Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

2.4 Structural Studies of Glutamine Synthetase (GS) from A. brasilensej35
Figure 2.8 Comparison of Mössbauer parameters–isomer
shift (IS, mm s 1 ; relative to a-Fe) and quadrupole splitting (QS,
mm s 1 ) – for different forms of [ 57 Co]-cobalt(II) in 57 Co II -doped
glutamine synthetase from A. brasilense Sp245 (1) in rapidly
frozen aqueous solution and in the solid (dried) state (2)
(measured at T ¼ 80 K).
cobalt(II) is possible [46–48], although in many proteins cobalt was found to have
preference for higher coordination numbers, i.e. 5 and 6 (see [47] and references
cited therein).
2.4.3.3 Conclusions and Outlook
The EMS technique, recently applied for the first time to probing cation binding in
the active centers of a bacterial enzyme doped with the radioactive 57 Co isotope
[36,44], has shown that each active center of glutamine synthetase from azospirilla
has two cation-binding sites with different affinities to cobalt(II) as an activating
cation and with different coordination symmetry. The results obtained are in good
agreement with the current literature data on the structural organization of the active
centers in bacterial adenylylatable glutamine synthetases [39,41].
For future structural investigations of the active centers in metal-containing
biocomplexes and enzymes, the advantages of using the highly sensitive and selective
emission variant of Mössbauer spectroscopy can hardly be overestimated. This
nuclear chemistry technique has recently been shown to be sensitive also (i) to the
effects of competitive binding of different activating cations (Mn 2þ þ 57 Co 2þ , with a
redistribution of the latter between the two sites in GS) at the active centers, showing
that heterobinuclear two-metal-ion catalysis by GS is principally possible, as well as
(ii) to fine structural changes induced by covalent modifications of the enzyme
molecule related to its activity [50]. The results obtained are highly promising for