title: Structural basis for electron and methyl-group transfer in a methyltransferase system operating in the reductive acetyl-CoA pathway.
abstract: Several anaerobic acetogenic, methanogenic, hydrogenogenic, and sulfate-reducing microorganisms are able to use the reductive acetyl-CoA (Wood-Ljungdahl) pathway to convert CO₂ into biomass. The reductive acetyl-CoA pathway consists of two branches connected by the Co/Fe-containing corrinoid iron-sulfur protein (CoFeSP), which transfers a methyl group from a methyltransferase (MeTr)/methyltetrahydrofolate (CH₃-H₄ folate) complex to the reduced Ni-Ni-[4Fe-4S] cluster (cluster A) of acetyl-CoA synthase. We investigated the CoFeSP and MeTr couple of the hydrogenogenic bacterium Carboxydothermus hydrogenoformans and show that the two proteins are able to catalyze the methyl-group transfer reaction from CH₃-H₄ folate to the Co(I) center of CoFeSP. We determined the crystal structures of both proteins. The structure of CoFeSP includes the previously unresolved N-terminal domain of the large subunit of CoFeSP, revealing a unique four-helix-bundle-like architecture in which a [4Fe-4S] cluster is shielded by hydrophobic amino acids. It further reveals that the corrinoid and the [4Fe-4S] cluster binding domains are mobile, which is mandatory for the postulated electron transfer between them. Furthermore, we solved the crystal structures of apo-MeTr, CH₃-H₄-folate-bound MeTr, and H₄-folate-bound MeTr, revealing a substrate-induced closure of the CH₃-H₄ folate binding cavity of MeTr. We observed three different conformations of Asn200 depending on the substrate bound in the active site, demonstrating its conformational modulation by hydrogen-bonding interactions with the substrate. The observed flexibility could be essential to stabilize the transition state during methyl-group transfer. The conformational space and role of Asn200 are likely conserved in homologous cobalamin-dependent MeTrs such as methionine synthase.
authors.affiliation: Institut für Biologie, Strukturbiologie/Biochemie, Humboldt-Universität zu Berlin, Leonor-Michaelis-Haus, 10115 Berlin, Germany.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Goetzl
authors.forename: Jae-Hun
authors.initials: JH
authors.lastname: Jeoung
authors.forename: Sandra Elisabeth
authors.initials: SE
authors.lastname: Hennig
authors.forename: Holger
authors.initials: H
authors.lastname: Dobbek
mesh_terms: D000105:Acetyl Coenzyme A / Q000378:metabolism*; D018360:Crystallography, X-Ray; D004583:Electrons; D006094:Gram-Positive Bacteria / Q000201:enzymology* / Q000378:metabolism; D007506:Iron-Sulfur Proteins / Q000737:chemistry* / Q000378:metabolism*; D053858:Metabolic Networks and Pathways; D008780:Methyltransferases / Q000737:chemistry* / Q000378:metabolism*; D008958:Models, Molecular; D010084:Oxidation-Reduction; D017434:Protein Structure, Tertiary
chemical_list: D007506:Iron-Sulfur Proteins; D000105:Acetyl Coenzyme A; D008780:Methyltransferases