Why Nature Uses Radical SAM Enzymes so Widely: Electron Nuclear Double Resonance Studies of Lysine 2,3-Aminomutase Show the 5′-dAdo• "free Radical" Is Never Free

Masaki Horitani, Amanda S. Byer, Krista A. Shisler, Tilak Chandra, Joan B. Broderick*, Brian M. Hoffman

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

35 Scopus citations

Abstract

Lysine 2,3-aminomutase (LAM) is a radical S-adenosyl-l-methionine (SAM) enzyme and, like other members of this superfamily, LAM utilizes radical-generating machinery comprising SAM anchored to the unique Fe of a [4Fe-4S] cluster via a classical five-membered N,O chelate ring. Catalysis is initiated by reductive cleavage of the SAM S-C5′ bond, which creates the highly reactive 5′-deoxyadenosyl radical (5′-dAdo•), the same radical generated by homolytic Co-C bond cleavage in B12 radical enzymes. The SAM surrogate S-3′,4′-anhydroadenosyl-l-methionine (anSAM) can replace SAM as a cofactor in the isomerization of l-α-lysine to l-β-lysine by LAM, via the stable allylic anhydroadenosyl radical (anAdo•). Here electron nuclear double resonance (ENDOR) spectroscopy of the anAdo• radical in the presence of 13C, 2H, and 15N-labeled lysine completes the picture of how the active site of LAM from Clostridium subterminale SB4 "tames" the 5′-dAdo• radical, preventing it from carrying out harmful side reactions: this "free radical" in LAM is never free. The low steric demands of the radical-generating [4Fe-4S]/SAM construct allow the substrate target to bind adjacent to the S-C5′ bond, thereby enabling the 5′-dAdo• radical created by cleavage of this bond to react with its partners by undergoing small motions, ∼0.6 Å toward the target and ∼1.5 Å overall, that are controlled by tight van der Waals contact with its partners. We suggest that the accessibility to substrate and ready control of the reactive C5′ radical, with "van der Waals control" of small motions throughout the catalytic cycle, is common within the radical SAM enzyme superfamily and is a major reason why these enzymes are the preferred means of initiating radical reactions in nature. (Figure Presented).

Original languageEnglish (US)
Pages (from-to)7111-7121
Number of pages11
JournalJournal of the American Chemical Society
Volume137
Issue number22
DOIs
StatePublished - Jun 10 2015

ASJC Scopus subject areas

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

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