Interaction of potassium cyanide with the [Ni-4Fe-5S] active site cluster of CO dehydrogenase from Carboxydothermus hydrogenoformans

Bioinspired Framework Catalysts: From Enzyme Immobilization to Biomimetic Catalysis

Enzymatic catalysis has fueled considerable interest from chemists due to its high efficiency and selectivity. However, the structural complexity and vulnerability hamper the application potentials of enzymes. Driven by the practical demand for chemical conversion, there is a long-sought quest for bioinspired catalysts reproducing and even surpassing the functions of natural enzymes. As nanoporous materials with high surface areas and crystallinity, metal-organic frameworks (MOFs) represent an exquisite case of how natural enzymes and their active sites are integrated into porous solids, affording bioinspired heterogeneous catalysts with superior stability and customizable structures. In this review, we comprehensively summarize the advances of bioinspired MOFs for catalysis, discuss the design principle of various MOF-based catalysts, such as MOF-enzyme composites and MOFs embedded with active sites, and explore the utility of these catalysts in different reactions. The advantages of MOFs as enzyme mimetics are also highlighted, including confinement, templating effects, and functionality, in comparison with homogeneous supramolecular catalysts. A perspective is provided to discuss potential solutions addressing current challenges in MOF catalysis.

Growth and Product Formation of Clostridium ljungdahlii in Presence of Cyanide

Cyanide is a minor constituent of crude syngas whose content depends on the feedstock and gasification procedure. It is a known poison to metal catalysts and inhibits iron-containing enzymes like carbon monoxide dehydrogenase of acetogenic organisms. Therefore, it is considered a component that has to be removed from the gas stream prior to use in chemical synthesis or syngas fermentation. We show that the growth rate and maximum biomass concentration of Clostridium ljungdahlii are unaffected by cyanide at concentrations of up to 1.0 mM with fructose as a carbon source and up to 0.1 mM with syngas as a carbon source. After the culture is adapted to cyanide it shows no growth inhibition. While the difference in growth is an increasing lag-phase with increasing cyanide concentrations, the product spectrum shifts from 97% acetic acid and 3% ethanol at 0 mM cyanide to 20% acetic acid and 80% ethanol at 1.0 mM cyanide for cultures growing on (fructose) and 80% acetic acid and 20% ethanol at 0.1 mM cyanide (syngas).

Gas fermentation: cellular engineering possibilities and scale up

Low carbon fuels and chemicals can be sourced from renewable materials such as biomass or from industrial and municipal waste streams. Gasification of these materials allows all of the carbon to become available for product generation, a clear advantage over partial biomass conversion into fermentable sugars. Gasification results into a synthesis stream (syngas) containing carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2) and nitrogen (N2). Autotrophy-the ability to fix carbon such as CO2 is present in all domains of life but photosynthesis alone is not keeping up with anthropogenic CO2 output. One strategy is to curtail the gaseous atmospheric release by developing waste and syngas conversion technologies. Historically microorganisms have contributed to major, albeit slow, atmospheric composition changes. The current status and future potential of anaerobic gas-fermenting bacteria with special focus on acetogens are the focus of this review.

When the inhibitor tells more than the substrate: the cyanide-bound state of a carbon monoxide dehydrogenase

An integral approach including experimental and theoretical analysis has been carried out with the wild-type and engineered CODHII_Ch variant to assess the parameters that control the C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N stretching frequency.

Carbon monoxide dehydrogenase (CODH) is a key enzyme for reversible CO interconversion. To elucidate structural and mechanistic details of CO binding at the CODH active site (C-cluster), cyanide is frequently used as an iso-electronic substitute and inhibitor. However, previous studies revealed conflicting results on the structure of the cyanide-bound complex and the mechanism of cyanide-inhibition. To address this issue in this work, we have employed IR spectroscopy, crystallography, site directed mutagenesis, and theoretical methods to analyse the cyanide complex of the CODH from Carboxydothermus hydrogenoformans (CODHII_Ch). IR spectroscopy demonstrates that a single cyanide binds to the Ni ion. Whereas the inhibitor could be partially removed at elevated temperature, irreversible degradation of the C-cluster occurred in the presence of an excess of cyanide on the long-minute time scale, eventually leading to the formation of [Fe(CN)₆]^4– and [Ni(CN)₄]^2– complexes. Theoretical calculations based on a new high-resolution structure of the cyanide-bound CODHII_Ch indicated that cyanide binding to the Ni ion occurs upon dissociation of the hydroxyl ligand from the Fe₁ subsite of the C-cluster. The hydroxyl group is presumably protonated by Lys563 which, unlike to His93, does not form a hydrogen bond with the cyanide ligand. A stable deprotonated ε-amino group of Lys563 in the cyanide complex is consistent with the nearly unchanged C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N stretching in the Lys563Ala variant of CODHII_Ch. These findings support the view that the proton channel connecting the solution phase with the active site displays a strict directionality, controlled by the oxidation state of the C-cluster.

Investigations by Protein Film Electrochemistry of Alternative Reactions of Nickel-Containing Carbon Monoxide Dehydrogenase

Protein film electrochemistry has been used to investigate reactions of highly active nickel-containing carbon monoxide dehydrogenases (CODHs). When attached to a pyrolytic graphite electrode, these enzymes behave as reversible electrocatalysts, displaying CO2 reduction or CO oxidation at minimal overpotential. The O2 sensitivity of CODH is suppressed by adding cyanide, a reversible inhibitor of CO oxidation, or by raising the electrode potential. Reduction of N2O, isoelectronic with CO2, is catalyzed by CODH, but the reaction is sluggish, despite a large overpotential, and results in inactivation. Production of H2 and formate under highly reducing conditions is consistent with calculations predicting that a nickel-hydrido species might be formed, but the very low rates suggest that such a species is not on the main catalytic pathway.

Bioinorganic modeling chemistry of carbon monoxide dehydrogenases: description of model complexes, current status and possible future scopes

Carbon monoxide dehydrogenases (CODHs) use CO as their sole source of carbon and energy and are found in both aerobic and anaerobic carboxidotrophic bacteria. Reversible transformation of CO to CO2 is catalyzed by a bimetallic [Mo-(μ2-S)-Cu] system in aerobic and by a highly asymmetric [Ni-Fe-S] cluster in anaerobic CODH active sites. The CODH activity in the microorganisms effects the removal of almost 10(8) tons of CO annually from the lower atmosphere and earth and thus help to maintain a sub-toxic concentration of CO. Despite an appreciable amount of work, the mechanism of CODH activity is not clearly understood yet. Moreover, biomimetic chemistry directed towards the active sites of CODHs faces several synthetic challenges. The synthetic problems associated with the modeling chemistry and strategies adopted to overcome those problems are discussed along with their limitations. A critical analysis of the exciting results delineating the present status of CODH modeling chemistry and its future prospects are presented.

Investigations of the efficient electrocatalytic interconversions of carbon dioxide and carbon monoxide by nickel-containing carbon monoxide dehydrogenases

Carbon monoxide dehydrogenases (CODH) play an important role in utilizing carbon monoxide (CO) or carbon dioxide (CO2) in the metabolism of some microorganisms. Two distinctly different types of CODH are distinguished by the elements constituting the active site. A Mo-Cu containing CODH is found in some aerobic organisms, whereas a Ni-Fe containing CODH (henceforth simply Ni-CODH) is found in some anaerobes. Two members of the simplest class (IV) of Ni-CODH behave as efficient, reversible electrocatalysts of CO2/CO interconversion when adsorbed on a graphite electrode. Their intense electroactivity sets an important benchmark for the standard of performance at which synthetic molecular and material electrocatalysts comprised of suitably attired abundant first-row transition elements must be able to operate. Investigations of CODHs by protein film electrochemistry (PFE) reveal how the enzymes respond to the variable electrode potential that can drive CO2/CO interconversion in each direction, and identify the potential thresholds at which different small molecules, both substrates and inhibitors, enter or leave the catalytic cycle. Experiments carried out on a much larger (Class III) enzyme CODH/ACS, in which CODH is complexed tightly with acetyl-CoA synthase, show that some of these characteristics are retained, albeit with much slower rates of interfacial electron transfer, attributable to the difficulty in making good electronic contact at the electrode. The PFE results complement and clarify investigations made using spectroscopic investigations.

Investigations of two bidirectional carbon monoxide dehydrogenases from Carboxydothermus hydrogenoformans by protein film electrochemistry

Carbon monoxide dehydrogenases (CODHs) catalyse the reversible conversion between CO and CO2 . Several small molecules or ions are inhibitors and probes for different oxidation states of the unusual [Ni-4 Fe-4 S] cluster that forms the active site. The actions of these small probes on two enzymes-CODH ICh and CODH IICh -produced by Carboxydothermus hydrogenoformans have been studied by protein film voltammetry to compare their behaviour and to establish general characteristics. Whereas CODH ICh is, so far, the better studied of the two isozymes in terms of its electrocatalytic properties, it is CODH IICh that has been characterised by X-ray crystallography. The two isozymes, which share 58.3% sequence identity and 73.9% sequence similarity, show similar patterns of behaviour with regard to selective inhibition of CO2 reduction by CO (product) and cyanate, potent and selective inhibition of CO oxidation by cyanide, and the action of sulfide, which promotes oxidative inactivation of the enzyme. For both isozymes, rates of binding of substrate analogues CN(-) (for CO) and NCO(-) (for CO2 ) are orders of magnitude lower than turnover, a feature that is clearly revealed through hysteresis of cyclic voltammetry. Inhibition by CN(-) and CO is much stronger for CODH IICh than for CODH ICh, a property that has relevance for applying these enzymes as model catalysts in solar-driven CO2 reduction.

In vivo activation of methyl-coenzyme M reductase by carbon monoxide

Methyl-coenzyme M reductase (MCR) from methanogenic archaea catalyzes the rate-limiting and final step in methane biosynthesis. Using coenzyme B as the two-electron donor, MCR reduces methyl-coenzyme M (CH₃-SCoM) to methane and the mixed disulfide, CoBS-SCoM. MCR contains an essential redox-active nickel tetrahydrocorphinoid cofactor, Coenzyme F₄₃₀, at its active site. The active form of the enzyme (MCR_red1) contains Ni(I)-F₄₃₀. Rapid and efficient conversion of MCR to MCR_red1 is important for elucidating the enzymatic mechanism, yet this reduction is difficult because the Ni(I) state is subject to oxidative inactivation. Furthermore, no in vitro methods have yet been described to convert Ni(II) forms into MCR_red1. Since 1991, it has been known that MCR_red1 from Methanothermobacter marburgensis can be generated in vivo when cells are purged with 100% H₂. Here we show that purging cells or cell extracts with CO can also activate MCR. The rate of in vivo activation by CO is about 15 times faster than by H₂ (130 and 8 min^-1, respectively) and CO leads to twofold higher MCR_red1 than H₂. Unlike H₂-dependent activation, which exhibits a 10-h lag time, there is no lag for CO-dependent activation. Based on cyanide inhibition experiments, carbon monoxide dehydrogenase is required for the CO-dependent activation. Formate, which also is a strong reductant, cannot activate MCR in M. marburgensis in vivo.

A unified electrocatalytic description of the action of inhibitors of nickel carbon monoxide dehydrogenase

Several small molecules and ions, notably carbon monoxide, cyanide, cyanate, and hydrogen sulfide, are potent inhibitors of Ni-containing carbon monoxide dehydrogenases (Ni-CODH) that catalyze very rapid, efficient redox interconversions of CO(2) and CO. Protein film electrochemistry, which probes the dependence of steady-state catalytic rate over a wide potential range, reveals how these inhibitors target particular oxidation levels of Ni-CODH relating to intermediates (C(ox), C(red1), and C(red2)) that have been established for the active site. The following properties are thus established: (1) CO suppresses CO(2) reduction (CO is a product inhibitor), but its binding affinity decreases as the potential becomes more negative. (2) Cyanide totally inhibits CO oxidation, but its effect on CO(2) reduction is limited to a narrow potential region (between -0.5 and -0.6 V), below which CO(2) reduction activity is restored. (3) Cyanate is a strong inhibitor of CO(2) reduction but inhibits CO oxidation only within a narrow potential range just above the CO(2)/CO thermodynamic potential--EPR spectra confirm that cyanate binds selectively to C(red2). (4) Hydrogen sulfide (H(2)S/HS(-)) inhibits CO oxidation but not CO(2) reduction--the complex on/off characteristics are consistent with it binding at the same oxidation level as C(ox) and forming a modified version of this inactive state rather than reacting directly with C(red1). The results provide a new perspective on the properties of different catalytic intermediates of Ni-CODH--uniting and clarifying many previous investigations.

Metal centers in the anaerobic microbial metabolism of CO and CO2

Carbon dioxide and carbon monoxide are important components of the carbon cycle. Major research efforts are underway to develop better technologies to utilize the abundant greenhouse gas, CO(2), for harnessing 'green' energy and producing biofuels. One strategy is to convert CO(2) into CO, which has been valued for many years as a synthetic feedstock for major industrial processes. Living organisms are masters of CO(2) and CO chemistry and, here, we review the elegant ways that metalloenzymes catalyze reactions involving these simple compounds. After describing the chemical and physical properties of CO and CO(2), we shift focus to the enzymes and the metal clusters in their active sites that catalyze transformations of these two molecules. We cover how the metal centers on CO dehydrogenase catalyze the interconversion of CO and CO(2) and how pyruvate oxidoreductase, which contains thiamin pyrophosphate and multiple Fe(4)S(4) clusters, catalyzes the addition and elimination of CO(2) during intermediary metabolism. We also describe how the nickel center at the active site of acetyl-CoA synthase utilizes CO to generate the central metabolite, acetyl-CoA, as part of the Wood-Ljungdahl pathway, and how CO is channelled from the CO dehydrogenase to the acetyl-CoA synthase active site. We cover how the corrinoid iron-sulfur protein interacts with acetyl-CoA synthase. This protein uses vitamin B(12) and a Fe(4)S(4) cluster to catalyze a key methyltransferase reaction involving an organometallic methyl-Co(3+) intermediate. Studies of CO and CO(2) enzymology are of practical significance, and offer fundamental insights into important biochemical reactions involving metallocenters that act as nucleophiles to form organometallic intermediates and catalyze C-C and C-S bond formations.

A role for nickel-iron cofactors in biological carbon monoxide and carbon dioxide utilization

Ni-Fe containing enzymes are involved in the biological utilization of carbon monoxide, carbon dioxide, and hydrogen. Interest in these enzymes has increased in recent years due to hydrogen fuel initiatives and concerns over development of new methods for CO2 sequestration. One Ni-Fe enzyme called carbon monoxide dehydrogenase (CODH) is a key player in the global carbon cycle and carries out the interconversion of the environmental pollutant CO and the greenhouse gas CO2. The Ni-Fe center responsible for this important chemistry, the C-cluster, has been the source of much controversy, but several recent structural studies have helped to direct the field toward a unifying mechanism. Here we summarize the current state of understanding of this fascinating metallocluster.

Towards a black-box for biological EXAFS data analysis. II. Automatic BioXAS Refinement and Analysis (ABRA)

In biological systems, X-ray absorption spectroscopy (XAS) can determine structural details of metal binding sites with high resolution. Here a method enabling an automated analysis of the corresponding EXAFS data is presented, utilizing in addition to least-squares refinement the prior knowledge about structural details and important fit parameters. A metal binding motif is characterized by the type of donor atoms and their bond lengths. These fit results are compared by bond valance sum analysis and target distances with established structures of metal binding sites. Other parameters such as the Debye-Waller factor and shift of the Fermi energy provide further insights into the quality of a fit. The introduction of mathematical criteria, their combination and calibration allows an automated analysis of XAS data as demonstrated for a number of examples. This presents a starting point for future applications to all kinds of systems studied by XAS and allows the algorithm to be transferred to data analysis in other fields.

Complexation and toxicity of copper in higher plants. II. Different mechanisms for copper versus cadmium detoxification in the copper-sensitive cadmium/zinc hyperaccumulator Thlaspi caerulescens (Ganges Ecotype)

The cadmium/zinc hyperaccumulator Thlaspi caerulescens is sensitive toward copper (Cu) toxicity, which is a problem for phytoremediation of soils with mixed contamination. Cu levels in T. caerulescens grown with 10 microm Cu(2+) remained in the nonaccumulator range (<50 ppm), and most individuals were as sensitive toward Cu as the related nonaccumulator Thlaspi fendleri. Obviously, hyperaccumulation and metal resistance are highly metal specific. Cu-induced inhibition of photosynthesis followed the "sun reaction" type of damage, with inhibition of the photosystem II reaction center charge separation and the water-splitting complex. A few individuals of T. caerulescens were more Cu resistant. Compared with Cu-sensitive individuals, they recovered faster from inhibition, at least partially by enhanced repair of chlorophyll-protein complexes but not by exclusion, since the content of Cu in their shoots was increased by about 25%. Extended x-ray absorption fine structure (EXAFS) measurements on frozen-hydrated leaf samples revealed that a large proportion of Cu in T. caerulescens is bound by sulfur ligands. This is in contrast to the known binding environment of cadmium and zinc in the same species, which is dominated by oxygen ligands. Clearly, hyperaccumulators detoxify hyperaccumulated metals differently compared with nonaccumulated metals. Furthermore, strong features in the Cu-EXAFS spectra ascribed to metal-metal contributions were found, in particular in the Cu-resistant specimens. Some of these features may be due to Cu binding to metallothioneins, but a larger proportion seems to result from biomineralization, most likely Cu(II) oxalate and Cu(II) oxides. Additional contributions in the EXAFS spectra indicate complexation of Cu(II) by the nonproteogenic amino acid nicotianamine, which has a very high affinity for Cu(II) as further characterized here.

Crystallographic snapshots of cyanide- and water-bound C-clusters from bifunctional carbon monoxide dehydrogenase/acetyl-CoA synthase

Nickel-containing carbon monoxide dehydrogenases (CODHs) reversibly catalyze the oxidation of carbon monoxide to carbon dioxide and are of vital importance in the global carbon cycle. The unusual catalytic CODH C-cluster has been crystallographically characterized as either a NiFe₄S₄ or a NiFe₄S₅ metal center, the latter containing a fifth, additional sulfide that bridges Ni and a unique Fe site. To determine whether this bridging sulfide is catalytically relevant and to further explore the mechanism of the C-cluster, we obtained crystal structures of the 310 kDa bifunctional CODH/acetyl-CoA synthase complex from Moorella thermoacetica bound both with a substrate H₂O/OH⁻ molecule and with a cyanide inhibitor. X-ray diffraction data were collected from native crystals and from identical crystals soaked in a solution containing potassium cyanide. In both structures, the substrate H₂O/OH⁻ molecule exhibits binding to the unique Fe site of the C-cluster. We also observe cyanide binding in a bent conformation to Ni of the C-cluster, adjacent the substrate H₂O/OH⁻ molecule. Importantly, the bridging sulfide is not present in either structure. As these forms of the C-cluster represent the coordination environment immediately before the reaction takes place, our findings do not support a fifth, bridging sulfide playing a catalytic role in the enzyme mechanism. The crystal structures presented here, along with recent structures of CODHs from other organisms, have led us toward a unified mechanism for CO oxidation by the C-cluster, the catalytic center of an environmentally important enzyme.

Nickel-based Enzyme Systems

Of the eight known nickel enzymes, all but glyoxylase I catalyze the use and/or production of gases central to the global carbon, nitrogen, and oxygen cycles. Nickel appears to have been selected for its plasticity in coordination and redox chemistry and is able to cycle through three redox states (1+, 2+, 3+) and to catalyze reactions spanning ∼1.5 V. This minireview focuses on the catalytic mechanisms of nickel enzymes, with an emphasis on the role(s) of the metal center. The metal centers vary from mononuclear to complex metal clusters and catalyze simple hydrolytic to multistep redox reactions.

Distorted octahedral coordination of tungstate in a subfamily of specific binding proteins

Bacteria and archaea import molybdenum and tungsten from the environment in the form of the oxyanions molybdate (MoO(4) (2-)) and tungstate (WO(4) (2-)). These substrates are captured by an external, high-affinity binding protein, and delivered to ATP binding cassette transporters, which move them across the cell membrane. We have recently reported a crystal structure of the molybdate/tungstate binding protein ModA/WtpA from Archaeoglobus fulgidus, which revealed an octahedrally coordinated central metal atom. By contrast, the previously determined structures of three bacterial homologs showed tetracoordinate molybdenum and tungsten atoms in their binding pockets. Until then, coordination numbers above four had only been found for molybdenum/tungsten in metalloenzymes where these metal atoms are part of the catalytic cofactors and coordinated by mostly non-oxygen ligands. We now report a high-resolution structure of A. fulgidus ModA/WtpA, as well as crystal structures of four additional homologs, all bound to tungstate. These crystal structures match X-ray absorption spectroscopy measurements from soluble, tungstate-bound protein, and reveal the details of the distorted octahedral coordination. Our results demonstrate that the distorted octahedral geometry is not an exclusive feature of the A. fulgidus protein, and suggest distinct binding modes of the binding proteins from archaea and bacteria.

Cysteine residues in the C-terminal tail of the human BK(Ca)alpha subunit are important for channel sensitivity to carbon monoxide

In the presence of oxygen (O(2)), carbon monoxide (CO) is synthesised from heme by endogenous hemeoxygenases, and is a powerful activator of BK(Ca) channels. This transduction pathway has been proposed to contribute to cellular O(2) sensing in rat carotid body. In the present study we have explored the role that four cysteine residues (C820, C911, C995 and C1028), located in the vicinity of the "calcium bowl" of C-terminal of human BK(Ca)-alphasubunit, have on channel CO sensitivity. Mutant BK(Ca)-alphasubunits were generated by site-directed mutagenesis (single, double and triple cysteine residue substitutions with glycine residues) and were transiently transfected into HEK 293 cells before subsequent analysis in inside-out membrane patches. Potassium cyanide (KCN) completely abolished activation of wild type BK(Ca) channels by the CO donor, tricarbonyldichlororuthenium (II) dimer, at 100microM. In the absence of KCN the CO donor increased wild-type channel activity in a concentration-dependent manner, with an EC(50) of ca. 50microM. Single cysteine point mutations of residues C820, C995 and C1028 affected neither channel characteristics nor CO EC(50) values. In contrast, the CO sensitivity of the C911G mutation was significantly decreased (EC(50) ca. 100 M). Furthermore, all double and triple mutants which contained the C911G substitution exhibited reduced CO sensitivity, whilst those which did not contain this mutation displayed essentially unaltered CO EC(50) values. These data highlight that a single cysteine residue is crucial to the activation of BK(Ca) by CO. We suggest that CO may bind to this channel subunit in a manner similar to the transition metal-dependent co-ordination which is characteristic of several enzymes, such as CO dehydrogenase.

Acetogenesis and the Wood-Ljungdahl pathway of CO(2) fixation

Conceptually, the simplest way to synthesize an organic molecule is to construct it one carbon at a time. The Wood-Ljungdahl pathway of CO(2) fixation involves this type of stepwise process. The biochemical events that underlie the condensation of two one-carbon units to form the two-carbon compound, acetate, have intrigued chemists, biochemists, and microbiologists for many decades. We begin this review with a description of the biology of acetogenesis. Then, we provide a short history of the important discoveries that have led to the identification of the key components and steps of this usual mechanism of CO and CO(2) fixation. In this historical perspective, we have included reflections that hopefully will sketch the landscape of the controversies, hypotheses, and opinions that led to the key experiments and discoveries. We then describe the properties of the genes and enzymes involved in the pathway and conclude with a section describing some major questions that remain unanswered.

13C NMR characterization of an exchange reaction between CO and CO2 catalyzed by carbon monoxide dehydrogenase

Carbon monoxide dehydrogenase (CODH) catalyzes the reversible oxidation of CO to CO2 at a nickel-iron-sulfur cluster (the C-cluster). CO oxidation follows a ping-pong mechanism involving two-electron reduction of the C-cluster followed by electron transfer through an internal electron transfer chain to external electron acceptors. We describe 13C NMR studies demonstrating a CODH-catalyzed steady-state exchange reaction between CO and CO2 in the absence of external electron acceptors. This reaction is characterized by a CODH-dependent broadening of the 13CO NMR resonance; however, the chemical shift of the 13CO resonance is unchanged, indicating that the broadening is in the slow exchange limit of the NMR experiment. The 13CO line broadening occurs with a rate constant (1080 s-1 at 20 degrees C) that is approximately equal to that of CO oxidation. It is concluded that the observed exchange reaction is between 13CO and CODH-bound 13CO2 because 13CO line broadening is pH-independent (unlike steady-state CO oxidation), because it requires a functional C-cluster (but not a functional B-cluster) and because the 13CO2 line width does not broaden. Furthermore, a steady-state isotopic exchange reaction between 12CO and 13CO2 in solution was shown to occur at the same rate as that of CO2 reduction, which is approximately 750-fold slower than the rate of 13CO exchange broadening. The interaction between CODH and the inhibitor cyanide (CN-) was also probed by 13C NMR. A functional C-cluster is not required for 13CN- broadening (unlike for 13CO), and its exchange rate constant is 30-fold faster than that for 13CO. The combined results indicate that the 13CO exchange includes migration of CO to the C-cluster, and CO oxidation to CO2, but not release of CO2 or protons into the solvent. They also provide strong evidence of a CO2 binding site and of an internal proton transfer network in CODH. 13CN- exchange appears to monitor only movement of CN- between solution and its binding to and release from CODH.

Carbon dioxide activation at the Ni,Fe-cluster of anaerobic carbon monoxide dehydrogenase

Anaerobic CO dehydrogenases catalyze the reversible oxidation of CO to CO2 at a complex Ni-, Fe-, and S-containing metal center called cluster C. We report crystal structures of CO dehydrogenase II from Carboxydothermus hydrogenoformans in three different states. In a reduced state, exogenous CO2 supplied in solution is bound and reductively activated by cluster C. In the intermediate structure, CO2 acts as a bridging ligand between Ni and the asymmetrically coordinated Fe, where it completes the square-planar coordination of the Ni ion. It replaces a water/hydroxo ligand bound to the Fe ion in the other two states. The structures define the mechanism of CO oxidation and CO2 reduction at the Ni-Fe site of cluster C.

Nickel and the carbon cycle

This article, dedicated to Edward Stiefel, reviews three nickel enzymes that play important roles in the carbon cycle: CO dehydrogenase, acetyl-CoA synthase, and methyl-coenzyme M reductase. After a short discussion of the carbon cycle, the structures of the active centers of the proteins and their proposed mechanisms are discussed. A brief description of future research areas is presented for each enzyme system. A short perspective on future research on nickel enzymes ends this contribution.