Self-sacrifice in radical S-adenosylmethionine proteins

Abstract

The radical SAM superfamily of metalloproteins catalyze the reductive cleavage of S-adenosyl-l-methionine to generate a 5'-deoxyadenosyl radical (5'-dA*) intermediate that is obligate for turnover. The 5'-dA* acts as a potent oxidant, initiating turnover by abstracting a hydrogen atom from an appropriate substrate. A special class of these enzymes use this strategy to functionalize unactivated C-H bonds by insertion of sulfur atoms. This review will describe the characterization of three members of this class - biotin synthase, lipoyl synthase, and MiaB protein - each of which has been shown to cannibalize itself during turnover.

Figure 1

Formation of methionine and a 5′-deoxyadenosyl radical via the reductive cleavage of S-adenosylmethionine. The iron–sulfur cluster shown is that which is ligated by protein cysteine thiolates that reside in the CX₃CX₂C motif that is common among radical SAM proteins.

Figure 2

Reactions catalyzed by radical SAM enzymes that are known to involve sulfur insertion. SAM, S-adenosylmethionine; Met, methionine; 5′-dAH, 5′-deoxyadenosine. Hydrogens shown in red are those that are removed by the 5′-deoxyadenosyl radical (A) Biotin synthase reaction (B) Lipoyl synthase reaction (C) MiaB reaction

Figure 3

X-ray structure of biotin synthase (PDB 1R30). (A) Structure of homodimer showing the (α/β)₈ barrel fold. The active site [4Fe–4S] and [2Fe–2S] clusters as well as dethiobiotin and SAM are shown in stick format. (B) View of the active site. Sulfur atoms are shown in yellow. Iron atoms are shown in black. The structure was prepared using the Pymol Molecular Graphics System (http://www.pymol.org).

Figure 4

Working model for catalysis by BS. The 6-pro-S hydrogen is shown in red.

Figure 5

Working model for catalysis by LS. The 6-pro-R hydrogen is shown in red.