Structural characterization of glutamine synthetase from Azospirillum brasilense

CD spectroscopic study of the secondary structure of partly adenylylated glutamine synthetase (GS) of the bacterium Azospirillum brasilense showed both the native and cation-free (EDTA-treated) enzyme to be highly structured (58 and 49% as (alpha)-helices, 10 and 20% as (beta)-structure, respectively). Mg^2+, Mn^2+, or Co^ 2+, when added to the native GS, had little effect on its CD spectrum, whereas their effects on the cation-free GS were more pronounced. Emission (57Co) Mossbauer spectroscopic (EMS) study of 57Co^2+-doped cation-free GS in frozen solution and in the dried state gave similar spectra and Mossbauer parameters for the corresponding spectral components, reflecting the ability of the Co^2+-enzyme complex to retain its properties upon drying. The EMS data show that (a) A. brasilense GS has 2 cation-binding sites per active center and (b) one site has a higher affinity to Co^2+ than the other, in line with the data on other bacterial GSs.