High-resolution tandem mass spectrometry of carbonic anhydrase.

Electrospray ionization of carbonic anhydrase with ion dissociation yields a mass spectrum from which the masses of > 100 isotopic clusters are determined accurately, with the number of charges assigned directly from resolved isotopic peaks. Of these clusters, 80% correspond to fragmentation at or near the amino acid proline. The masses of combinations of two, three, and four of these clusters sum to the molecular mass with 0.1-Da accuracy, while further fragment ion dissociation provides additional sequence information. The capability of this methodology to detect sequence variations is illustrated with an isozyme having a single amino acid replacement.