Free response · Unit 4 Beta

A blocked step in a G protein pathway

Conceptual Analysis · 4 points

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The question

Name: ______________________   Date: ____________   A blocked step in a G protein pathway (4 points)

Experimental setup

Epinephrine and a broken G protein

In liver cells, epinephrine binds a G protein-coupled receptor. The activated receptor makes a G protein release GDP and bind GTP. The active G protein switches on adenylyl cyclase, which converts ATP into cAMP. cAMP switches on protein kinase A, which starts a cascade of kinases ending with glycogen phosphorylase, the enzyme that breaks glycogen down to glucose. Phosphodiesterase converts cAMP into inactive AMP.

Researchers studied liver cells from a mouse with a mutation in the G protein. The mutant G protein binds GDP normally but cannot release it, so it cannot bind GTP.

  1. (a) Describe the role of the G protein in the normal pathway. [1 point]

  2. (b) Explain how the binding of a small number of epinephrine molecules can lead to the release of a large amount of glucose. [1 point]

  3. (c) Predict the effect of the G protein mutation on the cAMP level and on glucose release when epinephrine is added to the mutant cells. Justify your prediction. [1 point]

  4. (d) The researchers give the mutant cells epinephrine together with a drug that blocks phosphodiesterase. Predict how glucose release compares with mutant cells given epinephrine alone. Justify your prediction. [1 point]