Peptide mass determination. You have isolated a protein from...

Peptide mass determination. You have isolated a protein from the bacterium $E .$ coli and seek to confirm its identity by trypsin digestion and mass spectrometry. Determination of the masses of several peptide fragments has enabled you to deduce the identity of the protein. However, there is a discrepancy with one of the peptide fragments, which you believe should have the sequence MLNSFK and an $(\mathrm{M}+\mathrm{H})^{+}$ value of $739.38 .$ In your experiments, you repeatedly obtain an $(\mathrm{M}+\mathrm{H})^{+}$ value of $767.38 .$ What is the cause of this discrepancy and what does it tell you about the region of the protein from which this peptide is derived?

Key Concepts

Peptide Mass Fingerprinting

Peptide mass fingerprinting is a method for identifying proteins by digesting them into peptides, measuring the masses of these fragments, and matching the resulting pattern to theoretical patterns generated from protein databases. Any discrepancies in expected versus observed peptide masses can hint at the presence of modifications or unusual processing events.

N-Terminal Modifications in Bacteria

In bacteria, protein synthesis typically initiates with a formylated methionine (formylmethionine). Although the formyl group is often removed during post-translational processing, its retention results in an increased peptide mass, typically by about 28 Da. This modification indicates that the peptide is derived from the very N-terminal region of the protein, providing insight into the processing status of that region.

Proteolytic Digestion

Proteolytic digestion involves the enzymatic cleavage of proteins into smaller peptide fragments. Enzymes like trypsin cleave at specific amino acid residues, and the predictable nature of these cleavages allows for the calculation of expected peptide masses, which are then used as part of the protein identification process.

Mass Spectrometry

Mass spectrometry is an analytical technique used to measure the masses of molecules, including peptides, with high accuracy. In proteomics, it enables the identification of proteins by comparing the measured masses of peptide fragments to theoretical values computed from known protein sequences.

Transcript

Please give Ace some feedback