title: Deamidation and transamidation of substance P by tissue transglutaminase revealed by electron-capture dissociation fourier transform mass spectrometry.
abstract: Tissue transglutaminase (tTGase) catalyzes both deamidation and transamidation of peptides and proteins by using a peptidyl glutamine as primary substrate. A precise consensus sequence for the enzyme is unknown and the ratio between deamidated and transamidated (or cross-linked) reaction products is highly substrate-dependent. Due to its overlapping body distribution with tTGase and ease of manipulation with tandem mass spectrometry, we used the neuropeptide substance P as a model to investigate the associated enzymatic kinetics and reaction products. Online liquid-chromatography Fourier-transform ion-cyclotron-resonance mass spectrometry (FT-ICR MS) combined with electron-capture dissociation (ECD) was employed to study the tTGase-induced modifications of substance P. A particular strength of ECD for peptide-enzyme reaction product monitoring is its ability to distinguish isomeric amino acids, for example, Glu and iso-Glu, by signature product ions. Our studies show that the primary reaction observed is deamidation, with the two consecutive glutamine residues converted sequentially into glutamate: first Gln(5) , and subsequently Gln(6) . We then applied ECD FT-ICR MS to identify the transamidation site on an enzymatically cross-linked peptide, which turned out to correspond to Gln(5) . Three populations of substance-P dimers were detected that differed by the number of deamidated Gln residues. The higher reactivity of Gln(5) over Gln(6) was further confirmed by cross-linking SP with monodansylcadaverine (MDC). Overall, our approach described herein is of a general importance for mapping both enzymatically induced post-translational protein modifications and cross-linking. Finally, in vitro Ca-signaling assays revealed that the main tTGase reaction product, the singly deamidated SP (RPKPEQFFGLM-NH(2) ), has increased agonist potency towards its natural receptor, thus confirming the biologically relevant role of deamidation.
authors.affiliation: Biomolecular Mass Spectrometry Laboratory, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland.
authors.forename: Luca
authors.initials: L
authors.lastname: Fornelli
authors.forename: Adrien W
authors.initials: AW
authors.lastname: Schmid
authors.forename: Luigino
authors.initials: L
authors.lastname: Grasso
authors.forename: Horst
authors.initials: H
authors.lastname: Vogel
authors.forename: Yury O
authors.initials: YO
authors.lastname: Tsybin
mesh_terms: D000577:Amides / Q000737:chemistry*; D000595:Amino Acid Sequence; D000818:Animals; D005973:Glutamine / Q000032:analysis / Q000378:metabolism; D006168:Guinea Pigs; D006801:Humans; D008099:Liver / Q000201:enzymology; D012697:Serine Endopeptidases / Q000378:metabolism; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods; D013211:Staphylococcus aureus / Q000201:enzymology; D013373:Substance P / Q000737:chemistry* / Q000378:metabolism; D053719:Tandem Mass Spectrometry / Q000379:methods*; D011503:Transglutaminases / Q000737:chemistry* / Q000378:metabolism
chemical_list: D000577:Amides; D005973:Glutamine; D013373:Substance P; D011503:Transglutaminases; D012697:Serine Endopeptidases; C028344:glutamyl endopeptidase