Metal site conformational states of vanadyl(IV) human serotransferrin complexes

Publisher Website
Google Scholar
Abstract
The conformational states of the 2 metal sites in the human serum transferrin molecule were investigated. The 9.2 GHz EPR spectra of frozen solutions of divanadyl(IV) transferrin consist of a superposition of 2 sets of resonances, A and B, due to the magnetically nonequivalent binding environments of the VO2+ ion. Examination of the intensities of the A and B resonances as a function of pH from 6.0-10.7 reveals that they arise from 2 conformational states of the metal sites in which the geometrical arrangement and/or identity of 1 or more ligands in the 1st coordination sphere are different. From pH 7.5-9.0, the metal sites exist in A and B conformations but above pH 9.0 the A conformation undergoes a transition to the B conformation. This transformation is coupled to the ionization of an apparently noncoordinating protein functional group with a pK = 10.0 .+-. 0.1. Below pH 7.0, binding in the B conformation is rapidly lost, driven in part by the protonation of a functional group, possibly the anion, with a pK = 6.6 .+-. 0.1. In 90% D2O, this pK is elevated to 7.8 .+-. 0.1. At pH 6.0 in H2O, essentially one VO2+ ion remains bound to the protein with the metal site in the A conformation. Experiments with mixed VO2+-Fe3+ transferrin complexes indicate that the same may be true of Fe3+. At pH 10.7, a new set of VO2+ resonances, labeled C, are observed; they possibly arise from a 3rd conformation of the metal site. One bicarbonate or carbonate is required per VO2+ ion bound to the protein; 2.7 H+ are released per VO2+ bound in the A or B conformations. The results are discussed in terms of the equivalence and nonequivalence of the metal sites.

This publication has 1 reference indexed in Scilit: