Zinc transfer from the embryo-specific metallothionein EC from wheat: a case study

Abstract

The direct observation of binding and release of spectroscopically silent metal ions such as Zn²⁺ and Ca²⁺ by proteins has been challenging before the advent of native electrospray ionisation mass spectrometry. This report highlights the powerful capability of ESI-MS to provide insight into metalloprotein speciation that is independent of any spectroscopic property. Using the zinc-binding plant metallothionein E_C from wheat as a study case, we show that ESI-MS is unique amongst other techniques in capturing intermediary metallospecies that evolve during the course of metal transfer to the chelator EDTA, as a model reaction to mimic the biological function of the protein as a zinc donor. Zinc release from the two-domain protein E_C appears to be extremely rapid and non-cooperative, and progresses with loss of one zinc ion from the fully loaded Zn₆ species, and a transient build-up of Zn₅ and Zn₄ species, which further react to give species with 0-3 zinc ions bound. ¹H NMR data has provided further insights into the different behaviour of the two domains upon metal depletion.