Trapping a cross-linked lysine–tryptophan radical in the catalytic cycle of the radical SAM enzyme SuiB
Aidin R. Balo, Alessio Caruso, Lizhi Tao, Dean J. Tantillo, Mohammad R. Seyedsayamdost, R. David Britt
Abstract
Significance The catalytic cycles of metalloenzymes comprise fleeting intermediates, which are challenging to trap and characterize. Their detection, however, can provide invaluable insights into reaction mechanisms. Herein, we focus on SuiB, a radical S -adenosylmethionine (rSAM) enzyme that introduces a carbon–carbon bond at unactivated positions onto its peptide substrate. We present evidence for an unprecedented intermediate, a transient lysine-cross-linked tryptophan radical and its oxidation by a SuiB-bound Fe-S cluster. Our studies provide evidence for a radical electrophilic aromatic substitution pathway, a mechanistic paradigm that is likely common in metalloenzyme-catalyzed carbon–carbon bond formations. They also carry broad implications for the 165,000 rSAM enzymes that contain auxiliary Fe-S clusters.