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The ability to understand the function of a protein often relies on knowledge about its detailed structure. Sometimes, seemingly insignificant changes in the primary structure of a protein, like an amino acid substitution, can completely disrupt a protein's function. Long-lived proteins (LLPs), which can be found in critical areas of the human body, like the brain and eye, are especially susceptible to primary sequence alterations in the form of isomerization and epimerization. Because long-lived proteins do not have the corrective regeneration capabilities of most other proteins, points of isomerism and epimerization that accumulate within the proteins can severely hamper their functions and can lead to serious diseases like Alzheimer's disease, cancer and cataracts. Whereas tandem mass spectrometry (MS/MS) in the form of collision-induced dissociation (CID) generally excels at peptide characterization, MS/MS often struggles to pinpoint modifications within LLPs, especially when the differences are only isomeric or epimeric in nature. One of the most prevalent and difficult-to-identify modifications is that of aspartic acid between its four isomeric forms: l -Asp, l -isoAsp, d -Asp, and d -isoAsp. In this study, peptides containing isomers of Asp were analyzed by charge transfer dissociation (CTD) mass spectrometry to identify spectral features that could discriminate between the different isomers. For the four isomers of Asp in three model peptides, CTD produced diagnostic ions of the form c n +57 on the N-terminal side of iso-Asp residues, but not on the N-terminal side of Asp residues. Using CTD, the l - and d forms of Asp and isoAsp could also be differentiated based on the relative abundance of y - and z ions on the C-terminal side of Asp residues. Differentiation was accomplished through a chiral discrimination factor, R , which compares an ion ratio in a spectrum of one epimer or isomer to the same ion ratio in the spectrum of a different epimer or isomer. The R values obtained using CTD are as robust and statistically significant as other fragmentation techniques, like radical directed dissociation (RDD). In summary, the extent of backbone and side-chain fragments produced by CTD enabled the differentiation of isomers and epimers of Asp in a variety of peptides.
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Two-dimensional identification and localization of isomers in crystallin peptides using TWIM-MS
Recent studies have illuminated connections between spontaneous chemical reactions that cause isomerization at specific protein residues and various age-related diseases including cataracts and Alzheimer's. These discoveries provide impetus for better analytical methods to detect and characterize isomerization in proteins, which will enable a more complete understanding of the underlying relationship between these modifications and biology. Herein we employ a two-dimensional approach for identification of peptides isomers that also includes pinpointing of the modified residue. Collision-induced dissociation is used to fragment ions in the first dimension, followed by separation of the fragments with travelling-wave ion mobility. By comparing data obtained from both isomers, differences in either fragment-ion intensities or arrival-time distributions can be used to identify isomeric forms and the specific site of modification within the peptides. Synthetic peptide standards with sequences derived from long-lived proteins in the eye lens and isomerization at serine, aspartic acid, and glutamic acid were examined. Although both dimensions are capable of isomer identification, ion mobility is much better at determining the site of modification. In general, separation of isomeric forms by ion mobility is possible but does not follow predictable trends dictated by sequence or fragment-ion length. In most cases, however, the site of isomerization can be precisely determined.
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- Award ID(s):
- 1904577
- PAR ID:
- 10174898
- Date Published:
- Journal Name:
- The Analyst
- ISSN:
- 0003-2654
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0AN01036G
