title: Unexpected electron transfer mechanism upon AdoMet cleavage in radical SAM proteins.
abstract: Radical S-adenosine-L-methionine (SAM or AdoMet) proteins are involved in chemically difficult reactions including the synthesis of cofactors, the generation of protein radicals, and the maturation of complex organometallic catalytic sites. In the first and common step of the reaction, a conserved [Fe4S4] cluster donates an electron to perform the reductive cleavage of AdoMet into methionine and a reactive radical 5'-dA. species. The latter extracts a hydrogen atom from substrate eliciting one of the about 40 reactions so far characterized for this family of proteins. It has been suggested that the radical-generating mechanism differs depending on whether AdoMet is a cofactor or a substrate. It has also been speculated that electron transfer from the [Fe4S4] cluster to AdoMet is sulfur-based. Here we have used protein crystallography and theoretical calculations to show that regardless whether AdoMet serves as a cofactor or a substrate, the 5'-dA. generating mechanism should be common to the radical SAM proteins studied so far, and that electron transfer is mediated by a unique Fe from the conserved [Fe4S4] cluster. This unusual electron transfer is determined by the sulfonium ion in AdoMet.
authors.affiliation: Laboratoire de Cristallographie et Cristallogenèse des Protéines, Institut de Biologie Structurale J.P. Ebel, Commissariat à l'Energie Atomique, Centre National de la Recherche Scientifique, Université Joseph Fourier, 41 rue Jules Horowitz, 38027 Grenoble, France.
authors.forename: Yvain
authors.initials: Y
authors.lastname: Nicolet
authors.forename: Patricia
authors.initials: P
authors.lastname: Amara
authors.forename: Jean-Marie
authors.initials: JM
authors.lastname: Mouesca
authors.forename: Juan C
authors.initials: JC
authors.lastname: Fontecilla-Camps
mesh_terms: D018360:Crystallography, X-Ray; D004579:Electron Transport; D005609:Free Radicals / Q000378:metabolism; D006864:Hydrogenase / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D015394:Molecular Structure; D011485:Protein Binding; D012436:S-Adenosylmethionine / Q000737:chemistry* / Q000378:metabolism*; D020124:Thermotoga maritima / Q000201:enzymology*
chemical_list: D005609:Free Radicals; D012436:S-Adenosylmethionine; D006864:Hydrogenase
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