Biochemistry : Regulating Protein Degradation

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Regulating Protein Degradation

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?

Possible Answers:

WGIRAMSAKYMPL

GIRAMSAKYMPLW

MPLWGIRAMSAKY

SAKYMPLWGIRAM

AMSAKYMPLWGIR

Correct answer:

SAKYMPLWGIRAM

Explanation:

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. 

Example Question #2 : Regulating Protein Degradation

Which of the following describes the unfolded protein response?

Possible Answers:

Stopping protein translation

All of these answers

Triggering cell death

Targeting misfolded proteins for degradation by the 26S proteasome

Increasing amount of local chaperones

Correct answer:

All of these answers

Explanation:

There are 4 main steps in the unfolded protein response: (1) Translation of proteins is stopped, (2) Chaperones are recruited to the location of misfolded proteins, (3) Misfolded proteins are "tagged" with ubiquitin chains for degradation by the 26S proteasome, (4) if the above steps fail, the cell undergoes programmed cell death. Thus, all of the answers choices are affiliated with the unfolded protein response.

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