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A case study (see Strub et al., 2003, Structure, 11, 1359-1367)

The extend of incorporation of selenium atoms in Protegrin-3 precursor (ProS) overexpressed in the BL21(DE3)Sel strain was monitored by ES-MS (J.M. Strub in A. Van Dorsselaer lab., LSMBO, Strasbourg).

Native ProS (4 cysteine residues) and HisTagged-ProS (1 methionine and 4 cysteine residues) as well as their selenium derivatives overexpressed in the cysteine auxotroph E. coli strain BL21(DE3)SelD.

    The spectrum clearly indicates that the main compound contains five atoms of selenium corresponding to the four cysteines and the single methionine of the sequence.
       Notice that the replacement of a sulfur atom (32.06 Da) for a selenium one (78.96 Da) increases the molecular weight of 46.9 Da. An approximate percentage of selenium incorporation was calculated for each protein.


* For each spectrum the percentages were calculated from the intensity of peaks belonging to the highest group of identically charged ions.

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