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Corrinoids are cobalt‐containing tetrapyrroles. They include adenosylcobalamin (vitamin B12) and cobamides that function as cofactors and coenzymes for methyl transfer, radical‐dependent and redox reactions. Though cobamides are the most complex cofactors in nature, they are essential in the acetyl‐CoA pathway, thought to be the most ancient CO2‐fixation pathway, where they perform a pterin‐to‐cobalt‐to‐nickel methyl transfer reaction catalyzed by the corrinoid iron–sulphur protein (CoFeS). CoFeS occurs in H2‐dependent archaeal methanogens, the oldest microbial lineage by measure of physiology and carbon isotope data, dating corrinoids to ca. 3.5 billion years. However, CoFeS and cobamides are also essential in the acetyl‐CoA pathway of H2‐dependent bacterial acetogens. To determine whether corrin biosynthesis was established before archaea and bacteria diverged, whether the pathways arose independently or whether cobamide biosynthesis was transferred from the archaeal to the bacterial lineage (or vice versa) during evolution, we investigated phylogenies and structural data for 26 enzymes of corrin ring and lower ligand biosynthesis. The data trace cobamide synthesis to the common ancestor of bacteria and archaea, placing it in the last universal common ancestor of all lifeforms (LUCA), while pterin‐dependent methyl synthesis pathways likely arose independently post‐LUCA in the lineages leading to bacteria and archaea. Enzymes of corrin biosynthesis were recruited from preexisting ancient pathways. Evolutionary forerunners of CoFeS function were likely Fe‐, Ni‐ and Co‐containing solid‐state surfaces, which, in the laboratory, catalyze the reactions of the acetyl‐CoA pathway from CO2 to pyruvate under serpentinizing hydrothermal conditions. The data suggest that enzymatic corrin biosynthesis replaced insoluble solid‐state catalysts that tethered primordial CO2 assimilation to the Earth's crust, suggesting a role for corrin synthesis in the origin of free‐living cells. Show less

Life is an exergonic chemical reaction. The same was true when the very first cells emerged at life's origin. In order to live, all cells need a source of carbon, energy, and electrons to drive their overall reaction network (metabolism). In most cells, these are separate pathways. There is only one biochemical pathway that serves all three needs simultaneously: the acetyl-CoA pathway of CO₂ fixation. In the acetyl-CoA pathway, electrons from H₂ reduce CO₂ to pyruvate for carbon supply, while methane or acetate synthesis are coupled to energy conservation as ATP. This simplicity and thermodynamic favorability prompted Georg Fuchs and Erhard Stupperich to propose in 1985 that the acetyl-CoA pathway might mark the origin of metabolism, at the same time that Steve Ragsdale and Harland Wood were uncovering catalytic roles for Fe, Co, and Ni in the enzymes of the pathway. Subsequent work has provided strong support for those proposals.In the presence of Fe, Co, and Ni in their native metallic state as catalysts, aqueous H₂ and CO₂ react specifically to formate, acetate, methane, and pyruvate overnight at 100 °C. These metals (and their alloys) thus replace the function of over 120 enzymes required for the conversion of H₂ and CO₂ to pyruvate via the pathway and its cofactors, an unprecedented set of findings in the study of biochemical evolution. The reactions require alkaline conditions, which promote hydrogen oxidation by proton removal and are naturally generated in serpentinizing (H₂-producing) hydrothermal vents. Serpentinizing hydrothermal vents furthermore produce natural deposits of native Fe, Co, Ni, and their alloys. These are precisely the metals that reduce CO₂ with H₂ in the laboratory; they are also the metals found at the active sites of enzymes in the acetyl-CoA pathway. Iron, cobalt and nickel are relicts of the environments in which metabolism arose, environments that still harbor ancient methane- and acetate-producing autotrophs today. This convergence indicates bedrock-level antiquity for the acetyl-CoA pathway. In acetogens and methanogens growing on H₂ as reductant, the acetyl-CoA pathway requires flavin-based electron bifurcation as a source of reduced ferredoxin (a 4Fe4S cluster-containing protein) in order to function. Recent findings show that H₂ can reduce the 4Fe4S clusters of ferredoxin in the presence of native iron, uncovering an evolutionary precursor of flavin-based electron bifurcation and suggesting an origin of FeS-dependent electron transfer in proteins. Traditionally discussed as catalysts in early evolution, the most common function of FeS clusters in metabolism is one-electron transfer, also in radical SAM enzymes, a large and ancient enzyme family. The cofactors and active sites in enzymes of the acetyl-CoA pathway uncover chemical antiquity in metabolism involving metals, methyl groups, methyl transfer reactions, cobamides, pterins, GTP, S-adenosylmethionine, radical SAM enzymes, and carbon-metal bonds. The reaction sequence from H₂ and CO₂ to pyruvate on naturally deposited native metals is maximally simple. It requires neither nitrogen, sulfur, phosphorus, RNA, ion gradients, nor light. Solid-state metal catalysts tether the origin of metabolism to a H₂-producing, serpentinizing hydrothermal vent. Show less

Bioisosteres are a useful approach to address pharmacokinetic liabilities and improve drug-like properties. Specific to developing metalloenzyme inhibitors, metal-binding pharmacophores (MBPs) have been combined with bioisosteres, to produce metal-binding isosteres (MBIs) as alternative scaffolds for use in fragment-based drug discovery (FBDD). Picolinic acid MBIs have been reported and evaluated for their metal-binding ability, pharmacokinetic properties, and enzyme inhibitory activity. However, their structural, electronic, and spectroscopic properties with metal ions other than Zn(II) have not been reported, which might reveal similarities and differences between MBIs and the parent MBPs. To this end, [M(TPA)(MBI)]⁺ (M = Ni(II) and Co(II), TPA = tris(2-pyridylmethyl)amine) is presented as a bioinorganic model system for investigating picolinic acid, four heterocyclic MBIs, and 2,2'-bipyridine. These complexes were characterized by X-ray crystallography as well as NMR, IR, and UV-vis spectroscopies, and their magnetic moments were accessed. In addition, [(Tp^Ph,Me)Co(MBI)] (Tp^Ph,Me = hydrotris(3,5-phenylmethylpyrazolyl)borate) was used as a second model compound, and the limitations and attributes of the two model systems are discussed. These results demonstrate that bioinorganic model complexes are versatile tools for metalloenzyme inhibitor design and can provide insights into the broader use of MBIs. Show less

The association of proteins with metals, metalation, is challenging because the tightest binding metals are rarely the correct ones. Inside cells, correct metalation is enabled by controlled bioavailability plus extra mechanisms for tricky combinations such as iron and manganese. In this issue [1], Grāve, Högbom and colleagues address a tremendously important challenge: How do proteins acquire the correct metals? This is important because almost a half of enzymes are estimated to require metals [2, 3]. This is a Show less

Met80, one of the heme iron ligands in cytochrome c (cyt c), is readily oxidized to Met sulfoxide (Met-SO) by several biologically relevant oxidants. The modification has been suggested to affect both the electron-transfer (ET) and apoptotic functions of this metalloprotein. The coordination of the heme iron in Met-oxidized cyt c (Met-SO cyt c) is critical for both of these functions but has remained poorly defined. We present electronic absorption, NMR, and EPR spectroscopic investigations as well as kinetic studies and mutational analyses to identify the heme iron ligands in yeast iso-1 Met-SO cyt c. Similar to the alkaline form of native cyt c, Lys73 and Lys79 ligate to the ferric heme iron in the Met80-oxidized protein, but this coordination takes place at much lower pH. The ferrous heme iron is ligated by Met-SO, implying the redox-linked ligand switch in the modified protein. Binding studies with the model peptide microperoxidase-8 provide a rationale for alterations in ligation and for the role of the polypeptide packing in native and Met-SO cyt c. Imidazole binding experiments have revealed that Lys dissociation from the ferric heme in K73A/K79G/M80K (M80K^#) and Met-SO is more than 3 orders of magnitude slower than the opening of the heme pocket that limits Met80 replacement in native cyt c. The Lys-to-Met-SO ligand substitution gates ET of ferric Met-SO cyt c with Co(terpy)₂²⁺. Owing to the slow Lys dissociation step, ET reaction is slow but possible, which is not the case for nonswitchable M80A and M80K^#. Acidic conditions cause Lys replacement by a water ligand in Met-SO cyt c (p K_a = 6.3 ± 0.1), increasing the intrinsic peroxidase activity of the protein. This pH-driven ligand switch may be a mechanism to boost peroxidase function of cyt c specifically in apoptotic cells. Show less