Second messengers Transitional Reasoning Download PDF

Questions

1. 1.

   Researchers investigating a hormonal signaling pathway in liver cells observe that binding of an extracellular hormone to a cell-surface G protein-coupled receptor leads to rapid production of cyclic AMP (cAMP) and subsequent glycogen breakdown. When the researchers microinject an engineered binding agent that selectively sequesters cytosolic cAMP prior to hormone administration, receptor binding remains normal, but downstream activation of protein kinase A and glycogen degradation are completely blocked. Based on these observations, which statement best explains the role of cAMP in this signaling pathway?

   • A) It functions as an extracellular first messenger that directly enters the nucleus to initiate metabolic gene expression.
   • B) It serves as a transmembrane anchor protein that physically couples the cell-surface hormone directly to glycogen phosphorylase.
   • C) It acts as a catalytic enzyme that directly transfers phosphate groups to glycogen without requiring intermediate kinases.
   • D) It acts as a diffusible intracellular second messenger that relays and amplifies the extracellular signal to activate protein kinase A.