Glutamine synthetase of Mycobacterium tuberculosis: extracellular release and characterization of its enzymatic activity.
We have investigated the activity and extracellular release of glutamine synthetase [L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2] of Mycobacterium tuberculosis. The purified, homogeneous M. tuberculosis glutamine synthetase appears to consist of 12 most likely identical subunits of M(r) 58,000, arranged in two superimpose hexagons. In the catalysis of L-glutamine, the enzyme has an apparent Km for L-glutamate of approximately 3 mM at the pH optimum of 7.5. M. tuberculosis releases a large proportion (approximately 30%) of its total measurable enzyme activity into the culture medium, a feature that is highly specific for pathogenic mycobacteria. Immunogold electron microscopy revealed that M. tuberculosis also releases the enzyme into its phagosome in infected human monocytes. Two potentially important roles for glutamine synthase in the pathogenesis of M. tuberculosis infection are (i) the synthesis of L-glutamine, a major component of the cell wall of pathogenic but not nonpathogenic mycobacteria, and (ii) the modulation of the ammonia level in the M. tuberculosis phagosome, which may in turn influence phagosomal pH and phagosomelysosome fusion.
Harth
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Department of Medicine, School of Medicine, University of California, Los Angeles 90024.
Clemens
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Horwitz
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D000595:Amino Acid Sequence; D001409:Bacillus cereus / Q000201:enzymology; D001412:Bacillus subtilis / Q000201:enzymology; D002850:Chromatography, Gel; D002852:Chromatography, Ion Exchange; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000201:enzymology; D005974:Glutamate-Ammonia Ligase / Q000302:isolation & purification / Q000378:metabolism* / Q000648:ultrastructure; D007700:Kinetics; D008854:Microscopy, Electron; D016253:Microscopy, Immunoelectron; D008969:Molecular Sequence Data; D008970:Molecular Weight; D009000:Monocytes / Q000382:microbiology / Q000648:ultrastructure; D009161:Mycobacterium / Q000201:enzymology*; D009163:Mycobacterium bovis / Q000201:enzymology; D009168:Mycobacterium phlei / Q000201:enzymology; D009169:Mycobacterium tuberculosis / Q000201:enzymology*; D017386:Sequence Homology, Amino Acid
D005974:Glutamate-Ammonia Ligase