases, this species is an exogenous molecule of substrate, for example, L-tyrosine in the [FeFe] hydrogenase maturase, HydG. In other cases, the target is a proteinaceous residue as in all the glycyl radical forming enzymes. The generation of this initial radical species and the subsequent chemistry involving downstream radical intermediates is meticulously controlled by the enzyme so as to prevent unwanted reactions. But the manner in which this control is exerted is unknown. Electron paramagnetic resonance (EPR) spectroscopy has proven to be a valuable tool used to gain insight into these mechanisms. In this Account, we summarize efforts to trap such radical intermediates in radical SAM enzymes and highlight four examples in which EPR spectroscopic results have shed significant light on the corresponding mechanism. For lysine 2,3-aminomutase, nearly each possible intermediate, from an analogue of the initial 5'-dA(•) to the product radical L-?-lysine, has been explored. A paramagnetic intermediate observed in biotin synthase is shown to involve an auxiliary [FeS] cluster whose bridging sulfide is a co-substrate for the final step in the biosynthesis of vitamin B7. In HydG, the L-tyrosine substrate is converted in unprecedented fashion to a 4-oxidobenzyl radical on the way to generating CO and CN(-) ligands for the [FeFe] cluster of hydrogenase. And finally, EPR has confirmed a mechanistic proposal for the antibiotic resistance protein Cfr, which methylates the unactivated sp(2)-hybridized C8-carbon of an adenosine base of 23S ribosomal RNA. These four systems provide just a brief survey of the ever-growing set of radical SAM enzymes. The diverse chemistries catalyzed by these enzymes make them an intriguing target for continuing study, and EPR spectroscopy, in particular, seems ideally placed to contribute to our understanding.

Dataset Information

Reversible Formation of Alkyl Radicals at [Fe4S4] Clusters and Its Implications for Selectivity in Radical SAM Enzymes.

Publications

Reversible Formation of Alkyl Radicals at [Fe<sub>4</sub>S<sub>4</sub>] Clusters and Its Implications for Selectivity in Radical SAM Enzymes.

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