title: A novel ferredoxin-dependent glutamate synthase from the hydrogen-oxidizing chemoautotrophic bacterium Hydrogenobacter thermophilus TK-6.
abstract: Glutamate synthases are classified according to their specificities for electron donors. Ferredoxin-dependent glutamate synthases had been found only in plants and cyanobacteria, whereas many bacteria have NADPH-dependent glutamate synthases. In this study, Hydrogenobacter thermophilus, a hydrogen-oxidizing chemoautotrophic bacterium, was shown to possess a ferredoxin-dependent glutamate synthase like those of phototrophs. This is the first observation, to our knowledge, of a ferredoxin-dependent glutamate synthase in a nonphotosynthetic organism. The purified enzyme from H. thermophilus was shown to be a monomer of a 168-kDa polypeptide homologous to ferredoxin-dependent glutamate synthases from phototrophs. In contrast to known ferredoxin-dependent glutamate synthases, the H. thermophilus glutamate synthase exhibited glutaminase activity. Furthermore, this glutamate synthase did not react with a plant-type ferredoxin (Fd3 from this bacterium) containing a [2Fe-2S] cluster but did react with bacterial ferredoxins (Fd1 and Fd2 from this bacterium) containing [4Fe-4S] clusters. Interestingly, the H. thermophilus glutamate synthase was activated by some of the organic acids in the reductive tricarboxylic acid cycle, the central carbon metabolic pathway of this organism. This type of activation has not been reported for any other glutamate synthases, and this property may enable the control of nitrogen assimilation by carbon metabolism.
authors.affiliation: Department of Biotechnology, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan.
authors.forename: Masafumi
authors.initials: M
authors.lastname: Kameya
authors.forename: Takeshi
authors.initials: T
authors.lastname: Ikeda
authors.forename: Miyuki
authors.initials: M
authors.lastname: Nakamura
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Arai
authors.forename: Masaharu
authors.initials: M
authors.lastname: Ishii
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Igarashi
mesh_terms: D001419:Bacteria / Q000145:classification / Q000201:enzymology* / Q000235:genetics; D001426:Bacterial Proteins / Q000378:metabolism; D017931:DNA Primers; D004578:Electron Spin Resonance Spectroscopy; D005288:Ferredoxins / Q000378:metabolism*; D005970:Glutamate Synthase / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D006859:Hydrogen / Q000378:metabolism; D007700:Kinetics; D008969:Molecular Sequence Data; D010802:Phylogeny; D011994:Recombinant Proteins / Q000302:isolation & purification / Q000378:metabolism; D013053:Spectrophotometry
chemical_list: D001426:Bacterial Proteins; D017931:DNA Primers; D005288:Ferredoxins; D011994:Recombinant Proteins; D006859:Hydrogen; D005970:Glutamate Synthase