Iron–Sulfur Clusters in “Radical SAM” Enzymes: Spectroscopy and Coordination
Résumé
There is increasing evidence for the versatility of the coordination of iron–sulfur clusters in biology. In addition to cysteine residues as the most favored ligand and providing sulfur coordination, oxygenbased (aspartate, tyrosinate …) and nitrogen-based (histidine, arginine …) residues have also been observed as ligands to the clusters. Furthermore, low-molecular-weight substrates (citrate in the case of aconitase) and cofactors (S-adenosylmethionine, SAM, in the case of “Radical SAM” enzymes) have been shown to bind to one of the iron atoms of the [4Fe–4S] clusters where they are then activated. In this chapter we discuss the potential as well as the limitations of ENDOR and HYSCORE spectroscopy for characterizing metalloprotein coordination and, more specifically, the cluster–SAM complexes that are essential intermediates in pyruvate formate lyase-activating enzyme, lysine 2,3 aminomutase, and ribonucleotide reductase activating enzyme. These three systems are prototypes for the “Radical SAM” enzyme superfamily, whose chemistry seems to be extensively utilized in the metabolism of all living organisms.