In signal transduction pathways, water soluble hormones often
A. bind to a receptor in the plasma membrane that activates cAMP.
B. pass through the plasma membrane and bind with a G protein.
C. bind to a receptor in the plasma membrane that activates a G protein.
D. pass through the plasma membrane and bind with cAMP.
The correct answer and explanation is :
The correct answer is C. bind to a receptor in the plasma membrane that activates a G protein.
Explanation:
Water-soluble hormones (also called hydrophilic hormones) cannot pass through the lipid bilayer of the plasma membrane because they are not lipid-soluble. As a result, they cannot directly enter the target cells. Instead, water-soluble hormones interact with receptors located on the cell surface, typically embedded in the plasma membrane. This interaction triggers a cascade of intracellular signaling events, which can lead to a physiological response.
Here’s a breakdown of the key steps involved in water-soluble hormone signal transduction:
- Hormone Binding to Receptor: The water-soluble hormone binds to a specific receptor located on the surface of the target cell, usually a G protein-coupled receptor (GPCR). These receptors span the plasma membrane, with an extracellular domain that binds the hormone and an intracellular domain that interacts with intracellular signaling molecules.
- Activation of G Protein: Upon hormone binding, the receptor undergoes a conformational change that activates an intracellular G protein (a guanine nucleotide-binding protein). The G protein consists of three subunits: alpha (α), beta (β), and gamma (γ). The activation process typically involves the exchange of GDP for GTP on the G protein’s alpha subunit, which then dissociates from the beta-gamma subunits.
- Intracellular Signaling Cascade: The activated G protein can then interact with various downstream effectors. For example, the G protein can activate or inhibit adenylyl cyclase, leading to changes in the levels of cAMP (cyclic adenosine monophosphate), a secondary messenger that amplifies the signal and activates other intracellular proteins such as protein kinases.
- Cellular Response: The activation of protein kinases leads to phosphorylation of target proteins inside the cell, ultimately resulting in changes in cellular activities such as gene expression, metabolism, or secretion.
Thus, water-soluble hormones like peptide hormones (e.g., insulin, glucagon) or catecholamines (e.g., adrenaline) typically follow this process to transmit their signals, as they cannot enter cells directly due to their hydrophilic nature.