Biochemistry : Ubiquitination Pathway

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #1 : Protein Degradation

If a protein is bonded to ubiquitin, this tells the cell that the protein should be __________.

Possible Answers:






Correct answer:



When a protein is damaged, it can be tagged with the molecule, ubiquitin. This signals to the cell that the protein is no longer functioning properly and needs to be degraded.

Example Question #2 : Protein Degradation

HMGCoA reductase (3-hydroxy-3-methyl-glutaryl-CoA reductase) is the rate-limiting enzyme in cholesterol synthesis. Which of the following are true about the ubiquitination of this enzyme?

I. When cholesterol levels in the cell are high, the reductase binds to insulin-induced gene 1 proteins.

II. Binding to insulin induced gene 1 proteins leads to ubiquitination and proteasomal degradation of reductase.

III. Ubiquitination occurs through the binding of the C-terminal glycine of ubiquitin to the amino group of a lysine on the reductase.

IV. The enzyme tagged with ubiquitin is recognized by the proteasome where proteolysis occurs.

Possible Answers:

I, II, III, and IV

II, III, and IV

II and III

II and IV

I and II

Correct answer:

I, II, III, and IV


HMGCoA reductase is the rate-limiting enzyme in cholesterol synthesis. The reductase is present on the endoplasmic reticulum membrane. When levels of its product, cholesterol, are high, the enzyme gets ubiquitinated and degraded in smaller peptides and amino acids. It first binds to insulin-induced gene 1 protein before ubiquitination.

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