Determination of unique amino acid substitutions in protein variants by peptide mass mapping with FT-ICR MS

Peptide mass mapping plays a central role in the structural characterization of protein variants with single amino acid substitutions. Among the 20 standard amino acids found in living organisms, 18, all but Leu and Ile, differ from each other in molecular mass. The mass differences between amino acids range from 0.0364 to 129.0578 Da. The mass of the mutated peptide or the difference between normal and mutated peptides uniquely determines the type of substitution in some cases, and even pinpoints the position of the mutation when the involved residue is found only once in the peptide. Among 75 pairs of amino acid residues that are exchangeable via a single nucleotide replacement, 53 show specific change in exact mass, while only 25 in nominal mass. On the other hand, precise measurement, at least to the third decimal place, greatly enhances the capacity of the peptide mass mapping strategy for structural characterization. This notion was verified by an analysis of three Hb variants using MALDI-FTICR MS. In addition, the baseline resolution of two 1 kDa peptides with a single amino acid difference, Lys or Gln, which have the smallest (0.0364 Da) difference among residues, was achieved by measurement at a mass resolving power of 342,000. The results indicated that the smallest difference, 0.0040 Da between [Delta29.9742 for Glu-Val] and [Delta29.9782 for Trp-Arg], among all types of amino acid substitutions derived from a single nucleotide replacement can be discriminated at the present performance level. Therefore, FTICR MS is capable of identifying all 53 types of substitutions, each of which is associated with a unique mass difference, except for the Leu and Ile isomers.