Substrate-Dependent Cleavage Site Selection by Unconventional Radical S-Adenosylmethionine Enzymes in Diphthamide Biosynthesis

Min Dong, Masaki Horitani, Boris Dzikovski, Jack H. Freed, Steven E. Ealick, Brian M. Hoffman, Hening Lin*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

13 Scopus citations


S-Adenosylmethionine (SAM) has a sulfonium ion with three distinct C-S bonds. Conventional radical SAM enzymes use a [4Fe-4S] cluster to cleave homolytically the C5′,adenosine-S bond of SAM to generate a 5′-deoxyadenosyl radical, which catalyzes various downstream chemical reactions. Radical SAM enzymes involved in diphthamide biosynthesis, such as Pyrococcus horikoshii Dph2 (PhDph2) and yeast Dph1-Dph2 instead cleave the Cγ,Met-S bond of methionine to generate a 3-amino-3-carboxylpropyl radical. We here show radical SAM enzymes can be tuned to cleave the third C-S bond to the sulfonium sulfur by changing the structure of SAM. With a decarboxyl SAM analogue (dc-SAM), PhDph2 cleaves the Cmethyl-S bond, forming 5′-deoxy-5′-(3-aminopropylthio) adenosine (dAPTA, 1). The methyl cleavage activity, like the cleavage of the other two C-S bonds, is dependent on the presence of a [4Fe-4S]+ cluster. Electron-nuclear double resonance and mass spectroscopy data suggests that mechanistically one of the S atoms in the [4Fe-4S] cluster captures the methyl group from dc-SAM, forming a distinct EPR-active intermediate, which can transfer the methyl group to nucleophiles such as dithiothreitol. This reveals the [4Fe-4S] cluster in a radical SAM enzyme can be tuned to cleave any one of the three bonds to the sulfonium sulfur of SAM or analogues, and is the first demonstration a radical SAM enzyme could switch from an Fe-based one electron transfer reaction to a S-based two electron transfer reaction in a substrate-dependent manner. This study provides an illustration of the versatile reactivity of Fe-S clusters.

Original languageEnglish (US)
Pages (from-to)5680-5683
Number of pages4
JournalJournal of the American Chemical Society
Issue number16
StatePublished - Apr 26 2017

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry


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