Most metabolic enzymes form oligomeric complexes, but the catalytic consequences of different oligomerization states are not always clear. Here, we characterize the structure-function consequences of different oligomeric states of human glutamine synthetase (GS) by cryo-EM and biochemical characterization. GS is predominantly found in a decameric form (a dimer of pentamers) but can also be found in pentameric and filament structures. While GS filaments are a form of negative regulation of the enzyme, little is known about the consequences of the pentameric form. Here, we stabilized the pentamer state by mutation (P155A) and found that this variant displayed catalytic defects. Cryo-EM reconstruction of the pentameric state of GS P155A suggests high flexibility for the catalytic loop and adjacent loops that are relatively more stable in the decameric form. To understand the molecular basis of active site loop stabilization in the decamer we used 3D variability analysis and MD ensemble refinement to explore the conformational space of the GS decamer under turnover conditions to reveal a global rotational motion coupled to loop stability. Mutations that impact global rotation motion display subtle catalytic changes. Taken together, these data demonstrate large oligomeric-state dependent conformational changes to pivotal catalytic loops in glutamine synthetase. Glutamine synthetase plays a pivotal role in the global nitrogen cycle and is essential in humans as it converts neurotoxic ammonia into glutamine, and our data reveal the need to consider oligomeric state as a potent regulator of enzymatic activity.
Tecson et al. (Sun,) studied this question.