Visualizing the gas channel of a monofunctional carbon monoxide dehydrogenase

Coupling CO2 Reduction and Acetyl-CoA Formation: The Role of a CO Capturing Tunnel in Enzymatic Catalysis

The bifunctional CO-dehydrogenase/acetyl-CoA synthase (CODH/ACS) complex couples the reduction of CO2 to the condensation of CO with a methyl moiety and CoA to acetyl-CoA. Catalysis occurs at two sites connected by a tunnel transporting the CO. In this study, we investigated how the bifunctional complex and its tunnel support catalysis using the CODH/ACS from Carboxydothermus hydrogenoformans as a model. Although CODH/ACS adapted to form a stable bifunctional complex with a secluded substrate tunnel, catalysis and CO transport is even more efficient when two monofunctional enzymes are coupled. Efficient CO channeling appears to be ensured by hydrophobic binding sites for CO, which act in a bucket-brigade fashion rather than as a simple tube. Tunnel remodeling showed that opening the tunnel increased activity but impaired directed transport of CO. Constricting the tunnel impaired activity and CO transport, suggesting that the tunnel evolved to sequester CO rather than to maximize turnover.

Tunnel engineering of gas-converting enzymes for inhibitor retardation and substrate acceleration

Carbon monoxide dehydrogenase (CODH), formate dehydrogenase (FDH), hydrogenase (H2ase), and nitrogenase (N2ase) are crucial enzymatic catalysts that facilitate the conversion of industrially significant gases such as CO, CO2, H2, and N2. The tunnels in the gas-converting enzymes serve as conduits for these low molecular weight gases to access deeply buried catalytic sites. The identification of the substrate tunnels is imperative for comprehending the substrate selectivity mechanism underlying these gas-converting enzymes. This knowledge also holds substantial value for industrial applications, particularly in addressing the challenges associated with separation and utilization of byproduct gases. In this comprehensive review, we delve into the emerging field of tunnel engineering, presenting a range of approaches and analyses. Additionally, we propose methodologies for the systematic design of enzymes, with the ultimate goal of advancing protein engineering strategies.

Current status of carbon monoxide dehydrogenases (CODH) and their potential for electrochemical applications

Anthropogenic carbon dioxide (CO2) levels are rising to alarming concentrations in earth's atmosphere, causing adverse effects and global climate changes. In the last century, innovative research on CO2 reduction using chemical, photochemical, electrochemical and enzymatic approaches has been addressed. In particular, natural CO2 conversion serves as a model for many processes and extensive studies on microbes and enzymes regarding redox reactions involving CO2 have already been conducted. In this review we focus on the enzymatic conversion of CO2 to carbon monoxide (CO) as the chemical conversion downstream of CO production render CO particularly attractive as a key intermediate. We briefly discuss the different currently known natural autotrophic CO2 fixation pathways, focusing on the reversible reaction of CO2, two electrons and protons to CO and water, catalyzed by carbon monoxide dehydrogenases (CODHs). We then move on to classify the different type of CODHs, involved catalyzed chemical reactions and coupled metabolisms. Finally, we discuss applications of CODH enzymes in photochemical and electrochemical cells to harness CO2 from the environment transforming it into commodity chemicals.

Class III hybrid cluster protein homodimeric architecture shows evolutionary relationship with Ni, Fe-carbon monoxide dehydrogenases

Hybrid cluster proteins (HCPs) are Fe-S-O cluster-containing metalloenzymes in three distinct classes (class I and II: monomer, III: homodimer), all of which structurally related to homodimeric Ni, Fe-carbon monoxide dehydrogenases (CODHs). Here we show X-ray crystal structure of class III HCP from Methanothermobacter marburgensis (Mm HCP), demonstrating its homodimeric architecture structurally resembles those of CODHs. Also, despite the different architectures of class III and I/II HCPs, [4Fe-4S] and hybrid clusters are found in equivalent positions in all HCPs. Structural comparison of Mm HCP and CODHs unveils some distinct features such as the environments of their homodimeric interfaces and the active site metalloclusters. Furthermore, structural analysis of Mm HCP C67Y and characterization of several Mm HCP variants with a Cys67 mutation reveal the significance of Cys67 in protein structure, metallocluster binding and hydroxylamine reductase activity. Structure-based bioinformatics analysis of HCPs and CODHs provides insights into the structural evolution of the HCP/CODH superfamily.

Carbon Monoxide Production during Bio-Waste Composting under Different Temperature and Aeration Regimes

Despite the development of biorefinery processes, the possibility of coupling the "conventional" composting process with the production of biochemicals is not taken into account. However, net carbon monoxide (CO) production has been observed during bio-waste composting. So far, O₂ concentration and temperature have been identified as the main variables influencing CO formation. This study aimed to investigate CO net production during bio-waste composting under controlled laboratory conditions by varying aeration rates and temperatures. A series of composting processes was carried out in conditions ranging from mesophilic to thermophilic (T = 35, 45, 55, and 65 °C) and an aeration rate of 2.7, 3.4, 4.8, and 7.8 L·h^-1. Based on the findings of this study, suggestions for the improvement of CO production throughout the composting process have been developed for the first time. The highest concentrations of CO in each thermal variant was achieved with an O₂ deficit (aeration rate 2.7 L·h^-1); additionally, CO levels increased with temperature, reaching ~300 ppm at 65 °C. The production of CO in mesophilic and thermophilic conditions draws attention to biological CO formation by microorganisms capable of producing the CODH enzyme. Further research on CO production efficiency in these thermal ranges is necessary with the characterization of the microbial community and analysis of the ability of the identified bacteria to produce the CODH enzyme and convert CO from CO₂.

Structural and biochemical elucidation of class I hybrid cluster protein natively extracted from a marine methanogenic archaeon

Whilst widespread in the microbial world, the hybrid cluster protein (HCP) has been paradoxically a long-time riddle for microbiologists. During three decades, numerous studies on a few model organisms unravelled its structure and dissected its metal-containing catalyst, but the physiological function of the enzyme remained elusive. Recent studies on bacteria point towards a nitric oxide reductase activity involved in resistance during nitrate and nitrite reduction as well as host infection. In this study, we isolated and characterised a naturally highly produced HCP class I from a marine methanogenic archaeon grown on ammonia. The crystal structures of the enzyme in a reduced and partially oxidised state, obtained at a resolution of 1.45 and 1.36-Å, respectively, offered a precise picture of the archaeal enzyme intimacy. There are striking similarities with the well-studied enzymes from Desulfovibrio species regarding sequence, kinetic parameters, structure, catalyst conformations, and internal channelling systems. The close phylogenetic relationship between the enzymes from Methanococcales and many Bacteria corroborates this similarity. Indeed, Methanococcales HCPs are closer to these bacterial homologues than to any other archaeal enzymes. The relatively high constitutive production of HCP in M. thermolithotrophicus, in the absence of a notable nitric oxide source, questions the physiological function of the enzyme in these ancient anaerobes.

Structural Insights into Microbial One-Carbon Metabolic Enzymes Ni-Fe-S-Dependent Carbon Monoxide Dehydrogenases and Acetyl-CoA Synthases

Ni-Fe-S-dependent carbon monoxide dehydrogenases (CODHs) are enzymes that interconvert CO and CO₂ by using their catalytic Ni-Fe-S C-cluster and their Fe-S B- and D-clusters for electron transfer. CODHs are important in the microbiota of animals such as humans, ruminants, and termites because they can facilitate the use of CO and CO₂ as carbon sources and serve to maintain redox homeostasis. The bifunctional carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS) is responsible for acetate production via the Wood-Ljungdahl pathway, where acetyl-CoA is assembled from two CO₂-derived one-carbon units. A Ni-Fe-S A-cluster is key to this chemistry. Whereas acetogens use the A- and C-clusters of CODH/ACS to produce acetate from CO₂, methanogens use A- and C-clusters of an acetyl-CoA decarbonylase/synthase complex (ACDS) to break down acetate en route to CO₂ and methane production. Here we review some of the recent advances in understanding the structure and mechanism of CODHs, CODH/ACSs, and ACDSs, their unusual metallocofactors, and their unique metabolic roles in the human gut and elsewhere.

Programming xenon diffusion in maltose-binding protein

Protein interiors contain void space that can bind small gas molecules. Determination of gas pathways and kinetics in proteins has been an intriguing and challenging task. Here, we combined computational methods and the hyperpolarized xenon-129 chemical exchange saturation transfer (hyper-CEST) NMR technique to investigate xenon (Xe) exchange kinetics in maltose-binding protein (MBP). A salt bridge ∼9 Å from the Xe-binding site formed upon maltose binding and slowed the Xe exchange rate, leading to a hyper-CEST 129Xe signal from maltose-bound MBP. Xe dissociation occurred faster than dissociation of the salt bridge, as shown by 13C NMR spectroscopy and variable-B1 hyper-CEST experiments. "Xe flooding" molecular dynamics simulations identified a surface hydrophobic site, V23, that has good Xe binding affinity. Mutations at this site confirmed its role as a secondary exchange pathway in modulating Xe diffusion. This shows the possibility for site-specifically controlling xenon protein-solvent exchange. Analysis of the available MBP structures suggests a biological role of MBP's large hydrophobic cavity to accommodate structural changes associated with ligand binding and protein-protein interactions.

Second and Outer Coordination Sphere Effects in Nitrogenase, Hydrogenase, Formate Dehydrogenase, and CO Dehydrogenase

Gases like H₂, N₂, CO₂, and CO are increasingly recognized as critical feedstock in "green" energy conversion and as sources of nitrogen and carbon for the agricultural and chemical sectors. However, the industrial transformation of N₂, CO₂, and CO and the production of H₂ require significant energy input, which renders processes like steam reforming and the Haber-Bosch reaction economically and environmentally unviable. Nature, on the other hand, performs similar tasks efficiently at ambient temperature and pressure, exploiting gas-processing metalloenzymes (GPMs) that bind low-valent metal cofactors based on iron, nickel, molybdenum, tungsten, and sulfur. Such systems are studied to understand the biocatalytic principles of gas conversion including N₂ fixation by nitrogenase and H₂ production by hydrogenase as well as CO₂ and CO conversion by formate dehydrogenase, carbon monoxide dehydrogenase, and nitrogenase. In this review, we emphasize the importance of the cofactor/protein interface, discussing how second and outer coordination sphere effects determine, modulate, and optimize the catalytic activity of GPMs. These may comprise ionic interactions in the second coordination sphere that shape the electron density distribution across the cofactor, hydrogen bonding changes, and allosteric effects. In the outer coordination sphere, proton transfer and electron transfer are discussed, alongside the role of hydrophobic substrate channels and protein structural changes. Combining the information gained from structural biology, enzyme kinetics, and various spectroscopic techniques, we aim toward a comprehensive understanding of catalysis beyond the first coordination sphere.