I seem to be consistently getting a mass of 34167.5 instead of 34298.7 for my recombinantly expressed intact protein ( expressed in E. coli). This corresponds to an exact loss of a methionine residue. Is this a common occurence?
Hi Bharat,
Yes, it is. During N-terminal sequence analysis (Edman degradation) of recombinant protein preparations, we have seen many samples with and without the Methionine as first residue.
Best,
Hediye
Thank you Hediye! Do u know what might be a plausible explanation for such missing methionine. Is it some enzyme that specifically cleaves the N-term methionine? Thank you for your response.
Yes, I believe that it is the methionine aminopeptidase enzyme that is responsible for this event taking place.
Here is a link regarding research on the subject:
Hello Bharath,
i discovered the same mass shift problem in my protein and was also thinking about a missing methionine. But the mass of methionine is 149.21 Da and my mass shift was always around 135-138 Da. I saw that you had a mass shift of 131.2 Da. Can you explain me how this mass can fit for methionine?
Hope you can help me further!
Best regards,
Anastassja
Hi Anastassja,
I am not sure about 135-138 Da but 131.2 Da would correspond to the mass of a methionine in a protein corresponding to the loss of 16+1+1 Daltons corresponding to the carboxylic OH and amino terminus H ( the latter depending on the pH at which one is operating).
In your case, the numbers seem up by 4-5 Daltons even after the loss of a methionine residue. What pH are you working in?
Regards,
Bharath
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