Covalent Attachment and Dissociative Loss of Sinapinic Acid to/from Cysteine-Containing Proteins from Bacterial Cell Lysates Analyzed by MALDI-TOF-TOF Mass Spectrometry

Western Regional Research Center, Agricultural Research Service, United States Department of Agriculture, Albany, California 94710, USA.
Journal of the American Society for Mass Spectrometry (Impact Factor: 2.95). 05/2010; 21(5):819-32. DOI: 10.1016/j.jasms.2010.01.013
Source: PubMed


We report covalent attachment via a thiol ester linkage of 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid or SA) to cysteine-containing protein biomarkers from bacterial cell lysates of E. coli analyzed by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry when using SA as the matrix. Evidence to support this conclusion is the appearance of additional peaks in the MS spectra when using SA, which are absent when using alpha-cyano-4-hydroxycinnamic acid (HCCA). The additional peaks appear at a mass-to-charge (m/z) approximately 208 greater to the m/z of a more abundant protein ion peak. Protein biomarkers were identified by tandem mass spectrometry (MS/MS) using a MALDI time-of-flight/time-of-flight (TOF-TOF) mass spectrometer and top-down proteomics. Three protein biomarkers, HdeA, HdeB, and homeobox or YbgS (each containing two cysteine residues) were identified as having reactivity to SA. Non-cysteine-containing protein biomarkers showed no evidence of reactivity to SA. MS ions and MS/MS fragment ions were consistent with covalent attachment of SA via a thiol ester linkage to the side-chain of cysteine residues. MS/MS of a protein biomarker ion with a covalently attached SA revealed fragment ion peaks suggesting dissociative loss SA. We propose dissociative loss of SA is facilitated by a pentacyclic transition-state followed by proton abstraction of the beta-hydrogen of the bound SA by a sulfur lone pair followed by dissociative loss of 3-(4-hydroxy-3,5-dimethoxyphenyl)prop-2-ynal. The apparent reactivity of SA to cysteine/disulfide-containing proteins may complicate identification of such proteins, however the apparent differential reactivity of SA and HCCA toward cysteine/disulfide-containing proteins may be exploited for identification of unknown cysteine-containing proteins.

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    • "cysteine to an a,b-unsaturated matrix was confirmed by the MS 2 data. Inversely, under acidic conditions reported for identification of cysteine-containing proteins, [19] only minor adducts were observed, and the formation of these adducts was influenced by the peptide amino acid composition and the matrix. Our study is of great importance for controlling the peptide–matrix adducts in biochemical analysis of cysteinecontaining proteins by MALDI-TOF MS. "

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