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The biosynthesis of the molybdenum cofactor (Moco) has been intensively studied, in addition to its insertion into molybdoenzymes. In particular, a link between the assembly of molybdoenzymes and the biosynthesis of FeS clusters has been identified in the recent years: 1) the synthesis of the first intermediate in Moco biosynthesis requires an FeS-cluster containing protein, 2) the sulfurtransferase for the dithiolene group in Moco is also involved in the synthesis of FeS clusters, thiamin and thiolated tRNAs, 3) the addition of a sulfido-ligand to the molybdenum atom in the active site additionally involves a sulfurtransferase, and 4) most molybdoenzymes in bacteria require FeS clusters as redox active cofactors. In this review we will focus on the biosynthesis of the molybdenum cofactor in bacteria, its modification and insertion into molybdoenzymes, with an emphasis to its link to FeS cluster biosynthesis and sulfur transfer. (C) 2014 Elsevier B.V. All rights reserved.
The bioelectrocatalytic sulfite oxidation by human sulfite oxidase (hSO) on indium tin oxide (ITO) is reported, which is facilitated by functionalizing of the electrode surface with polyethylenimine (PEI)-entrapped CdS nanoparticles and enzyme. hSO was assembled onto the electrode with a high surface loading of electroactive enzyme. In the presence of sulfite but without additional mediators, a high bioelectrocatalytic current was generated. Reference experiments with only PEI showed direct electron transfer and catalytic activity of hSO, but these were less pronounced. The application of the polyelectrolyte-entrapped quantum dots (QDs) on ITO electrodes provides a compatible surface for enzyme binding with promotion of electron transfer. Variations of the buffer solution conditions, e.g., ionic strength, pH, viscosity, and the effect of oxygen, were studied in order to understand intramolecular and heterogeneous electron transfer from hSO to the electrode. The results are consistent with a model derived for the enzyme by using flash photolysis in solution and spectroelectrochemistry and molecular dynamic simulations of hSO on monolayer-modified gold electrodes. Moreover, for the first time a photoelectrochemical electrode involving immobilized hSO is demonstrated where photoexcitation of the CdS/hSO-modified electrode lead to an enhanced generation of bioelectrocatalytic currents upon sulfite addition. Oxidation starts already at the redox potential of the electron transfer domain of hSO and is greatly increased by application of a small overpotential to the CdS/hSO-modified ITO.
A direct electron transfer (DET) based sulphite/oxygen biofuel cell is reported that utilises human sulphite oxidase (hSOx) and Myrothecium verrucaria bilirubin oxidase (MvBOx) and nanostructured gold electrodes. For bioanode construction, the nanostructured gold microelectrodes were further modified with 3,3'-dithiodipropionic acid di(N-hydroxysuccinimide ester) to which polyethylene imine was covalently attached. hSOx was adsorbed onto this chemically modified nanostructured electrode with high surface loading of electroactive enzyme and in presence of sulphite high anodic bioelectrocatalytic currents were generated with an onset potential of 0.05 V vs. NHE. The biocathode contained MyBOx directly adsorbed to the deposited gold nanoparticles for cathodic oxygen reduction starting at 0.71 V vs. NHE. Both enzyme electrodes were integrated to a DET-type biofuel cell. Power densities of 8 and 1 mu W cm(-2) were achieved at 0.15 V and 0.45 V of cell voltages, respectively, with the membrane based biodevices under aerobic conditions. (C) 2014 Elsevier B.V. All rights reserved.