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:

Thanks a lot Hediye! your answers were extremely helpful.

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

Dear Bharath,

i don't think that something is lost. I found out that this is just the monoisotopic mass of Methionine. And in the protein mass spectrum we would always go for the monoisotopic peak of the protein.

Best regards,

Anastassja