Molecular Biology Of The Cell 6th Edition by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter

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You have isolated the proteins from two adjacent spots after two-dimensional polyacrylamide-gel electro- phoresis and digested them with trypsin. When the masses of the peptides were measured by MALDI-TOF mass spec- trometry, the peptides from the two proteins were found to be identical except for one (Figure Q8-2). For this peptide, the mass-to-charge (m/z) values differed by 80, a value that does not correspond to a difference in amino acid sequence. (For example, glutamic acid instead of valine at one position would give an m/z difference of around 30.) Can you suggest a possible difference between the two peptides that might account for the observed m/z differ- ence?