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B. subtilis glutamine synthetase structures reveal large active site conformational changes and basis for isoenzyme specific regulation: form two of GS-1
Structural highlights
FunctionGLN1A_BACSU Glutamine synthetase (GS) is an unusual multitasking protein that functions as an enzyme, a transcription coregulator, and a chaperone in ammonium assimilation and in the regulation of genes involved in nitrogen metabolism (PubMed:25691471). It catalyzes the ATP-dependent biosynthesis of glutamine from glutamate and ammonia (PubMed:24158439). Feedback-inhibited GlnA interacts with and regulates the activity of the transcriptional regulator TnrA (PubMed:11719184, PubMed:12139611). During nitrogen limitation, TnrA is in its DNA-binding active state and turns on the transcription of genes required for nitrogen assimilation (PubMed:11719184, PubMed:12139611, PubMed:25691471). Under conditions of nitrogen excess, feedback-inhibited GlnA forms a stable complex with TnrA, which inhibits its DNA-binding activity (PubMed:11719184, PubMed:12139611, PubMed:25691471). In contrast, feedback-inhibited GlnA acts as a chaperone to stabilize the DNA-binding activity of GlnR, which represses the transcription of nitrogen assimilation genes (PubMed:25691471).[1] [2] [3] [4] Publication Abstract from PubMedGlutamine synthetase (GS), which catalyzes the production of glutamine (Q), plays essential roles in nitrogen metabolism. There are two main bacterial GS isoenzymes, GSI-alpha and GSI-beta. GSI-alpha enzymes, which have not been structurally characterized, are uniquely feedback inhibited by Q. To gain insight into GSI-alpha function, we performed biochemical and cellular studies and obtained structures for all GSI-alpha catalytic and regulatory states. GSI-alpha forms a massive 600 kDa dodecameric machine. Unlike other characterized GS, the B. subtilis enzyme undergoes dramatic intersubunit conformational alterations during formation of the transition state. Remarkably, these changes are required for active site construction. Feedback inhibition arises from a hydrogen-bond network between Q, the catalytic glutamate and GSI-alpha specific residue, Arg62, from an adjacent subunit. Notably, Arg62 must be ejected for proper active site reorganization. Consistent with these findings, a R62A mutation abrogates Q-feedback inhibition but does not affect catalysis. Thus, these data reveal a heretofore unseen restructuring of an enzyme active site that is coupled with an isoenzyme-specific regulatory mechanism. This GSI-alpha-specific regulatory network could be exploited for inhibitor design against Gram-positive pathogens. Structures of the B. subtilis glutamine synthetase dodecamer reveal large intersubunit catalytic conformational changes linked to a unique feedback inhibition mechanism.,Murray DS, Chinnam N, Tonthat NK, Whitfill T, Wray LV, Fisher SH, Schumacher MA J Biol Chem. 2013 Oct 24. PMID:24158439[5] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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