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

doi:10.1073/pnas.0601420103

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

*Laboratorium Proteinkristallographie,
^†Lehrstuhl für Mikrobiologie, and
^‡Bayreuther Zentrum für Molekulare Biowissenschaften, Universität Bayreuth, 95440 Bayreuth, Germany

Edited by Martha L. Ludwig, University of Michigan, Ann Arbor, MI, and approved August 4, 2006 (received for review February 20, 2006)

Abstract

The cobalt- and iron-containing corrinoid iron–sulfur protein (CoFeSP) is functional in the acetyl-CoA (Ljungdahl–Wood) pathway of autotrophic carbon fixation in various bacteria and archaea, where it is essential for the biosynthesis of acetyl-CoA. CoFeSP acts in two methylation reactions: the transfer of a methyl group from methyltransferase (MeTr)-bound methyltetrahydrofolate to the cob(I)amide of CoFeSP and the transfer of the methyl group of methyl-cob(III)amide to the reduced Ni-Ni-[4Fe-4S] active site cluster A of acetyl-CoA synthase (ACS). We have solved the crystal structure of as-isolated CoFeSP_Ch from the CO-oxidizing hydrogenogenic bacterium Carboxydothermus hydrogenoformans at 1.9-Å resolution. The heterodimeric protein consists of two tightly interacting subunits with pseudo-twofold symmetry. The large CfsA subunit comprises three domains, of which the N-terminal domain binds the [4Fe-4S] cluster, the middle domain is a (βα)₈-barrel, and the C-terminal domain shows an open fold and binds Coβ-aqua-(5,6-dimethylbenzimidazolylcobamide) in a “base-off” state without a protein ligand at the cobalt ion. The small CfsB subunit also displays a (βα)₈-barrel fold and interacts with the upper side of the corrin macrocycle. Structure-based alignments show that both (βα)₈-barrel domains are related to the MeTr in the acetyl-CoA pathway and to the folate domain of methionine synthase. We suggest that the C-terminal domain of the large subunit is the mobile element that allows the necessary interaction of CoFeSP_Ch with the active site of ACS_Ch and the methyltetrahydrofolate carrying MeTr. The conformation in the crystal structure shields the two open coordinations of cobalt and likely represents a resting state.

Footnotes

^§To whom correspondence should be addressed. E-mail: holger.dobbek{at}uni-bayreuth.de
Author contributions: T.S. and V.S. contributed equally to this work; V.S., O.M., and H.D. designed research; T.S., V.S., and H.D. performed research; H.D. analyzed data; and H.D. wrote the paper.
The authors declare no conflict of interest.
This paper was submitted directly (Track II) to the PNAS office.
Data deposition: The atomic coordinates and structure factors have been deposited in the Protein Data Bank, www.pdb.org (PDB ID code 2H9A).
Abbreviations:

ACS,

acetyl-CoA synthase;

MeTr,

methyltransferase.