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Comparative assessment of site assignments in CID and electron transfer dissociation spectra of phosphopeptides discloses limited relocation of phosphate groups.

ABSTRACT: In large scale mass spectrometry-based phosphoproteomics, a current bottleneck is the unambiguous assignment of the phosphorylation site within the peptide. An additional problem is that it has been reported that under conditions wherein peptide ions are collisionally activated the phosphate group may migrate to a nearby phosphate group acceptor, thus causing ambiguity in site assignment. Here, we generated and analyzed a statistically significant number of phosphopeptides. Starting with a human cell lysate, we obtained via strong cation exchange fractionation nearly pure phosphopeptide pools from trypsin and Lys-N digestions. These pools were subjected to nano-LC-MS using an Orbitrap mass spectrometer that is equipped with both CID and electron transfer dissociation with supplemental activation (ETcaD) functionality. We configured a method to obtain sequentially both ETcaD and CID spectra for each peptide ion. We exploited the resistant nature of ETcaD toward rearrangement of phosphate groups to evaluate whether there is potentially phosphate group relocation occurring during CID. We evaluated a number of peptide and spectral annotation properties and found that for ∼75% of the sequenced phosphopeptides the assigned phosphosite was unmistakably identical for both the ETcaD and CID spectra. For the remaining 25% of the sequenced phosphopeptides, we also did not observe evident signs of relocation, but these peptides exhibited signs of ambiguity in site localization, predominantly induced by factors such as poor fragmentation, sequences causing inefficient fragmentation, and generally poor spectrum quality. Our data let us derive the conclusion that both for trypsin- and Lys-N-generated peptides there is little relocation of phosphate groups occurring during CID.

SUBMITTER: Mischerikow N

PROVIDER: S-EPMC2953911 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Data mining was performed on 28 330 unique peptide tandem mass spectra for which sequences were assigned with high confidence. By dividing the spectra into different sets based on structural features and charge states of the corresponding peptides, chemical interactions involved in promoting specific cleavage patterns in gas-phase peptides were characterized. Pairwise fragmentation maps describing cleavages at all Xxx-Zzz residue combinations for b and y ions reveal that the difference in basicity between Arg and Lys results in different dissociation patterns for singly charged Arg- and Lys-ending tryptic peptides. While one dominant protonation form (proton localized) exists for Arg-ending peptides, a heterogeneous population of different protonated forms or more facile interconversion of protonated forms (proton partially mobile) exists for Lys-ending peptides. Cleavage C-terminal to acidic residues dominates spectra from singly charged peptides that have a localized proton and cleavage N-terminal to Pro dominates those that have a mobile or partially mobile proton. When Pro is absent from peptides that have a mobile or partially mobile proton, cleavage at each peptide bond becomes much more prominent. Whether the above patterns can be found in b ions, y ions, or both depends on the location of the proton holder(s) in multiply protonated peptides. Enhanced cleavages C-terminal to branched aliphatic residues (Ile, Val, Leu) are observed in both b and y ions from peptides that have a mobile proton, as well as in y ions from peptides that have a partially mobile proton; enhanced cleavages N-terminal to these residues are observed in b ions from peptides that have a partially mobile proton. Statistical tools have been designed to visualize the fragmentation maps and measure the similarity between them. The pairwise cleavage patterns observed expand our knowledge of peptide gas-phase fragmentation behaviors and may be useful in algorithm development that employs improved models to predict fragment ion intensities.

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Comparative assessment of site assignments in CID and electron transfer dissociation spectra of phosphopeptides discloses limited relocation of phosphate groups.

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