Regulating Protein Degradation - Biochemistry

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Question

A polypeptide when treated with trypsin yielded the following fragments:

(AM) (SAK) (YMPLWGIR)

The same polypeptide treated with chymotrypsin yielded the following fragments:

(MPLW) (GIRAM) (SAKY)

Which of the following displays the original sequence of the polypeptide?

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Answer

This problem requires knowledge of the endopeptidase properties. Trypsin cleaves after the amino acids lysine and arginine, and chymotrypsin cleaves after the amino acids phenylalanine, tyrosine, and tryptophan.

Using this information, the slashes below indicate where each enzyme cleaved the polypeptide.

(AM) (SAK)/ (YMPLWGIR)/ trypsin

(MPLW)/ (GIRAM) (SAKY)/ chymotrypsin

Because the (AM) and (GIRAM) fragments have no slash after M in both the first and second treatment, one can conclude that this piece is at the end of the polypeptide. The other two pieces are sequenced by realizing that S starts the (SAK) and (SAKY) fragments and is never found in the middle of a fragment.

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