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901.
Schmid B Ribbe MW Einsle O Yoshida M Thomas LM Dean DR Rees DC Burgess BK 《Science (New York, N.Y.)》2002,296(5566):352-356
One of the most complex biosynthetic processes in metallobiochemistry is the assembly of nitrogenase, the key enzyme in biological nitrogen fixation. We describe here the crystal structure of an iron-molybdenum cofactor-deficient form of the nitrogenase MoFe protein, into which the cofactor is inserted in the final step of MoFe protein assembly. The MoFe protein folds as a heterotetramer containing two copies each of the homologous alpha and beta subunits. In this structure, one of the three alpha subunit domains exhibits a substantially changed conformation, whereas the rest of the protein remains essentially unchanged. A predominantly positively charged funnel is revealed; this funnel is of sufficient size to accommodate insertion of the negatively charged cofactor. 相似文献
902.
903.
The nature of the first generation of stars in the universe remains largely unknown. Observations imply the existence of massive primordial stars early in the history of the universe, and the standard theory for the growth of cosmic structure predicts that structures grow hierarchically through gravitational instability. We have developed an ab initio computer simulation of the formation of primordial stars that follows the relevant atomic and molecular processes in a primordial gas in an expanding universe. The results show that primeval density fluctuations left over from the Big Bang can drive the formation of a tiny protostar with a mass 1% that of the Sun. The protostar is a seed for the subsequent formation of a massive primordial star. 相似文献
904.
Hibino T Hashimoto A Inoue T Tokuno J Yoshida S Sano M 《Science (New York, N.Y.)》2000,288(5473):2031-2033
The performance of a single-chamber solid oxide fuel cell was studied using a ceria-based solid electrolyte at temperatures below 773 kelvin. Electromotive forces of approximately 900 millivolts were generated from the cell in a flowing mixture of ethane or propane and air, where the solid electrolyte functioned as a purely ionic conductor. The electrode-reaction resistance was negligibly small in the total internal resistances of the cell. The resulting peak power density reached 403 and 101 milliwatts per square centimeter at 773 and 623 kelvin, respectively. 相似文献
905.
906.
Leaf area index (LAI) is one of the major determinants of crop photosynthesis. The objectives of this study were to clarify the relationship between LAI development and crop growth in diverse rice genotypes grown under widely different climate conditions and to develop a model explaining genotypic and environmental variation in LAI dynamics based on environmental and plant factors. Cross-locational experiments were conducted with nine different rice genotypes at eight locations in Asia covering a wide climate range under irrigated conditions with sufficient nitrogen application. The LAI observed at the heading stage ranged from 0.85 to 8.77 among the genotypes grown at the eight locations. A fairly stable allometric relationship was observed between LAI development and above-ground biomass growth during the period from transplanting to 2 weeks before heading over all the genotypes, sites and years (r = 0.91). The allometric relationship was, however, under the influence of leaf nitrogen content per unit leaf area (LNC, g m−2 leaf) and air temperature. On the basis of these results, we modeled the LAI development as a function of relative crop growth rate (RGR), LNC and air temperature. The rate of LAI decrease associated with leaf senescence was also described as a function of LNC. 相似文献
907.
H. Takahashi Y. Kurose Y. Suzuki M. Kojima T. Yamaguchi Y. Yoshida M. Ogino K. Hodate Y. Azuma T. Sugino M. Kojima K. Kangawa Y. Hasegawa S. Kobayashi 《Domestic animal endocrinology》2009
The effect of energy balance on the growth hormone (GH) secretory responsiveness to growth hormone-releasing hormone (GHRH) has not been determined in ruminant animals. Therefore, we examined the effects of intravenous injections of 0, 3.3, and 6.6 μg ghrelin/kg body weight (BW), with and without GHRH at 0.25 μg/kg BW, on GH secretory responsiveness in both the fed and fasted sheep. The injections were carried out at 48 h (Fasting state) and 3 h (Satiety state) after feeding. Blood samples were taken every 10 minutes, from 30 minutes before to 120 minutes after the injection. Low (3.3 μg/kg BW) and high (6.6 μg/kg BW) doses of ghrelin stimulated GH secretion significantly (P < .05) greater in the Satiety state than in the Fasting state. Growth hormone-releasing hormone plus both doses of ghrelin stimulated GH secretion significantly (P < .05) greater in the Satiety state than in the Fasting state. Ghrelin and GHRH exerted a synergistic effect in the Satiety state, but not in the Fasting state. Plasma ghrelin levels were maintained significantly (P < .05) greater in the Fasting state than in the Satiety state except the temporal increases after ghrelin administration. Plasma free fatty acid (FFA) concentrations were significantly (P < .01) greater in the Fasting state than in the Satiety state. In conclusion, the present study has demonstrated for the first time that ghrelin differentially modulates GH secretory response to GHRH according to feeding states in ruminant animals. 相似文献
908.
Ogasawara Islands are important ecosystems sustaining many indigenous spices. To clarify the indigenous soil environments of Ogasawara Islands, we studied the chemistry of the soils. Many surface soils were low in bio-available P (0 to 0.55 g P2O5 kg−1, average: 0.04 g P2O5 kg−1 as Bray II P, n = 22), but several soils were found to contain extremely large amounts of bio-available P (1.36 to 6.98 g P2O5 kg−1, average: 2.93 g P2O5 kg−1, n = 5). From soil profile analyses, the authors concluded that the extremely large amount of bio-available P could not be explained by the effects of parent materials with high P contents nor the effect of fertilizations by human activity, but the effects of natural seabird activities in the past could be the cause. The soil profiles with large amounts of bio-available P indicate deep migration of soil materials from A horizons, which could be a result of intensive mixing of upper horizons by seabird activities. The intensive mixing was supported by the low mechanical impedance of the horizons for the P-accumulating soils (8.17 ± 2.54 kg cm−2, n = 8) than those for the non-P-accumulating soils (17.46 ± 3.52 kg cm−2, n = 36). It is likely that in the past seabirds, such as shearwaters, made burrows in the soils for nesting and propagating and inadvertently transported a large amount of P from the sea to the soils, resulting in the extremely large amounts of bio-available P in the present soils. 相似文献
909.
Masashi Ando Yoko Nakagishi Keiko Yoshida Masashi Nakao Takayuki Nakagawa Yasuo Makinodan Yasuyuki Tsukamasa Ken-Ichi Kawasaki 《Fisheries Science》2006,72(5):1104-1108
Pyridinoline (Pyr), one of the mature crosslinks of collagen, was determined in muscular collagen of three species of fish.
The amounts of muscular Pyr in red sea bream, yellowtail, and tiger puffer were 3.4, 8.8, and 50.3 mmol/mol collagen, respectively,
indicating that the Pyr concentration in muscular collagen differs greatly among fish species. The Pyr concentration in tiger
puffer muscular collagen was the greatest, but it was only one-fourth that in rabbit muscle. As in mammalian skin collagen,
Pyr was not detected in skin collagen of red sea bream and yellowtail. However, tiger puffer skin contained Pyr (3.75 mmol/mol
collagen). The presence of Pyr would have a relationship to some features of tiger puffer skin, such as mechanical strength
and thickness. Pyr concentrations in acid-soluble collagen (ASC), pepsin-solubilized collagen (PSC), and insoluble collagen
(ISC) in muscles of three species of fish were determined. Pyr was found in ISC > PSC > ASC, from the highest to the lowest
concentration, and the concentration in ISC was 45–200 times that in ASC. Therefore, Pyr crosslinks that are formed between
collagen molecules would have a close relationship to collagen solubility. 相似文献
910.
Trimethylamine-N-oxide (TMAO) degrades to dimethylamine (DMA) and formaldehyde, which cause a decline in fish quality. Suppression of DMA
formation in frozen surimi was investigated using croaker, lizardfish, and walleye pollock. The leaching process in surimi
processing was shown to reduce not only the TMAO, iron, and taurine content, but also to reduce unidentified factors that
accelerate DMA formation in lizardfish muscle; in contrast, unidentified factors that suppress DMA formation were reduced
in croaker and pollock muscle. Sucrose, used as a cryoprotectant, was shown to decrease DMA formation in vitro, likely due
to the reduction in freezing-induced concentration of solutes. The effects of pH on DMA formation were different in minced
frozen meat among the three species. DMA formation was not observed in croaker when the pH varied between 6 and 8. On the
other hand, DMA was elevated in lizardfish under acidic conditions, and DMA formation in pollock was maximal when the pH of
the meat was neutral. Thus, the suppression of TMAO degradation by surimi processing results from the removal of TMAO, iron,
and reductants from fish meat; sucrose also reduces DMA formation. Furthermore, unidentified factors in croaker, lizardfish,
and pollock meat substantially affect DMA formation. 相似文献