Probing higher order multimers of pyruvate kinase with charge detection mass spectrometry

Pyruvate kinase multimers have been investigated by charge detection mass spectrometry (CDMS). In CDMS, the m/z and z are simultaneously measured for each ion, so the mass is determined directly. The measurements were made using a modified cone trap that incorporates an image charge detector with a cryogenically cooled preamplifier. With non-denaturing solution conditions, the tetrameric form of pyruvate kinase is observed along with aggregates of the tetramer. The time-of-flight m/z spectrum shows octamers and dodecamers. However, the lack of charge state resolution prevents identification of larger multimers. Multimers up to the 40-met are revealed by CDMS. Their intensities fall-off exponentially with size. Evidence supporting a non-specific, solution-based aggregation mechanism is presented. The relationship between the m/z and mass of the multimers is consistent with the predictions of the charge residue model. Pyruvate kinase ions are held in the cone trap for up to 129 ms. With this long trapping time the root mean square deviation in the charge determination is reduced to 1.3 elementary charges. (C) 2013 Elsevier B.V. All rights reserved.