The oxidation-reduction and electrocatalytic properties of CO dehydrogenase from Oligotropha carboxidovorans

Molybdenum-Copper Antagonism In Metalloenzymes And Anti-Copper Therapy

The connection between 3d (Cu) and 4d (Mo) via the "Mo-S-Cu" unit is called Mo-Cu antagonism. Biology offers case studies of such interactions in metalloproteins such as Mo/Cu-CO Dehydrogenases (Mo/Cu-CODH), and Mo/Cu Orange Protein (Mo/Cu-ORP). The CODH significantly maintains the CO level in the atmosphere below the toxic level by converting it to non-toxic CO2 for respiring organisms. Several models were synthesized to understand the structure-function relationship of these native enzymes. However, this interaction was first observed in ruminants, and they convert molybdate (MoO4 2- ) into tetrathiomolybdate (MoS4 2- ; TTM), reacting with cellular Cu to yield biological unavailable Mo/S/Cu cluster, then developing Cu-deficiency diseases. These findings inspire the use of TTM as a Cu-sequester drug, especially for treating Cu-dependent human diseases such as Wilson diseases (WD) and cancer. It is well known that a balanced Cu homeostasis is essential for a wide range of biological processes, but negative consequence leads to cell toxicity. Therefore, this review aims to connect the Mo-Cu antagonism in metalloproteins and anti-copper therapy.

Catalytic electrochemistry of the bacterial Molybdoenzyme YcbX

Molybdenum-dependent enzymes that can reduce N-hydroxylated substrates (e.g. N-hydroxyl-purines, amidoximes) are found in bacteria, plants and vertebrates. They are involved in the conversion of a wide range of N-hydroxylated organic compounds into their corresponding amines, and utilize various redox proteins (cytochrome b₅, cyt b₅ reductase, flavin reductase) to deliver reducing equivalents to the catalytic centre. Here we present catalytic electrochemistry of the bacterial enzyme YcbX from Escherichia coli utilizing the synthetic electron transfer mediator methyl viologen (MV²⁺). The electrochemically reduced form (MV^+.) acts as an effective electron donor for YcbX. To immobilize YcbX on a glassy carbon electrode, a facile protein crosslinking approach was used with the crosslinker glutaraldehyde (GTA). The YcbX-modified electrode showed a catalytic response for the reduction of a broad range of N-hydroxylated substrates. The catalytic activity of YcbX was examined at different pH values exhibiting an optimum at pH 7.5 and a bell-shaped pH profile with deactivation through deprotonation (pK_a1 9.1) or protonation (pK_a2 6.1). Electrochemical simulation was employed to obtain new biochemical data for YcbX, in its reaction with methyl viologen and the organic substrates 6-N-hydroxylaminopurine (6-HAP) and benzamidoxime (BA).

Microbial oxidation of atmospheric trace gases

The atmosphere has recently been recognized as a major source of energy sustaining life. Diverse aerobic bacteria oxidize the three most abundant reduced trace gases in the atmosphere, namely hydrogen (H₂), carbon monoxide (CO) and methane (CH₄). This Review describes the taxonomic distribution, physiological role and biochemical basis of microbial oxidation of these atmospheric trace gases, as well as the ecological, environmental, medical and astrobiological importance of this process. Most soil bacteria and some archaea can survive by using atmospheric H₂ and CO as alternative energy sources, as illustrated through genetic studies on Mycobacterium cells and Streptomyces spores. Certain specialist bacteria can also grow on air alone, as confirmed by the landmark characterization of Methylocapsa gorgona, which grows by simultaneously consuming atmospheric CH₄, H₂ and CO. Bacteria use high-affinity lineages of metalloenzymes, namely hydrogenases, CO dehydrogenases and methane monooxygenases, to utilize atmospheric trace gases for aerobic respiration and carbon fixation. More broadly, trace gas oxidizers enhance the biodiversity and resilience of soil and marine ecosystems, drive primary productivity in extreme environments such as Antarctic desert soils and perform critical regulatory services by mitigating anthropogenic emissions of greenhouse gases and toxic pollutants.

Electrochemically driven catalysis of the bacterial molybdenum enzyme YiiM

The Mo-dependent enzyme YiiM enzyme from Escherichia coli is a member of the sulfite oxidase family and shares many similarities with the well-studied human mitochondrial amidoxime reducing component (mARC). We have investigated YiiM catalysis using electrochemical and spectroscopic methods. EPR monitored redox potentiometry found the active site redox potentials to be Mo^VI/V -0.02 V and Mo^V/IV -0.12 V vs NHE at pH 7.2. In the presence of methyl viologen as an electrochemically reduced electron donor, YiiM catalysis was studied with a range of potential substrates. YiiM preferentially reduces N-hydroxylated compounds such as hydroxylamines, amidoximes, N-hydroxypurines and N-hydroxyureas but shows little or no activity against amine-oxides or sulfoxides. The pH optimum for catalysis was 7.1 and a bell-shaped pH profile was found with pK_a values of 6.2 and 8.1 either side of this optimum that are associated with protonation/deprotonations that modulate activity. Simulation of the experimental voltammetry elucidated kinetic parameters associated with YiiM catalysis with the substrates 6-hydroxyaminopurine and benzamidoxime.

Exploiting Aerobic Carboxydotrophic Bacteria for Industrial Biotechnology

Aerobic carboxydotrophic bacteria are a group of microorganisms which possess the unique trait to oxidize carbon monoxide (CO) as sole energy source with molecular oxygen (O₂) to produce carbon dioxide (CO₂) which subsequently is used for biomass formation via the Calvin-Benson-Bassham cycle. Moreover, most carboxydotrophs are also able to oxidize hydrogen (H₂) with hydrogenases to drive the reduction of carbon dioxide in the absence of CO. As several abundant industrial off-gases contain significant amounts of CO, CO₂, H₂ as well as O₂, these bacteria come into focus for industrial application to produce chemicals and fuels from such gases in gas fermentation approaches. Since the group of carboxydotrophic bacteria is rather unknown and not very well investigated, we will provide an overview about their lifestyle and the underlying metabolic characteristics, introduce promising members for industrial application, and give an overview of available genetic engineering tools. We will point to limitations and discuss challenges, which have to be overcome to apply metabolic engineering approaches and to utilize aerobic carboxydotrophs in the industrial environment.

QM/MM study of the binding of H2 to MoCu CO dehydrogenase: development and applications of improved H2 van der Waals parameters

The MoCu CO dehydrogenase enzyme not only transforms CO into CO₂ but it can also oxidise H₂. Even if its hydrogenase activity has been known for decades, a debate is ongoing on the most plausible mode for the binding of H₂ to the enzyme active site and the hydrogen oxidation mechanism. In the present work, we provide a new perspective on the MoCu-CODH hydrogenase activity by improving the in silico description of the enzyme. Energy refinement—by means of the BigQM approach—was performed on the intermediates involved in the dihydrogen oxidation catalysis reported in our previously published work (Rovaletti, et al. “Theoretical Insights into the Aerobic Hydrogenase Activity of Molybdenum–Copper CO Dehydrogenase.” Inorganics 7 (2019) 135). A suboptimal description of the H₂–HN(backbone) interaction was observed when the van der Waals parameters described in previous literature for H₂ were employed. Therefore, a new set of van der Waals parameters is developed here in order to better describe the hydrogen–backbone interaction. They give rise to improved binding modes of H₂ in the active site of MoCu CO dehydrogenase. Implications of the resulting outcomes for a better understanding of hydrogen oxidation catalysis mechanisms are proposed and discussed.