Figure 1.

Overview of PTM analysis at the protein and peptide level. The principal steps for protein level PTM enrichment and MS analysis are represented on the left. Peptide level phosphorylation enrichment strategies are illustrated on the right. Isomeric isobars are proteins or peptides that may reveal the same amino acid sequence with equal numbers of PTMs but with different PTM configurations. The development of algorithms that search for non-redundant ions representing all possible PTM configurations enable the identification, quantification, and localization of all PTMs.