Cryo-EM Structures of Azospirillum brasilense Glutamate Synthase in Its Oligomeric Assemblies.

Bacterial NADPH-dependent glutamate synthase (GltS) is a complex iron-sulfur flavoprotein that catalyzes the reductive synthesis of two L-Glu molecules from L-Gln and 2-oxo-glutarate. GltS functional unit hosts an α-subunit (αGltS) and a β-subunit (βGltS) that assemble in different αβ oligomers in solution. Here, we present the cryo-electron microscopy structures of Azospirillum brasilense GltS in four different oligomeric states (α ₄ β ₃ , α ₄ β ₄ , α ₆ β ₄ and α ₆ β ₆ , in the 3.5- to 4.1-Å resolution range). Our study provides a comprehensive GltS model that details the inter-protomeric assemblies and allows unequivocal location of the FAD cofactor and of two electron transfer [4Fe-4S] ^+1,+2 clusters within βGltS.
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