High resolution mass spectrometry of metalloproteins
Johnson, Keith Anthony
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ESI-FTICR mass spectrometry is used to determine the stoichiometry of iron-sulfur cluster proteins. The iron-sulfur clusters range from mononuclear iron-containing rubredoxin to ferredoxin that contains [2Fe-2S], [3Fe-4S] and [4Fe-4S] clusters as well as metal-substituted [3FeMn-4S] and [4Ga-4S] clusters. Positive and negative ionization mode mass spectra are compared for these metalloproteins. An increase in ion stability is observed in the mass spectra for some iron-sulfur cluster proteins in negative mode ESI compared to their spectra obtained in positive ion ESI.|High resolution and high mass accuracy are two characteristics of FTICR that make the technique useful for metalloprotein studies distinguishing between mass differences associated with disulfide bond formation, changes in oxidation state of the metal or metal cluster, and changes in mass due to post-translational modifications. The formation of disulfide bonds between free cysteine ligands upon metalloprotein denaturation leads to changes in protein molecular weight by as little as two mass units. The oxidation state of the metal or metal cluster in a metalloprotein is derived from the difference between the calculated and experimental masses, and the biologically relevant oxidation state of the metalloprotein is observed. Posttranslational modifications such as glycosylation can greatly affect the measured molecular weight, whereas modifications such as amidation have only a small effect. The high potential iron-sulfur protein from Chromatium vinosum embodies all of these complications.|The chemically and on-line electrochemically reduced forms of several metalloproteins are observed using ESI-FTICR. Normal sample handling procedures produce mass spectra of metalloproteins in their oxidized forms. After chemical or electrochemical reduction and rigorous exclusion of oxygen, the reduced forms of metalloproteins are detected in the gas phase. The oxidation state of the metal center is stable with respect to electrospray ionization in both positive and negative ionization modes.