Mediated electrochemistry of dimethyl sulfoxide reductase from Rhodobacter capsulatus

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.

Deconstructing the electron transfer chain in a complex molybdoenzyme: Assimilatory nitrate reductase from Neurospora crassa

Nitrate reductase (NR) from the fungus Neurospora crassa is a complex homodimeric metallo-flavoenzyme, where each protomer contains three distinct domains; the catalytically active terminal molybdopterin cofactor, a central heme-containing domain, and an FAD domain which binds with the natural electron donor NADPH. Here, we demonstrate the catalytic voltammetry of variants of N. crassa NRs on a modified Au electrode with the electrochemically reduced forms of benzyl viologen (BV²⁺) and anthraquinone sulfonate (AQS^-) acting as artificial electron donors. The biopolymer chitosan used to entrap NR on the electrode non-covalently and the enzyme film was both stable and highly active. Electrochemistry was conducted on two distinct forms; one lacking the FAD cofactor and the other lacking both the FAD and heme cofactors. While both enzymes showed catalytic nitrate reductase activity, removal of the heme cofactor resulted in a more significant effect on the rate of nitrate reduction. Electrochemical simulation was carried out to enable kinetic characterisation of both the NR:nitrate and NR:mediator reactions.

Extracellular electron transfer features of Gram-positive bacteria

Electroactive microorganisms possess the unique ability to transfer electrons to or from solid phase electron conductors, e.g., electrodes or minerals, through various physiological mechanisms. The processes are commonly known as extracellular electron transfer and broadly harnessed in microbial electrochemical systems, such as microbial biosensors, microbial electrosynthesis, or microbial fuel cells. Apart from a few model microorganisms, the nature of the microbe-electrode conductive interaction is poorly understood for most of the electroactive species. The interaction determines the efficiency and a potential scaling up of bioelectrochemical systems. Gram-positive bacteria generally have a thick electron non-conductive cell wall and are believed to exhibit weak extracellular electron shuttling activity. This review highlights reported research accomplishments on electroactive Gram-positive bacteria. The use of electron-conducting polymers as mediators is considered as one promising strategy to enhance the electron transfer efficiency up to application scale. In view of the recent progress in understanding the molecular aspects of the extracellular electron transfer mechanisms of Enterococcus faecalis, the electron transfer properties of this bacterium are especially focused on. Fundamental knowledge on the nature of microbial extracellular electron transfer and its possibilities can provide insight in interspecies electron transfer and biogeochemical cycling of elements in nature. Additionally, a comprehensive understanding of cell-electrode interactions may help in overcoming insufficient electron transfer and restricted operational performance of various bioelectrochemical systems and facilitate their practical applications.

Redox dependent metabolic shift in Clostridium autoethanogenum by extracellular electron supply

BACKGROUND

Microbial electrosynthesis is a novel approach that aims at shifting the cellular metabolism towards electron-dense target products by extracellular electron supply. Many organisms including several acetogenic bacteria have been shown to be able to consume electrical current. However, suitable hosts for relevant industrial processes are yet to be discovered, and major knowledge gaps about the underlying fundamental processes still remain.

RESULTS

In this paper, we present the first report of electron uptake by the Gram-positive, ethanol-producing acetogen, Clostridium autoethanogenum. Under heterotrophic conditions, extracellular electron supply induced a significant metabolic shift away from acetate. In electrically enhanced fermentations on fructose, acetate production was cut by more than half, while production of lactate and 2,3-butanediol increased by 35-fold and threefold, respectively. The use of mediators with different redox potential revealed a direct dependency of the metabolic effect on the redox potential at which electrons are supplied. Only electrons delivered at a redox potential low enough to reduce ferredoxin caused the reported effect.

CONCLUSIONS

Production in acetogenic organisms is usually challenged by cellular energy limitations if the target product does not lead to a net energy gain as in the case of acetate. The presented results demonstrate a significant shift of carbon fluxes away from acetate towards the products, lactate and 2,3-butanediol, induced by small electricity input (~0.09 mol of electrons per mol of substrate). This presents a simple and attractive method to optimize acetogenic fermentations for production of chemicals and fuels using electrochemical techniques. The relationship between metabolic shift and redox potential of electron feed gives an indication of possible electron-transfer mechanisms and helps to prioritize further research efforts.

Electrochemically mediated enantioselective reduction of chiral sulfoxides

The respiratory DMSO reductase from Rhodobacter capsulatus catalyzes the reduction of dimethyl sulfoxide to dimethyl sulfide. Herein, we have utilized this Mo enzyme as an enantioselective catalyst to generate optically pure sulfoxides (methyl p-tolyl sulfoxide, methyl phenyl sulfoxide and phenyl vinyl sulfoxide) from racemic starting materials. A hexaaminecobalt coordination compound in its divalent oxidation state was employed as the mediator of electron transfer between the working electrode and DMSO reductase to continually reactivate the enzyme after turnover. In all cases, chiral HPLC analysis of the reaction mixture revealed that the S-sulfoxide was reduced more rapidly leading to enrichment or isolation of the R isomer.

Electrochemically driven catalysis of Rhizobium sp. NT-26 arsenite oxidase with its native electron acceptor cytochrome c552

We describe the catalytic voltammograms of the periplasmic arsenite oxidase (Aio) from the chemolithoautotrophic bacterium Rhizobium sp. str. NT-26 that oxidizes arsenite to arsenate. Electrochemistry of the enzyme was accomplished using its native electron transfer partner, cytochrome c552 (cyt c552), as a mediator. The protein cyt c552 adsorbed on a mercaptoundecanoic acid (MUA) modified Au electrode exhibited a stable, reversible one-electron voltammetric response at +275mV vs NHE (pH6). In the presence of arsenite and Aio the voltammetry of cyt c552 is transformed from a transient response to an amplified sigmoidal (steady state) wave consistent with an electro-catalytic system. Digital simulation was performed using a single set of parameters for all catalytic voltammetries obtained at different sweep rates and various substrate concentrations. The obtained kinetic constants from digital simulation provide new insight into the kinetics of the NT-26 Aio catalytic mechanism.

Fabrication and biosensing with CNT/aligned mesostructured silica core-shell nanowires

We report the synthesis of carbon nanotubes (CNTs)/mesostructured silica core-shell nanowires via an interfacial surfactant templating approach. The nanowires possess perpendicularly aligned and uniform accessible mesopores, high surface area and large pore volume. When dimethyl sulfoxide reductase (DMSOR) enzyme is immobilized on the core-shell nanowires, the complex can enhance the electrical communication between the active sites of the enzyme and the electrode surface in the presence of a mediator. The unique properties of the CNTs and the uniform accessible mesopores of the nanowires have made this material promising in the applications as carbon nanotubes field-effect transistors, electrochemical detection, and biosensors.

Dependence of the Properties of Cobalt(III) Cage Complex as a Function of the Derivatization of Amine Substituents

Control of redox properties of cobalt macrobicyclic hexaamine (cage) complexes by substituent modification is important for their use as electron-transfer agents, and the resultant derivatives can also change the lipophilicity of the complexes for a variety of biological and other applications. Such derivatization is also important for incorporating cage complexes into a range of redoxactive conjugates. Here, the derivatization of the amine groups in the 1 and 8 positions of [Co(sar)]3+ (sar = sarcophagine = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane) are reported. The synthesis and properties of methylamide (from the reactions with acetic anhydride), arylimine (from Schiff base reactions), benzylamine, phthalimido, and tosylate derivatives are described. These reactions provide synthons that have the potential to act as precursors for building a range of conjugates containing metal cage complexes, including dimers. The effects of the substituents on the ligand conformations, which affect other chemical and physical properties of the cage complexes, are discussed.