Evidence that NiNi Acetyl-CoA Synthase Is Active and that the CuNi Enzyme Is Not

The bifunctional CO dehydrogenase/acetyl-CoA synthase (CODH/ACS) plays a central role in the Wood-Ljungdahl pathway of autotrophic CO₂ fixation. One structure of the Moorella thermoacetica enzyme revealed that the active site of ACS (the A-cluster) consists of a [4Fe-4S] cluster bridged to a binuclear CuNi center with Cu at the proximal metal site (M_p) and Ni at the distal metal site (M_d). In another structure of the same enzyme, Ni or Zn was present at M_p. On the basis of a positive correlation between ACS activity and Cu content, we had proposed that the Cu-containing enzyme is active [Seravalli, J., et al. (2003) Proc. Natl. Acad. Sci. U.S.A. 100, 3689-3694]. Here we have reexamined this proposal. Enzyme preparations with a wider range of Ni (1.6-2.8) and Cu (0.2-1.1) stoichiometries per dimer were studied to reexamine the correlation, if any, between the Ni and Cu content and ACS activity. In addition, the effects of o-phenanthroline (which removes Ni but not Cu) and neocuproine (which removes Cu but not Ni) on ACS activity were determined. EXAFS results indicate that these chelators selectively remove Mp. Multifrequency EPR spectra (3-130 GHz) of the paramagnetic NiFeC state of the A-cluster were examined to investigate the electronic state of this proposed intermediate in the ACS reaction mechanism. The combined results strongly indicate that the CuNi enzyme is inactive, that the NiNi enzyme is active, and that the NiNi enzyme is responsible for the NiFeC EPR signal. The results also support an electronic structure of the NiFeC-eliciting species as a [4Fe-4S]²⁺ (net S = 0) cluster bridged to a Ni¹⁺ (S = 1/2) at M_p that is bridged to planar four-coordinate Ni²⁺ (S = 0) at M_d, with the spin predominantly on the Ni¹⁺. Furthermore, these studies suggest that M_p is inserted during cell growth. The apparent vulnerability of the proximal metal site in the A-cluster to substitution with different metals appears to underlie the heterogeneity observed in samples that has confounded studies of CODH/ACS for many years. On the basis of this principle, a protocol to generate nearly homogeneous preparations of the active NiNi form of ACS was achieved with NiFeC signals of ∼0.8 spin/mol.