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 ß-subunit (ßGltS) that assemble in different αß oligomers in solution. Here, we present the cryo-electron microscopy structures of Azospirillum brasilense GltS in four different oligomeric states (α4ß3, α4ß4, α6ß4 and α6ß6, 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 ßGltS.

The subnanometer resolution structure of the glutamate synthase 1.2-MDa hexamer by cryoelectron microscopy and its oligomerization behavior in solution: functional implications.

The three-dimensional structure of the hexameric (alphabeta)(6) 1.2-MDa complex formed by glutamate synthase has been determined at subnanometric resolution by combining cryoelectron microscopy, small angle x-ray scattering, and molecular modeling, providing for the first time a molecular model of this complex iron-sulfur flavoprotein. In the hexameric species, interprotomeric alpha-alpha and alpha-beta contacts are mediated by the C-terminal domain of the alpha subunit, which is based on a beta helical fold so far unique to glutamate synthases. The alphabeta protomer extracted from the hexameric model is fully consistent with it being the minimal catalytically active form of the enzyme. The structure clarifies the electron transfer pathway from the FAD cofactor on the beta subunit, to the FMN on the alpha subunit, through the low potential [4Fe-4S](1+/2+) centers on the beta subunit and the [3Fe-4S](0/1+) cluster on the alpha subunit. The (alphabeta)(6) hexamer exhibits a concentration-dependent equilibrium with alphabeta monomers and (alphabeta)(2) dimers, in solution, the hexamer being destabilized by high ionic strength and, to a lower extent, by the reaction product NADP(+). Hexamerization seems to decrease the catalytic efficiency of the alphabeta protomer only 3-fold by increasing the K(m) values measured for l-Gln and 2-OG. However, it cannot be ruled out that the (alphabeta)(6) hexamer acts as a scaffold for the assembly of multienzymatic complexes of nitrogen metabolism or that it provides a means to regulate the activity of the enzyme through an as yet unknown ligand.