The interaction of methane with unsupported and supported molybdenum compounds (Mo, MoO 2 , MoO 3 , Mo 2 C, and MoC (1−x) ) has been investigated at 973 K. ZSM-5 was used as a support. Reaction products were analyzed using gas chromatography. Changes in the composition of catalyst samples were followed by X-ray photoelectron spectroscopy. Molybdenum metal and oxides interacted strongly with methane at 973 K to give H 2 (Mo) and H 2 O and CO 2 (oxides), but only a trace amount of ethane. When these compounds were contacted with ZSM-5, the reaction pathway of methane initially was the same. Afterward, however, a dramatic change occurred in the product distribution: the formation of ethane, ethylene, and benzene came into prominence. This was particularly the case when these compounds were highly dispersed on ZSM-5. The selectivity to benzene was 80-85%. XPS analysis of Mo-containing catalysts demonstrated the formation of Mo carbides during the catalytic reaction. Unsupported Mo carbides behaved like metallic Mo; the dominant process was the decomposition of methane to hydrogen and carbon. The deposition of Mo 2 C on ZSM-5 in a well-dispersed state, however, produced a very active and selective catalyst for the conversion of methane into benzene. The results suggest that Mo 2 C is the active surface species in the Mo-containing catalysts, which converts methane into ethylene, the primary compound for the production of benzene on the zeolite surface.
The expression of A'-pyrroline-5-carboxylate reductase (P5CR) gene was found to be higher in soybean root nodules than in leaves and roots, and its expression in roots appeared to be osmoregulated Mol Gen Genet 221: 299-305).P5CR was purified to homogeneity as a monomeric protein of 29 kilodaltons by overexpression of a soybean P5CR cDNA clone in Escherichia coli. The pH optimum of the purified P5CR was altered by increasing the salt concentration, and maximum enzyme activity was attainable at a lower pH under high salt (0.2-1 molar NaCI). Kinetic studies of the purified enzyme suggested that nicotinamide adenine dinucleotide phosphate' inhibited P5CR activity, whereas nicotinamide adenine dinucleotide+ did not. Subcellular fractionation and antibodies raised against purified soybean P5CR were used to investigate location of the enzyme in different parts of soybean as well as in leaves of transgenic tobacco plants synthesizing soybean P5CR. P5CR activity was present in cytoplasm of soybean roots and nodules as well as in leaves, but in leaves, about 15% of the activity was detected in the plastid fraction. The location of P5CR was further confirmed by western blot assay of the proteins from cytosol and plastid fractions of different parts of the plant. Expression of soybean nodule cytosolic P5CR in transgenic tobacco under the control of cauliflower mosaic virus 35S promoter led to the accumulation of this protein exclusively in the cytoplasm, suggesting that the chloroplastic activity may be due to the presence of a plastid form of the enzyme. The different locations of P5CR in root and leaf suggested that proline may be synthesized in different subcellular compartments in root and leaf. Proline concentration was not significantly increased in transgenic plants exhibiting high level P5CR activity, indicating that reduction of P5C is not a rate-limiting step in proline production.Proline accumulation is one of the adaptations of plants to salinity and water deficit (4,22,34). The proline biosynthetic pathway in plants and its regulation in response to osmotic stress is, however, not well understood. A precise determination of the proline pathway and its compartmentalization in different tissues is essential not only for understanding the regulation of proline production, but also for establishing the role of this amino acid in conferring tolerance to salt and drought stresses. The proline pathway in plants has been proposed to be analogous to that in bacteria (7, 21), although proline can also be made from omithine (1).
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