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You have isolated the proteins from two adjacent spots after two-dimensional polyacrylamide-gel electrophoresis and digested them with trypsin. When the masses of the peptides were measured by MALDI-TOF mass spectrometry, the peptides from the two proteins were found to be identical except for one (Figure $Q 8-2$ ). 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 $m / z$ difference?

          You have isolated the proteins from two adjacent spots after two-dimensional polyacrylamide-gel electrophoresis and digested them with trypsin. When the masses of the peptides were measured by MALDI-TOF mass spectrometry, the peptides from the two proteins were found to be identical except for one (Figure $Q 8-2$ ). 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 $m / z$ difference?

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