The membrane-bound hydrogenase from Paracoccus denitrificans. Purification and molecular characterization

Influence of Hydrogen Electron Donor, Alkaline pH, and High Nitrate Concentrations on Microbial Denitrification: A Review

Bacterial respiration of nitrate is a natural process of nitrate reduction, which has been industrialized to treat anthropic nitrate pollution. This process, also known as “microbial denitrification”, is widely documented from the fundamental and engineering points of view for the enhancement of the removal of nitrate in wastewater. For this purpose, experiments are generally conducted with heterotrophic microbial metabolism, neutral pH and moderate nitrate concentrations (<50 mM). The present review focuses on a different approach as it aims to understand the effects of hydrogenotrophy, alkaline pH and high nitrate concentration on microbial denitrification. Hydrogen has a high energy content but its low solubility, 0.74 mM (1 atm, 30 °C), in aqueous medium limits its bioavailability, putting it at a kinetic disadvantage compared to more soluble organic compounds. For most bacteria, the optimal pH varies between 7.5 and 9.5. Outside this range, denitrification is slowed down and nitrite (NO₂⁻) accumulates. Some alkaliphilic bacteria are able to express denitrifying activity at pH levels close to 12 thanks to specific adaptation and resistance mechanisms detailed in this manuscript, and some bacterial populations support nitrate concentrations in the range of several hundred mM to 1 M. A high concentration of nitrate generally leads to an accumulation of nitrite. Nitrite accumulation can inhibit bacterial activity and may be a cause of cell death.

Studies on inhibition of transformation of 2,4,6-trinitrotoluene catalyzed by Fe-only hydrogenase from Clostridium acetobutylicum

The major enzyme in Clostridium acetobutylicum ATCC 824 leading to transformation of TNT has been reported to be the Fe-only hydrogenase. In this study, we examine the effect of inhibitors of hydrogenase on TNT reduction by Clostridial extracts. These experiments further demonstrate the major role of hydrogenase in TNT transformation. The C. acetobutylicum hydrogenase is closely related to that of C. pasteurianum; and can be fitted to the X-ray crystal structure with a root mean square deviation of 1.18 angstroms for the Calpha atoms of the generated 3D simulation model. The Hyd1, Hyd2, and Hyd3 antibodies generated against hydrogenase reacted with both the hydrogenase in cell extracts and with C. acetobutylicum hydrogenase expressed in Escherichia coli. Inhibition studies using antibodies against Fe-only hydrogenase from C. acetobutylicum indicated that the transformation of TNT by crude cell extracts was completely inhibited by Hyd2 antibody (to amino acid 415-428) whereas antibodies Hyd1 (to residues 1-16) and Hyd3 (to amino acid 424-448) inhibited less effectively. The TNT transforming activity of the cell extract was retained when Hyd2 antibody pretreated with purified but enzymatically inactive recombinant hydrogenase was added to the extract. Addition of the transition metal Cu2+ to extracts completely inhibited the transformation of TNT suggesting the destruction of [4Fe-4S] centers which are essential for transfer of electrons from the H2-activating site to TNT. Growth of C. acetobutylicum was also inhibited by 0.5 mM Cu2+ and Hg2+ ions. The triazine dye, procion red and the nitroimidazole drug, metronidazole inhibit TNT reduction. The inhibition studies using antibodies, procion red, metronidazole, and transition metals suggest that different portions of hydrogenase are required for effective TNT reduction.

A new ecological adaptation to high sulfide by a Hydrogenobacter sp. growing on sulfur compounds but not on hydrogen

Thermophilic bacteria were isolated from a sulfide-rich, neutral hot spring in Iceland on gelrite minimal medium with 16 mM thiosulfate. The isolates were aerobic, obligate chemolithoautotrophs and used thiosulfate and sulfur as electron donors, producing sulfate from both substrates. No growth was observed with hydrogen as the sole electron donor, and no hydrogenase activity was detected. The cells were gram-negative and usually single, 4-5 microm long and 0.7 microm in diameter and formed sulfur globules after a few days of incubation. By SSU rRNA sequence comparisons, the bacterium was placed in the genus Hydrogenobacter with the closest relative to be Calderobacterium hydrogenophilum with 98.3% sequence similarity. This novel bacterium shows an ecological adaptation to high sulfide springs and is differentiated from its closest known relatives by lack of H2 oxidation, deposition of sulfur and lower growth temperature.

Phase separation of biomolecules in polyoxyethylene glycol nonionic detergents

The advantage of aqueous two-phase systems based on polyoxyethylene detergents over other liquid-liquid two-phase systems lies in their capacity to fractionate membrane proteins simply by heating the solution over a biocompatible range of temperatures (20 to 37 degrees C). This permits the peripheral membrane proteins to be effectively separated from the integral membrane proteins, which remain in the detergent-rich phase due to the interaction of their hydrophobic domains with detergent micelles. Since the first reports of this special characteristic of polyoxyethylene glycol detergents in 1981, numerous reports have consolidated this procedure as a fundamental technique in membrane biochemistry and molecular biology. As examples of their use in these two fields, this review summarizes the studies carried out on the topology, diversity, and anomalous behavior of transmembrane proteins on the distribution of glycosyl-phosphatidylinositol-anchored membrane proteins, and on a mechanism to describe the pH-induced translocation of viruses, bacterial endotoxins, and soluble cytoplasmic proteins related to membrane fusion. In addition, the phase separation capacity of these polyoxyethylene glycol detergents has been used to develop quick fractionation methods with high recoveries, on both a micro- and macroscale, and to speed up or increase the efficiency of bioanalytical assays.

The structure and mechanism of iron-hydrogenases

Hydrogenases devoid of nickel and containing only Fe-S clusters have been found so far only in some strictly anaerobic bacteria. Four Fe-hydrogenases have been characterized: from Megasphaera elsdenii, Desulfovibrio vulgaris (strain Hildenborough), and two from Clostridium pasteurianum. All contain two or more [4Fe-4S]1+,2+ or F clusters and a unique type of Fe-S center termed the H cluster. The H cluster appears to be remarkably similar in all the hydrogenases, and is proposed as the site of H2 oxidation and H2 production. The F clusters serve to transfer electrons between the H cluster and the external electron carrier. In all of the hydrogenases the H cluster is comprised of at least three Fe atoms, and possibly six. In the oxidized state it contains two types of magnetically distinct Fe atoms, has an S = 1/2 spin state, and exhibits a novel rhombic EPR signal. The reduced cluster is diamagnetic (S = 0). The oxidized H cluster appears to undergo a conformation change upon reduction with H2 with an increase in Fe-Fe distances of about 0.5 A. Studies using resonance Raman, magnetic circular dichroism and electron spin echo spectroscopies suggest that the H cluster has significant non-sulfur coordination. The H cluster has two binding sites for CO, at least one of which can also bind O2. Binding to one site changes the EPR properties of the cluster and gives a photosensitive adduct, but does not affect catalytic activity. Binding to the other site, which only becomes exposed during the catalytic cycle, leads to loss of catalytic activity. Mechanisms of H2 activation and electron transfer are proposed to explain the effects of CO binding and the ability of one of the hydrogenases to preferentially catalyze H2 oxidation.