Quaternary structure of Azospirillum brasilense NADPH-dependent glutamate synthase in solution as revealed by synchrotron radiation x-ray scattering

Azospirillum brasilense glutamate synthase (GltS) is the prototype of bacterial NADPH-dependent enzymes, a class of complex iron-sulfur flavoproteins essential in ammonia assimilation processes. The catalytically active GltS alphabeta holoenzyme and its isolated alpha and beta subunits (162 and 52 kDa, respectively) were analyzed using synchrotron radiation x-ray solution scattering. The GltS alpha subunit and alphabeta holoenzyme were found to be tetrameric in solution, whereas the beta subunit was a mixture of monomers and dimers. Ab initio low resolution shapes restored from the scattering data suggested that the arrangement of subunits in the (alphabeta)(4) holoenzyme is similar to that in the tetrameric alpha(4) complex and that beta subunits occupy the periphery of the holoenzyme. The structure of alpha(4) was further modeled using the available crystallographic coordinates of the monomeric alpha subunit assuming P222 symmetry. To model the entire alphabeta holoenzyme, a putative alphabeta protomer was constructed from the coordinates of the alpha subunit and those of the N-terminal region of porcine dihydropyrimidine dehydrogenase, which is similar to the beta subunit. Rigid body refinement yielded a model of GltS with an arrangement of alpha subunits similar to that in alpha(4), but displaying contacts also between beta subunits belonging to adjacent protomers. The holoenzyme model allows for independent catalytic activity of the alphabeta protomers, which is consistent with the available biochemical evidence.