Inert Site in a Protein Zinc Cluster: Isotope Exchange by High Resolution Mass Spectrometry

Abstract
Using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry, we show that one Zn2+ ion in the Zn4 cluster of cyanobacterial metallothionein is inert to exchange with 67Zn2+. We suggest that this is zinc in site A, which together with the surrounding α and β secondary structure forms a zinc-finger fold.