Large-Scale Differentiation and Site Specific Discrimination of Hydroxyproline Isomers by Electron Transfer/Higher-Energy Collision Dissociation (EThcD) Mass Spectrometry.

3- and 4-Hydroxyprolines (HyP) are regioisomers that play different roles in various species and organs. Despite their distinct functions inside cells, they are generally considered indistinguishable using mass spectrometry due to their identical masses. Here, we demonstrate, for the first time, that characteristic w ions can be produced by electron-transfer/higher energy collision dissociation (EThcD) dual fragmentation technique to confidently discriminate 3-HyP/4-HyP isomers. An integrated and high throughput strategy was developed which combined online LC separation with EThcD for large-scale differentiation of 3-HyP/4-HyP in complex samples. An automated algorithm was developed for charge state dependent characterization of 3-HyP/4-HyP isomers. Using this combined discrimination approach, we identified 108 3-HyP sites and 530 4-HyP sites from decellularized pancreas, allowing more than 5-fold increase of both 3-HyP and 4-HyP identifications compared to previous reports. This approach outperformed ETD and HCD in the analysis of HyP-containing peptides with unique capacity to generate w ions for HyP discrimination, improved fragmentation of precursor ions, as well as unambiguous localization of modifications. A high content of 3-HyP was observed in the C-terminal (GPP) domain of human CO1A1, which was previously only identified in vertebrate fibrillar collagens from tendon. Unexpectedly, some unusual HyP sites at Xaa position in Gly-HyP-Ala, Gly-HyP-Val, Gly-HyP-Gln, Gly-HyP-Ser, and Gly-HyP-Arg were also confirmed to be 3-hydroxylated, whose functions and enzymes are yet to be discovered. Overall, this novel discrimination strategy can be readily implemented into de novo sequencing or other proteomic search engines.