Reversible and irreversible effects of nitric oxide on the soluble hydrogenase from Alcaligenes eutrophus H16

Fermentation of biomass-generated synthesis gas: Effects of nitric oxide

The production of renewable fuels, such as ethanol, has been steadily increasing owing to the need for a reduced dependency on fossil fuels. It was demonstrated previously that biomass-generated synthesis gas (biomass-syngas) can be converted to ethanol and acetic acid using a microbial catalyst. The biomass-syngas (primarily CO, CO(2), H(2), and N(2)) was generated in a fluidized-bed gasifier and used as a substrate for Clostridium carboxidivorans P7(T). Results showed that the cells stopped consuming H(2) when exposed to biomass-syngas, thus indicating that there was an inhibition of the hydrogenase enzyme due to some biomass-syngas contaminant. It was hypothesized that nitric oxide (NO) detected in the biomass-syngas could be the possible cause of this inhibition. The specific activity of hydrogenase was monitored with time under varying concentrations of H(2) and NO. Results indicated that NO (at gas concentrations above 40 ppm) was a non-competitive inhibitor of hydrogenase activity, although the loss of hydrogenase activity was reversible. In addition, NO also affected the cell growth and increased the amount of ethanol produced. A kinetic model of hydrogenase activity with inhibition by NO was demonstrated with results suggesting there are multiple binding sites of NO on the hydrogenase enzyme. Since other syngas-fermenting organisms utilize the same metabolic pathways, this study estimates that NO < 40 ppm can be tolerated by cells in a syngas-fermentation system without compromising the hydrogenase activity, cell growth, and product distribution.

Sustained photoevolution of molecular hydrogen in a mutant of Synechocystis sp. strain PCC 6803 deficient in the type I NADPH-dehydrogenase complex

The interaction between hydrogen metabolism, respiration, and photosynthesis was studied in vivo in whole cells of Synechocystis sp. strain PCC 6803 by continuously monitoring the changes in gas concentrations (H2, CO2, and O2) with an online mass spectrometer. The in vivo activity of the bidirectional [NiFe]hydrogenase [H2:NAD(P) oxidoreductase], encoded by the hoxEFUYH genes, was also measured independently by the proton-deuterium (H-D) exchange reaction in the presence of D2. This technique allowed us to demonstrate that the hydrogenase was insensitive to light, was reversibly inactivated by O2, and could be quickly reactivated by NADH or NADPH (+H2). H2 was evolved by cells incubated anaerobically in the dark, after an adaptation period. This dark H2 evolution was enhanced by exogenously added glucose and resulted from the oxidation of NAD(P)H produced by fermentation reactions. Upon illumination, a short (less than 30-s) burst of H2 output was observed, followed by rapid H2 uptake and a concomitant decrease in CO2 concentration in the cyanobacterial cell suspension. Uptake of both H2 and CO2 was linked to photosynthetic electron transport in the thylakoids. In the ndhB mutant M55, which is defective in the type I NADPH-dehydrogenase complex (NDH-1) and produces only low amounts of O2 in the light, H2 uptake was negligible during dark-to-light transitions, allowing several minutes of continuous H2 production. A sustained rate of photoevolution of H2 corresponding to 6 micro mol of H2 mg of chlorophyll(-1) h(-1) or 2 ml of H2 liter(-1) h(-1) was observed over a longer time period in the presence of glucose and was slightly enhanced by the addition of the O2 scavenger glucose oxidase. By the use of the inhibitors DCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea] and DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone), it was shown that two pathways of electron supply for H2 production operate in M55, namely photolysis of water at the level of photosystem II and carbohydrate-mediated reduction of the plastoquinone pool.

Kinetic analysis of the interaction of nitric oxide with the membrane-associated, nickel and iron-sulfur-containing hydrogenase from Azotobacter vinelandii

The effects of nitric oxide (NO) on the membrane-associated form of the nickel and iron-sulfur-containing hydrogenase from Azotobacter vinelandii have been investigated. In the presence of H2 and an electron acceptor (turnover conditions), NO acts as a noncompetitive inhibitor vs. methylene blue (Ki = 12 microM). There is no element of competition between NO and H2, implying that the site of NO action is not the H2-activating site of the hydrogenase. When the membrane-associated hydrogenase is incubated under non-turnover conditions, the enzyme is irreversibly inactivated by NO in a time-dependent process. The inactivation is a non-saturable, pseudo-first-order process which is consistent with a direct chemical reaction between NO and the hydrogenase. Kinetic evidence is presented which is compatible with an interaction between NO and a redox-active component other than the H2-activating site on the enzyme. The complex inhibition pattern of NO has been interpreted in terms of two distinct interactions of NO with iron-sulfur centers of the hydrogenase.

Role of hydrogen in the activation and regulation of hydrogen oxidation by the soluble hydrogenase from Alcaligenes eutrophus H16

The activation kinetics of the H2-oxidizing activity of the soluble hydrogenase from Alcaligenes eutrophus H16 were investigated. Activation with Na2S2O4 plus 101 kPa H2 resulted in a rapid increase in activity over 1 h and constant activity after 3 h incubation. Less-stable activations were achieved if enzyme was incubated with Na2S2O4 under 1 kPa H2 or 101 kPa N2. The enzyme could also be partly activated either with NADH alone or with H2 alone. The level of activity obtained with both 101 kPa H2 and NADH present was greater than that obtained with either 101 kPa H2 or NADH alone. Activation with H2 plus NADH was virtually independent of NADH concentration but highly dependent on H2 concentration. The effects of various concentrations of H2 and constant concentration of NADH on the level of activation were the same whether H2 oxidation was assayed by H2-dependent Methylene Blue or NAD+ reduction. Diaphorase activity did not require activation and was little affected by the treatments that activated H2-oxidizing activity. The results suggest that H2 plays an important role in regulating the level of H2-oxidizing activity in this soluble hydrogenase.