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.

The figure presents a simplified model of the epinephrine signaling pathway in muscle cells. A plasma membrane is shown with two transmembrane structures, a Beta-2 Adrenergic Receptor and Adenylyl Cyclase. Epinephrine is shown binding to the extracellular surface of the beta-2 adrenergic receptor. Bound to the cytosolic surface of the receptor is a G protein made up of alpha, beta, and gamma subunits. An arrow indicates that G D P that is bound to the alpha subunit of the G protein is now released from the subunit. A successive arrow points to the G protein subunits dissociated from the adrenergic receptor and the alpha subunit dissociated from a complex of the beta and gamma subunits. G T P is shown binding to the alpha subunit. A successive arrow points from the G T P-modified alpha subunit to the cytosolic region of adenylyl cyclase. Adenylyl cyclase intersects an arrow that points from A T P to Cyclic A M P. A series of arrows follows. An arrow from Cyclic A M P points to Protein Kinase A. An arrow from Protein Kinase A points to Phosphorylase Kinase and is paired with a curved arrow pointing from A T P to A D P. An arrow from Phosphorylase Kinase points to Glycogen Phosphorylase and is paired with a curved arrow pointing from A T P to A D P. An arrow from Glycogen Phosphorylase points to an arrow indicating the conversion of Glycogen to Glucose-1-Phosphate. A final arrow points from Glucose-1-Phosphate to Glycolysis.

Figure 1. A simplified model of the epinephrine signaling pathway in muscle cells

Which of the following outcomes will most likely result from the irreversible binding of GDP to the G protein?

Question

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.

The figure presents a simplified model of the epinephrine signaling pathway in muscle cells. A plasma membrane is shown with two transmembrane structures, a Beta-2 Adrenergic Receptor and Adenylyl Cyclase. Epinephrine is shown binding to the extracellular surface of the beta-2 adrenergic receptor. Bound to the cytosolic surface of the receptor is a G protein made up of alpha, beta, and gamma subunits. An arrow indicates that G D P that is bound to the alpha subunit of the G protein is now released from the subunit. A successive arrow points to the G protein subunits dissociated from the adrenergic receptor and the alpha subunit dissociated from a complex of the beta and gamma subunits. G T P is shown binding to the alpha subunit. A successive arrow points from the G T P-modified alpha subunit to the cytosolic region of adenylyl cyclase. Adenylyl cyclase intersects an arrow that points from A T P to Cyclic A M P. A series of arrows follows. An arrow from Cyclic A M P points to Protein Kinase A. An arrow from Protein Kinase A points to Phosphorylase Kinase and is paired with a curved arrow pointing from A T P to A D P. An arrow from Phosphorylase Kinase points to Glycogen Phosphorylase and is paired with a curved arrow pointing from A T P to A D P. An arrow from Glycogen Phosphorylase points to an arrow indicating the conversion of Glycogen to Glucose-1-Phosphate. A final arrow points from Glucose-1-Phosphate to Glycolysis.

Figure 1. A simplified model of the epinephrine signaling pathway in muscle cells

Which of the following outcomes will most likely result from the irreversible binding of GDP to the G protein?

BlackTom AI · Accepted Answer

To understand the effect of irreversible GDP binding to the G protein in the epinephrine signaling pathway, we must follow the signaling cascade starting at the receptor.
Option 1: The intracellular concentration of glycogen will increase. If GDP remains irreversibly bound to the G alpha subunit, the G protein cannot exchange GDP for GTP, so the G protein remains in an inactive state and cannot activate adenylyl cyclase. Without cAMP production, downstream activation of protein kinase A (PKA) and the phosphorylation cascade that stimulates glycogenolysis is blocked. As a result, glycogen breakdown is not promoted, and glycogen accumulates relative to the stimulated state. This makes an increase in glycogen plausible.
Option 2: The intracellular concentration of glucose-1-phosphate will increase. Glucose-1-phosphate is produced downstream of glycogen breakdown via glycogen phosphorylase. Since the G protein cannot be activated to promote glycogenolysis, the supply of glucose-1-phosphate would not rise; instead, it would stay the same or decrease compared to the stimulated condition. Therefore this option is unlikely.
Option 3: The intracellular concentration of cyclic AMP will increase. cAMP production is driven by adenylyl cyclase, which is activated by the G alpha subunit when GTP is bound after receptor stimulation. If GDP is irreversibly bound, the G alpha subunit cannot become GTP-bound, so adenylyl cyclase is not activated and cAMP does not rise. Thus this option contradicts the expected outcome of an inactive G protein.
Option 4: The intracellular concentration of activated protein kinase A will increase. PKA activation depends on elevated cAMP levels; since cAMP would not increase due to the inactive G protein, PKA would not be activated. Therefore this option is inconsistent with irreversible GDP binding.
In summary, blocking G protein activation at the GDP stage prevents the entire downstream cascade (adenylyl cyclase activation, cAMP production, PKA activation, glycogenolysis). The most consistent outcome with irreversible GDP binding is that glycogen remains high or accumulates, making glycogen concentration the most likely to increase among the options.

APBIO141-031 with labs S10 - CollegeBoard* AP Exam Prep: Multiple Choice

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