期刊
IUBMB LIFE
卷 60, 期 5, 页码 287-300出版社
WILEY
DOI: 10.1002/iub.52
关键词
nitrogen assimilation; amidotransferase; iron-sulfur clusters; flavoenzymes; enzyme activation; ammonia tunnel; glutamine; glutamate; intermediate channeling
Glutamate synthases play with glutamine synthetase an essential role in nitrogen assimilation processes in microorganisms, plants, and lower animals by catalyzing the net synthesis of one molecule of L-glutamate from L-glutamine and 2-oxoglutarate. They exhibit a modular architecture with a common subunit or region, which is responsible for the L-glutamine-dependent glutamate synthesis from 2-oxoglutarate. Here, a PurF- (Type II- or Ntn-) type amidotransferase domain is coupled to the synthase domain, a (beta/alpha)(8) barrel containing FMN and one [3Fe-4S](0,+1) cluster, through a similar to 30 angstrom-long intramolecular tunnel for the transfer of ammonia between the sites. In bacterial and eukaryotic GltS, reducing equivalents are provided by reduced pyridine nucleotides thanks to the stable association with a second subunit or region, which acts as a FAD-dependent NAD(P)H oxidoreductase and is responsible for the formation of the two low potential [4Fe-4S](+1,+2) clusters of the enzyme. In photosynthetic cells, reduced ferredoxin is the physiological reductant. This review focus on the mechanism of cross-activation of the synthase and glutaminase reactions in response to the bound substrates and the redox state of the enzyme cofactors, as well as on recent information on the structure of the alpha beta protomer of the NADPH-dependent enzyme, which sheds light on the intramolecular electron transfer pathway between the flavin cofactors. (C) 2008 IUBMB.
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