Glutamate dehydrogenases from hyperthermophiles

Ammonia Oxidation by the Arctic Terrestrial Thaumarchaeote Candidatus Nitrosocosmicus arcticus Is Stimulated by Increasing Temperatures

Climate change is causing arctic regions to warm disproportionally faster than those at lower latitudes, leading to alterations in carbon and nitrogen cycling, and potentially higher greenhouse gas emissions. It is thus increasingly important to better characterize the microorganisms driving arctic biogeochemical processes and their potential responses to changing conditions. Here, we describe a novel thaumarchaeon enriched from an arctic soil, Candidatus Nitrosocosmicus arcticus strain Kfb, which has been maintained for seven years in stable laboratory enrichment cultures as an aerobic ammonia oxidizer, with ammonium or urea as substrates. Genomic analyses show that this organism harbors all genes involved in ammonia oxidation and in carbon fixation via the 3-hydroxypropionate/4-hydroxybutyrate cycle, characteristic of all AOA, as well as the capability for urea utilization and potentially also for heterotrophic metabolism, similar to other AOA. Ca. N. arcticus oxidizes ammonia optimally between 20 and 28°C, well above average temperatures in its native high arctic environment (-13-4°C). Ammonia oxidation rates were nevertheless much lower than those of most cultivated mesophilic AOA (20-45°C). Intriguingly, we repeatedly observed apparent faster growth rates (based on marker gene counts) at lower temperatures (4-8°C) but without detectable nitrite production. Together with potential metabolisms predicted from its genome content, these observations indicate that Ca. N. arcticus is not a strict chemolithotrophic ammonia oxidizer and add to cumulating evidence for a greater metabolic and physiological versatility of AOA. The physiology of Ca. N. arcticus suggests that increasing temperatures might drastically affect nitrification in arctic soils by stimulating archaeal ammonia oxidation.

Purification and characterization of an iron-containing alcohol dehydrogenase in extremely thermophilic bacterium Thermotoga hypogea

Thermotoga hypogea is an extremely thermophilic anaerobic bacterium capable of growing at 90 degrees C. It uses carbohydrates and peptides as carbon and energy sources to produce acetate, CO(2), H(2), L-alanine and ethanol as end products. Alcohol dehydrogenase activity was found to be present in the soluble fraction of T. hypogea. The alcohol dehydrogenase was purified to homogeneity, which appeared to be a homodimer with a subunit molecular mass of 40 +/- 1 kDa revealed by SDS-PAGE analyses. A fully active enzyme contained iron of 1.02 +/- 0.06 g-atoms/subunit. It was oxygen sensitive; however, loss of enzyme activity by exposure to oxygen could be recovered by incubation with dithiothreitol and Fe(2+). The enzyme was thermostable with a half-life of about 10 h at 70 degrees C, and its catalytic activity increased along with the rise of temperature up to 95 degrees C. Optimal pH values for production and oxidation of alcohol were 8.0 and 11.0, respectively. The enzyme had a broad specificity to use primary alcohols and aldehydes as substrates. Apparent K (m) values for ethanol and 1-butanol were much higher than that of acetaldehyde and butyraldehyde. It was concluded that the physiological role of this enzyme is likely to catalyze the reduction of aldehydes to alcohols.

Purification and characterization of an NADH oxidase from extremely thermophilic anaerobic bacterium Thermotoga hypogea

Thermotoga hypogea is an extremely thermophilic anaerobic bacterium capable of growing at 90 degrees C. It was found to be able to grow in the presence of micromolar molecular oxygen (O2). Activity of NADH oxidase was detected in the cell-free extract of T. hypogea, from which an NADH oxidase was purified to homogeneity. The purified enzyme was a homodimeric flavoprotein with a subunit of 50 kDa, revealed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It catalyzed the reduction of O2 to hydrogen peroxide (H2O2), specifically using NADH as electron donor. Its catalytic properties showed that the NADH oxidase had an apparent Vmax value of 37 micromol NADH oxidized min(-1) mg(-1) protein. Apparent Km values for NADH and O2 were determined to be 7.5 microM and 85 microM, respectively. The enzyme exhibited a pH optimum of 7.0 and temperature optimum above 85 degrees C. The NADH-dependent peroxidase activity was also present in the cell-free extract, which could reduce H2O2 produced by the NADH oxidase to H2O. It seems possible that O2 can be reduced to H2O by the oxidase and peroxidase, but further investigation is required to conclude firmly if the purified NADH oxidase is part of an enzyme system that protects anaerobic T. hypogea from accidental exposure to O2.

Minimal sulfur requirement for growth and sulfur-dependent metabolism of the hyperthermophilic archaeon Staphylothermus marinus

Staphylothermus marinus is an anaerobic hyperthermophilic archaeon that uses peptides as carbon and energy sources. Elemental sulfur (S(o)) is obligately required for its growth and is reduced to H2S. The metabolic functions and mechanisms of S(o) reduction were explored by examining S(o)-dependent growth and activities of key enzymes present in this organism. All three forms of S(o) tested--sublimed S(o), colloidal S(o) and polysulfide--were used by S. marinus, and no other sulfur-containing compounds could replace S(o). Elemental sulfur did not serve as physical support but appeared to function as an electron acceptor. The minimal S(o) concentration required for optimal growth was 0.05% (w/v). At this concentration, there appeared to be a metabolic transition from H2 production to S reduction. Some enzymatic activities related to S(o)-dependent metabolism, including sulfur reductase, hydrogenase, glutamate dehydrogenase and electron transfer activities, were detected in cell-free extracts of S. marinus. These results indicate that S(o) plays an essential role in the heterotrophic metabolism of S. marinus. Reducing equivalents generated by the oxidation of amino acids from peptidolysis may be transferred to sulfur reductase and hydrogenase, which then catalyze the production of H2S and H2, respectively.