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Following the electrons: peculiarities in the catalytic cycles of radical SAM enzymes.

ABSTRACT: Radical SAM enzymes use S-adenosyl-l-methionine as an oxidant to initiate radical-mediated transformations that would otherwise not be possible with Lewis acid/base chemistry alone. These reactions are either redox neutral or oxidative leading to certain expectations regarding the role of SAM as either a reusable cofactor or the ultimate electron acceptor during each turnover. However, these expectations are frequently not realized resulting in fundamental questions regarding the redox handling and movement of electrons associated with these biological catalysts. Herein we provide a focused perspective on several of these questions and associated hypotheses with an emphasis on recently discovered radical SAM enzymes.

SUBMITTER: Ruszczycky MW

PROVIDER: S-EPMC6051909 | biostudies-literature | 2018 Jul

REPOSITORIES: biostudies-literature

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Following the electrons: peculiarities in the catalytic cycles of radical SAM enzymes.

Ruszczycky Mark W MW Zhong Aoshu A Liu Hung-Wen HW

Natural product reports 20180701 7

Radical SAM enzymes use S-adenosyl-l-methionine as an oxidant to initiate radical-mediated transformations that would otherwise not be possible with Lewis acid/base chemistry alone. These reactions are either redox neutral or oxidative leading to certain expectations regarding the role of SAM as either a reusable cofactor or the ultimate electron acceptor during each turnover. However, these expectations are frequently not realized resulting in fundamental questions regarding the redox handling ...[more]

PMID: 29485151

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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-β-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

Following the electrons: peculiarities in the catalytic cycles of radical SAM enzymes.

Publications

Following the electrons: peculiarities in the catalytic cycles of radical SAM enzymes.

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