Structural insights into methyltransfer reactions of a corrinoid iron–sulfur protein involved in acetyl-CoA synthesis

Abstract

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...

The Ether-Cleaving Methyltransferase System of the Strict Anaerobe Acetobacterium dehalogenans : Analysis and Expression of the Encoding Genes

“…Therefore, it is expected that anaerobes using this pathway for carbon assimilation should involve RACEs. An example of such an organism is Carboxydothermus hydrogenoformans (54), where the acs operon also was found to contain a RACE-encoding gene.…”

ATP-induced electron transfer by redox-selective partner recognition

“…The numerous interactions between the corrin ring and RACo are in contrast to the few interactions between the corrin ring and the protein matrix found in the uncomplexed structure of CoFeSP. The water molecule acting as the b-ligand of Co(II) in free CoFeSP 11,12 is replaced by a serine residue from RACo (Ser398) (Fig. 2a).…”

Redox-dependent complex formation by an ATP-dependent activator of the corrinoid/iron-sulfur protein

“…As the three oxidation-states of the cobalamin will only differ slightly in the structure of the corrin ring, a direct interaction of RACo with the corrinoid cofactor of CoFeSP appears mandatory for the specific recognition of Co(II)-CoFeSP. The crystal structure of Co (II)-CoFeSP shows that the corrin ring is sandwiched between the C-terminal domain of the large subunit and the small subunit with both axial coordination sites of Co shielded off by the protein matrix (22,32). A readout of the oxidation state appears to require a direct interaction of RACo with the corrin ring, making conformational changes of CoFeSP necessary when forming the encounter complex with RACo.…”

“…CoFeSP is an unusual corrinoid containing proteins because it occurs in the base-off/His-off coordination and has no protein ligand coordinating the cobalt ion (22,32). The absence of protein ligands is the probable reason why the redox potential of the Co 2þ ∕Co 1þ couple of CoFeSP is about 50-100 mV more positive than in other corrinoid containing methyltransferases (25,33), rendering it reducible by electrons generated from the oxidation of CO by carbon monoxide dehydrogenases (26).…”