Trafficking of hydrolysis-prone metals titanium(IV) and iron(III) by serum proteins and models.

摘要

Transferrins are a family of proteins that participate in Fe(III) binding and transport. Modern transferrins are typically bilobal and are believed to have evolved from an ancient gene duplication of a monolobal form. A monolobal transferrin, nicatransferrin (nicaTf), was identified in the primitive ascidian species Ciona intestinalis. Fe(III)-bound nicaTf displays important spectral differences from, and weaker affinity than other Fe(III)-bound transferrins, which could be due to differences in metal coordination. This work provides insight into the structure and function of primitive transferrins and suggests that a major advantage for the evolution of modern transferrins is stronger Fe(III) affinity.;Human serum transferrin (HsTf) can bind two equivalents of Ti(IV) in unhydrolyzed or hydrolyzed forms. Ti(IV) coordination by N,N'-di(o-hydroxybenzyl)ethylenediamine- N,N'-diacetic acid (HBED) at different pH values models the two forms of Ti2-HsTf spectrally and structurally. The crystal structure of TiHBED obtained at low pH models transferrin coordination of unhydrolyzed Ti(IV) in a distorted octahedral environment with bound carboxylate synergistic anion. In this form transferrin binds Ti(IV) with greater affinity than Fe(III). At higher pH the complex TiOHBED was isolated, which mimics hydrolyzed Ti(IV) bound transferrin. Thermal stability, competition and redox studies facilitate elucidation of the mechanism of Ti2-HsTf transport in cells.;The current work also explores other Ti(IV) interactions in serum. Two equivalents of Ti2-HsTf bind the transferrin receptor TfR1 with the tightest values yet measured for a metal ion other than Fe(III). The formation of a M2-HsTf complex of high affinity appears to be correlated with a lobe-closed conformation that leads to a favorable interaction with TfR1. Human serum albumin (HSA), an important serum competitor for metal binding, can bind up to twenty equivalents of Ti(IV) supplied in several forms. The dominant mode of HSA binding is via Ti(IV) complex form. Notably, HSA greatly stabilizes the titanocene moiety of the drug candidate Cp2TiCl2 with respect to hydrolysis and precipitation. SA binds Ti(citrate) and the aromatic-based tris(2,3-naphthalenediolato)titanate(IV) complex. The nature of the chelating ligand dictates where the metal ion will be bound. Each of these findings has implications for the metabolism of Ti(IV) in human serum.