B1 domain of staphylococcal protein G (GB1) is a widely used model protein for developing in vivo and in vitro protein structural determination methods based on paramagnetic nuclear magnetic resonance (NMR) such as pseudocontact chemical shift (PCS) and paramagnetic relaxation enhancement (PRE). However, few previous studies have investigated the interactions between GB1 and metal ions, especially paramagnetic ions. In this study, the interactions between GB1 and divalent/lanthanide metal ions were studied by NMR spectroscopy. It was found that GB1 weakly bound with paramagnetic lanthanide ions and paramagnetic divalent ions, including Cu2+, Mn2+ and Co2+. In contrast, GB1 did not bind with diamagnetic divalent ions, such as Ca2+, Mg2+ and Zn2+. Furthermore, it was demonstrated that there were two binding sites for Cu2+ in GB1, but only one for lanthanide ions and divalent ions Mn2+ and Co2+. The current study demonstrated that NMR spectroscopy is a powerful tool to study weak binding between protein and metal ions. And the results indicated that care must be taken to avoid possible interference to paramagnetic NMR data when using GB1 as the model protein.
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