Protein backbone cleavage

Photodegradation. UV/Vis exposure can induce protein oxidation, aggregation, and backbone cleavage. For example, oxidation has been observed in the histidine residue of human growth hormone (hGH) exposed to photostability conditions (6.7 x 106 lux hours). The proposed oxidation mechanism and product is shown in Figure 136. 

A few PTM reactions also involve changes in protein backbone. These reactions include the hydrolytic cleavage of the peptide backbone by proteases, the anchoring of proteins to glycosylphosphotidylinositol (GPl) or cholesterol, and... 

Electron-capture dissociation (ECD) [89], where the multiple-charge ion captures an electron to produce cationic products. The electron capture results in a radical site in the ion, leading to more and different backbone cleavages in the protein. It has been argued that the fragmentation actually takes place at the electron-capture site, i.e., prior to internal energy redistribution. 

To this end, different ion fragmentation tools have been characterized with respect to phosphopeptide fragmentation, e.g., electron-capture dissociation (ECD) [31] and infrared multiphoton dissociation (IRMPD) [32]. An application of ECD in PTM analysis is the top-down protein characterization (Ch. 18.3.5) of carbonic anhydrase [33]. IRMPD is applied in the study on protein kinase C phosphorylation [30]. Both ECD and IRMPD were applied in a subsequent nano-ESI-FT-ICR-MS study on protein kinase A phosphorylation [34]. Combined ECD and IRMPD for multistage MS-MS in FT-ICR-MS was applied for phosphopeptide characterization [35]. ECD provides backbone cleavages (c- and z -ions) without H3PO4 loss, whereas in IRMPD the loss of H3PO4 is prominent and only a few backbone cleavages (b- andy-ions) are observed cf. Ch. 17.6.1). 

T3) within the protein backbone of the thyroglobuiin protein in the follicular lumen. Endocytosis followed by proteolytic cleavage of thyroglobuiin releases the iodothyronines into the circulation. A schematic outline of iodine metabolism, with emphasis on the formation and secretion of thyroid hormones, is shown in Figure 52-3. 

A particular form of CNB is (2-nitrophenyl)glycine (Npg). This artificial amino acid (Npg, Fig. 3.2-ID) was successfully incorporated into ion channels like the nicotinic acetylcholine receptor [22] by nonsense suppression, a technique developed by Peter Schultz and coworkers [23-26]. Irradiation (4 h, >360 nm) of proteins containing Npg led to peptide backbone cleavage in Xenopus oocytes [22],... 

Fe [129-138], Fe [126, 138-144], Ni° [145, 146], and v [147-150]) were employed to promote site-selective oxidative degradation of folded proteins under non-denaturing conditions. In the presence of oxidants (e.g., O2 or H2O2), hydroxy radicals or in situ-generated metal-oxo species abstracted an H atom at Ca. This event was followed by oxidation at Ca and cleavage of the Ca-N or Ca-C (O) bond [118, 126, 141, 142]. In those methods, backbone cleavage sites were determined by the affinity to metal complexes [127]. 

Figure 2 Collision induced fragmentation scheme for peptides and proteins. The initial ionizing proton is not shown and proton transfers are not shown for backbone cleavages (see text).

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