ECD and ETD both involve the reaction of an electron with a highly protonated cation to form an odd-electron peptide. This process induces large amounts of backbone cleavage to yield c- and z-ions that are analogous to the b- and y-ions produced from CAD. Unlike CAD, ECD and ETD cleavage is very weakly affected by the composition and number of amino acids in the peptide and provides more fragmentation coverage than CAD alone. Both ECD and ETD also prevent cleavage of labile modifications and these PTMs are fully present on the c- and z-ions produced during cleavage. The aforementioned benefits make ECD and ETD well-suited for the "top-down" analysis of post-translationally modified proteins. A further benefit of ECD/ETD analysis is the quantitative information obtained from the spectral data. A full protein sequence is eluted into the spectrophotometer, so it is expected that the resulting ratios of peaks intensities will correspond quite closely to concentration ratios of the ions. If two proteins with slightly different modifications eluted at the same time, measurement of the peak intensity ratios can give a good approximation of the ratio of protein present in the sample.
Both ECD and ETD were used to extensively study the modifications on histone H3 [8]. The variety of modifications present on the N-terminus of the protein represents a "histone code" that must be fully characterized to understand the cellular processes in which H3 is involved. To fully appreciate the depth of potential modification combinations for histone H3, hydrophilic interaction chromatography (HILIC) was used in combination with gas-phase isolation of species with different parent masses. HILIC chromatography will separate the different H3 proteins mainly by number of acetyl groups and secondly by degree of methylation. Such a separation will allow for better validation of the spectral data by comparing the potential modifications with the elution time. Consequent analysis has revealed potential relationships between K14, K18, and K23 acetylations as well as interplay between K4 methylation and the previously mentioned acetylations.
In this work, we will present de novo methods based on mixed-integer linear programming (MILP) for the identification of post-translationally modified peptides using ETD and ECD tandem mass spectra, with a particular emphasis on the study of histone H3. Though a vast amount of de novo and database methods have been developed to analyze unmodified spectra [9-11], the ETD/ECD can illuminate certain in vivo features such as internally cleaved sequences and spliced sequences that cannot be determined once the original sample is digested with trypsin. To our knowledge, the only currently available software for the analysis of ETD spectra is the Open Mass Spectrometry Search Algorithm [12], which is a database method. We will present a de novo mixed-integer linear programming (MILP) model developed for the analysis of H3 histone that utilizes the total number of possible modifications, given modified parent mass, to derive the most probable set of PTMs. We will also present a generalized de novo method for the identification of modified and unmodified proteins using mixed-integer linear optimization and ETD/ECD tandem mass spectrometry.
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