2-Oxoquinoline 8-monooxygenase is a Rieske non-heme iron oxygenase that catalyzes the NADH-dependent oxidation of the N-heterocyclic aromatic compound 2-oxoquinoline to 8-hydroxy-2-oxoquinoline in the soil bacterium Pseudomonas putida 86. The crystal structure of the oxygenase component of 2-oxoquinoline 8-monooxygenase shows a ring-shaped, C3-symmetric arrangement in which the mononuclear Fe(II) ion active site of one monomer is at a distance of 13 A from the Rieske-[2Fe-2S] center of a second monomer. Structural analyses of oxidized, reduced, and substrate bound states reveal the molecular bases for a new function of Fe-S clusters. Reduction of the Rieske center modulates the mononuclear Fe through a chain of conformational changes across the subunit interface, resulting in the displacement of Fe and its histidine ligand away from the substrate binding site. This creates an additional coordination site at the mononuclear Fe(II) ion and can open a pathway for dioxygen to bind in the substrate-containing active site.

acety-CoA pathway ͉ Carboxydothermus hydrogenoformans ͉ methyltransferase ͉ cobalt B 12-dependent enzymes are widespread in nature and catalyze distinct reactions. They can be grouped in three major classes: the isomerases, reductive dehalogenases, and methyltransferases (MeTrs; ref. 1). The cobalamin-dependent MeTrs play important roles in the amino acid biosynthesis in many organisms, ranging from bacteria to humans, as well as in the one-carbon metabolism of bacteria and archaea. MeTrs typically catalyze the transfer of methyl groups between organic and inorganic donors and acceptors like S-adenosylmethionine, methyltetrahydrofolate, and cobalamins. A unique methyltransfer reaction is found within the acetyl-CoA (Ljungdahl-Wood) pathway of autotrophic carbon assimilation, which operates in the anaerobic acetogenic, sulfidogenic, methanogenic, and hydrogenogenic bacteria and archaea. In the final step of the pathway, acetyl-CoA is synthesized from a methyl cation, carbon monoxide (CO), and CoA, a reaction catalyzed by the Ni-and Fe-containing enzyme acetyl-CoA synthase (ACS; for a review, see ref.2). Key to this reaction (Eq. 1) is the action of the Coand Fe-containing corrinoid iron-sulfur protein (CoFeSP) that donates the methyl group from methyl-cob(III)amide to one of the two Ni ions in the active site cluster A of ACS (3). This is the only methyltransfer reaction known with metals as donors and acceptors of the methyl group: CH 3 OCoFeSP ϩ CO ϩ CoA 3 CH 3 (CO)CoA ϩ CoFeSP. [1]CoFeSP has been isolated and characterized from the acetogenic bacterium Moorella thermoacetica (4), the methanogenic archaeon Methanosarcina thermophila (5), and the hydrogenogenic bacterium Carboxydothermus hydrogenoformans (6).C. hydrogenoformans can use CO as a sole source of energy and carbon under anaerobic chemolithoautotrophic conditions. Protons serve as terminal electron acceptor (7), and the methyl branch of the acetyl-CoA pathway is used for the assimilation of carbon (6). The oxidation of CO in this bacterium is catalyzed by two monofunctional NiFe-containing CODHs (CODHI Ch and CODHII Ch ) that harbor the [Ni-4Fe-5S] active site cluster C (7). A monomeric ACS Ch , a 1:1 molar complex of ACS Ch and CODHIII Ch , and CoFeSP Ch are expressed in functional form during the growth of C. hydrogenoformans using CO as the only substrate (6). The CoFeSP Ch from C. hydrogenoformans is a heterodimeric protein composed of a large subunit (48.4 kDa, CfsA) and a small subunit (33.9 kDa, CfsB) and harbors one molecule of the cobalt-containing corrinoid cofactor and a single The CoFeSPs from acetogens and methanogens are heterodimeric proteins sharing sequence identities of 35-53% with CoFeSP Ch from C. hydrogenoformans. CoFeSP Mt from M. thermoacetica was shown to contain two cofactors, 5-methoxybenzimidazolylcobamide (a variant of B12) and a [4Fe-4S] 2ϩ/1ϩ cluster, which gave the protein its name. For the cobamide cofactor, it was shown that neither the 5-methoxybenzimidazolyl group nor a nitrogen atom from a protein side chain is coor...