Enzymatic characterization of recombinant nitrate reductase expressed and purified from Neurospora crassa

The Neurospora crassa molybdate transporter: Characterizing a novel transporter homologous to the plant MOT1 family

Molybdenum (Mo) is an essential element for animals, plants, and fungi. To achieve biological activity in eukaryotes, Mo must be complexed into the molybdenum cofactor (Moco). Cells are known to take up Mo in the form of the oxyanion molybdate. However, molybdate transporters are scarcely characterized in the fungal kingdom. In plants and algae, molybdate is imported into the cell via two families of molybdate transporters (MOT), MOT1 and MOT2. For the filamentous fungus Neurospora crassa, a sequence homologous to the MOT1 family was previously annotated. Here we report a characterization of this molybdate-related transporter, encoded by the ncmot-1 gene. We found that the deletion of ncmot-1 leads to an accumulation of total Mo within the mycelium and a roughly 51 % higher tolerance against high molybdate levels when grown on ammonium medium. The localization of a GFP tagged NcMOT-1 was identified among the vacuolar membrane. Thereby, we propose NcMOT-1 as an exporter, transporting molybdate out of the vacuole into the cytoplasm. Lastly, the heterologous expression of NcMOT-1 in Saccharomyces cerevisiae verifies the functionality of this protein as a MOT. Our results open the way towards understanding molybdate transport as part of Mo homeostasis and Moco-biosynthesis in fungi.

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).

Precise Quantification of Molybdate In Vitro by the FRET-Based Nanosensor 'MolyProbe'

Molybdenum (Mo) is an essential trace element in all kingdoms of life. Mo is bioavailable as the oxyanion molybdate and gains biological activity in eukaryotes when bound to molybdopterin, forming the molybdenum cofactor. The imbalance of molybdate homeostasis results in growth deficiencies or toxic symptoms within plants, fungi and animals. Recently, fluorescence resonance energy transfer (FRET) methods have emerged, monitoring cellular and subcellular molybdate distribution dynamics using a genetically encoded molybdate-specific FRET nanosensor, named MolyProbe. Here, we show that the MolyProbe system is a fast and reliable in vitro assay for quantitative molybdate determination. We added a Strep-TagII affinity tag to the MolyProbe protein for quick and easy purification. This MolyProbe is highly stable, resistant to freezing and can be stored for several weeks at 4 °C. Furthermore, the molybdate sensitivity of the assay peaked at low nM levels. Additionally, The MolyProbe was applied in vitro for quantitative molybdate determination in cell extracts of the plant Arabidopsis thaliana, the fungus Neurospora crassa and the yeast Saccharomyces cerevisiae. Our results show the functionality of the Arabidopsis thaliana molybdate transporter MOT1.1 and indicate that FRET-based molybdate detection is an excellent tool for measuring bioavailable Mo.

The structure of the Moco carrier protein from Rippkaea orientalis

The molybdenum cofactor (Moco) is the prosthetic group of all molybdenum-dependent enzymes except for nitrogenase. The multistep biosynthesis pathway of Moco and its function in molybdenum-dependent enzymes are already well understood. The mechanisms of Moco transfer, storage and insertion, on the other hand, are not. In the cell, Moco is usually not found in its free form and remains bound to proteins because of its sensitivity to oxidation. The green alga Chlamydomonas reinhardtii harbors a Moco carrier protein (MCP) that binds and protects Moco but is devoid of enzymatic function. It has been speculated that this MCP acts as a means of Moco storage and transport. Here, the search for potential MCPs has been extended to the prokaryotes, and many MCPs were found in cyanobacteria. A putative MCP from Rippkaea orientalis (RoMCP) was selected for recombinant production, crystallization and structure determination. RoMCP has a Rossmann-fold topology that is characteristic of nucleotide-binding proteins and a homotetrameric quaternary structure similar to that of the MCP from C. reinhardtii. In each protomer, a positively charged crevice was identified that accommodates up to three chloride ions, hinting at a potential Moco-binding site. Computational docking experiments supported this notion and gave an impression of the RoMCP-Moco complex.

Heterologous and Homologous Expression of Proteins from Haloarchaea: Denitrification as Case of Study

Haloarchaea (halophilic microbes belonging to the Archaea domain) are microorganisms requiring mid or even high salt concentrations to be alive. The molecular machinery of these organisms is adapted to such conditions, which are stressful for most life forms. Among their molecular adaptations, halophilic proteins are characterized by their high content of acidic amino acids (Aspartate (Asp) and glumate (Glu)), being only stable in solutions containing high salt concentration (between 1 and 4 M total salt concentration). Recent knowledge about haloarchaeal peptides, proteins, and enzymes have revealed that many haloarchaeal species produce proteins of interest due to their potential applications in biotechnology-based industries. Although proteins of interest are usually overproduced in recombinant prokaryotic or eukaryotic expression systems, these procedures do not accurately work for halophilic proteins, mainly if such proteins contain metallocofactors in their structures. This work summarizes the main challenges of heterologous and homologous expression of enzymes from haloarchaea, paying special attention to the metalloenzymes involved in the pathway of denitrification (anaerobic reduction of nitrate to dinitrogen), a pathway with significant implications in wastewater treatment, climate change, and biosensor design.