Reference no: EM13310845

You have purified a protein (which you call protein X) that you suspect to be covalently modified by the addition of a single ubiquitin molecule. You also know that your purified protein is very unique, in that it only contains one lysine amino acid in its entire primary sequence. The molecular weight of the unmodified form of protein X is 60,000 Daltons. Suppose you also know that the c-terminal sequence of ubiquitin is HOOC-Gly-Gly-Lys-Ile-Gln-etc. where the indicated lysine at position 3 is also the only lysine amino acid in the ubiquitin sequence. You are now trying to confirm that your protein does indeed carry this ubiquitin modification. Suppose you add to your purifed protein X the enzyme LysC, a serine protease that specifically recognizes the protonated amine group on lysine side chains, and hydrolyzes the peptide bond on the c-terminal side of lysine. Assume the solution is buffered to pH 7 and the pKa of the lysines in question is > 10. Based on the information above, answer the following two questions: i) If protein X is modified by ubiquitin, how many polypeptide products would you expect to create if it is treated with LysC? Briefly explain why. ii) What would be the approximate molecular weight of the different peptide products?