Characterization of the pH-dependent dissociation of a multimeric metalloprotein Streptomyces rubiginosus xylose isomerase by ESI FT-ICR mass spectrometry

We report an analysis of the pH-dependent dissociation of a multimeric metalloprotein, xylose isomerase from Streptomyces rubiginosus (XI), by electrospray ionization (ESI) Fourier transform ion cyclotron. resonance (FT-ICR) mass spectrometry. Xylose isomerases are industrially significant enzymes that catalyze interconversion of aldose and ketose sugars. XI is biologically active as a similar to 173-kDa tetrameric complex, comprised of four identical similar to 43-kDa subunits and eight metal cations, unequivocally identified as the Mg2+ cations in this work. ESI FT-ICR mass spectra of XI measured in the pH range of 3.0-6.9 indicated that the dissociation of the intact holo-tetramer is initiated by the loss of all eight Mg2+ cations at pH <= 5.0, followed by step-by-step dissociation of the remaining apo-tetramer to trimers, dimers and monomers. In addition, a similar to 346-kDa protein octamer was detected at pH 6.9. Copyright (c) 2008 John Wiley & Sons, Ltd.