Metal–metal bonds in biology

Abstract

Nickel-containing carbon monoxide dehydrogenases, acetyl-CoA synthases, nickel-iron hydrogenases, and diron hydrogenases are distinct metalloenzymes yet they share a number of important characteristics. All are O2-sensitive, with active-sites composed of iron and/or nickel ions coordinated primarily by sulfur ligands. In each case, two metals are juxtaposed at the “heart” of the active site, within range of forming metal-metal bonds. These active-site clusters exhibit multielectron redox abilities and must be reductively activated for catalysis. Reduction potentials are milder than expected based on formal oxidation state changes. When reductively activated, each cluster attacks an electrophilic substrate via an oxidative addition reaction. This affords a two-electron-reduced substrate bound to one or both metals of an oxidized cluster. M-M bonds have been established in hydrogenases where they serve to initiate the oxidative addition of protons and perhaps stabilize active sites in multiple redox states. The same may be true of the CODH and ACS active sites – Ni-Fe and Ni-Ni bonds in these sites may play critical roles in catalysis, stabilizing low-valence states and initiating oxidative addition of CO2 and methyl group cations, respectively. In this article, the structural and functional commonalities of these metalloenzyme active sites are described, and the case is made for the formation and use of metal-metal bonds in each enzyme mentioned. As a post-script, the importance of Fe-Fe bonds in the nitrogenase FeMoco active site is discussed.

NADP-Specific Electron-Bifurcating [FeFe]-Hydrogenase in a Functional Complex with Formate Dehydrogenase in Clostridium autoethanogenum Grown on CO

“…We found the novel enzyme complex in Clostridium autoethanogenum (28) growing on CO, which is fermented via the WoodLjungdahl pathway (29)(30)(31). Via the same pathway the organism can also grow on H 2 and CO 2 .…”

Extended X-ray absorption fine structure and multiple-scattering simulation of nickel dithiolene complexes Ni[S2C2(CF3)2]2n(n= −2, −1, 0) and an olefin adduct Ni[S2C2(CF3)2]2(1-hexene)

“…For example, Ni-Fe interactions are proposed for Ni-L NiFe hydrogenase (Kampa et al, 2013) and for carbon monoxide dehydrogenase (CODH) (Jeoung & Dobbek, 2009), Ni-Ni and Fe-Fe bonding are proposed for acetyl-CoA synthetase (Lindahl, 2012) and [FeFe]-H 2 ase (Nicolet et al, 2001). While these metal-metal interactions are typically in the range of $ 3 Å , metal-C interactions and MS parameters are also at $ 3 Å , complicating EXAFS studies on these important metal-metal interactions [by the existence of metal-C/metal-N interactions as well as MS scattering effects Table 2 Single-scattering fit parameters for Ni dithiolene complexes 1, 2, 3 and 4.…”

Electron Bifurcation Involved in the Energy Metabolism of the Acetogenic Bacterium Moorella thermoacetica Growing on Glucose or H 2 plus CO 2

“…Most of the enzymes involved in CO 2 reduction to acetic acid (Wood-Ljungdahl pathway) (reaction 7) have been well studied (38,39,58). What we know of the mechanism of the total synthesis The two cytoplasmic enzymes identified in this work as being electron bifurcating are highlighted by a blue disc and a green disc, respectively.…”