IST3G- The epinephrine signaling pathway plays a role in regulating glucose homeostasis in muscle cells. The signaling pathway is activated by the binding of epinephrine to the beta-2 adrenergic receptor. A simplified model of the epinephrine signaling pathway is represented in Figure 1.

Which of the following outcomes will most likely result from the irreversible binding of GDP

to the G protein?

Question

IST3G- The epinephrine signaling pathway plays a role in regulating glucose homeostasis in muscle cells. The signaling pathway is activated by the binding of epinephrine to the beta-2 adrenergic receptor. A simplified model of the epinephrine signaling pathway is represented in Figure 1.

Which of the following outcomes will most likely result from the irreversible binding of GDP

to the G protein?

BlackTom AI · Accepted Answer

In this question, we’re examining the consequence of irreversible GDP binding to the G protein within the epinephrine signaling pathway that involves the beta-2 adrenergic receptor and its Gs protein.

First, consider what GDP-bound G protein implies: the G protein is in an inactive state and cannot exchange GDP for GTP to become ACTIVE. Without active Gs, the receptor cannot stimulate adenylate cyclase, so cAMP production would drop and downstream PKA signaling would be reduced.

If GPCR signaling is blunted, the cascade that promotes glycogen breakdown (glycogenolysis) via phosphorylation events is diminished. In muscle (and liver) cells, epinephrine normally promotes glycogen breakdown to release glucose; this process is driven by Gs activation leading to increased cAMP and activation of glycogen phosphorylase. Therefore, blocking Gs activation would tilt the balance away from glycogenolysis.

As a result, the intracellular glycogen stores would be expected to increase or at least not be mobilized efficiently, since the signal to degrade glycogen is not transmitted.

Note on the provided options: the only listed option states that 'The intracellular concentration of glycogen will increase.' This aligns with the mechanistic expectation that irreversible GDP binding prevents GPCR-mediated glycogenolysis. If there were other options, they might include outcomes like decreased glycogen breakdown, reduced cAMP levels, or unchanged glycogen levels; however, none are presented here.

Thus, the reasoning supports the conclusion that irreversible GDP binding to the G protein would lead to an accumulation or preservation of glycogen in the cell due to inhibited epinephrine-induced glycogenolysis.

AP 2024-2025 4.1-4.4 MC Assessment

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