Metal-ion binding properties of the transferrins: a vanadium-51 NMR study

Transferrins can bind a wide range of di- and trivalent metal ions. They have a bilobal structure where each domain contains a deep cleft that binds a metal ion along with a synergistic anion. In this work, the binding of vanadate as VO2+ to the transferrins was studied by 51V quadrupolar central transition (QCT) NMR. Information about differences in the symmetry and motion of the bound metal ion was obtained from chemical shift and line width differences for serotransferrin (sTf), lactoferrin (lTf), and ovotransferrrin(oTf). The effects of pH, ionic strength, and temperature on the 51V QCT NMR spectra of the bound VO2+ cations showed that the N-lobe binding site of sTf is unique as compared to the other proteins. Properties of the quadrupolar central transition were also investigated, revealing that temperature, magnetic field strength, and NMR pulse angle all induce predictable changes on the second-order dynamic frequency shift, spectral line width, and optimal pulse angle in the 51V NMR spectra. Analysis of NMR spectra of V(V)2-oTf and V(V)2-sTf at three magnetic fields allowed an estimation of the quadrupolar coupling constants for these binding sites. This indicates that the degree of coordination symmetry in the binding sites is as follows: sTf N < sTf C < oTf N, C. Carbon-13 NMR studies revealed that VO2+ binding, in contrast to di- and trivalent metal ions, has no requirement for a synergistic anion.