An uncharged protein (P) has an Asp in position 23 with a molecular weight of 6501 Da, as determined by mass spectrometry. The uncharged mutant of this protein (P') contains a single amino acid substitution with Asn at position 23. The molecular weight of P', as determined by mass spectrometry (rounded off to one decimal place) is ____Da.
:)
That would make for a nice homework question..!
Given that the difference in molecular weight of aspartic acid (Asp) and asparagine (Asn) is 1 Da, the molecular weight of the mutant is 1 (or actually 0.98402) Da lower than that of the original protein.
The answer that should be filled in therefore is 6500.
And for some bonus information: the opposite conversion (from Asn to Asp) is observed quite often in proteins and is the result of a chemical reaction referred to as 'deamidation' leading to a mass increase of 1 Da.
It depends a little what you intend to achieve with this question. First, any molecule that is to be analysed by MS needs to have a charge. Second, in proteomic MS experiments, the eluate (or if you are not eluting from an LC – the “infusate” from a syringe pump) is acidified. Thus, the Asp will be protonated, that is uncharged, in a normal MS analysis. Third, in most proteomic experiments, the protein will be digested with a protease, commonly trypsin, and thus be cleaved into several peptides. Fourth, if you analyse the intact protein, it is approximately 100% sure that several protons will stick to the protein molecule, but the number needs not to be the same for every molecule. Thus, you will have a population of protein molecules where a few have 1 proton (these will exeedingly few), some more molecules will have 2 protons, even more will have 3 or 4 protons, and then fewer will have 5 or 6 or 7 protons. The distribution will depend on several factors, including how many Arg or Lys there are in the protein. You can find the mass M by the formula m/z = (M + nxprotons)/n where n is the charge. You must always remember that you are not measuring the mass of a neutral molecule in MS, it is the m/z value. In any case, if the only change in the protein is Asp -> Asn , you have a mass loss of 0.98 Da.
I agree with the two last answers! Also at this mass range and if you use electrospray, take care with the use of monoisotopic or average masses!!
- Badges
- Science topic
- Similar topics
- Biological Science
- Molecular Biology
- Biomolecules
- Proteins