For lipophobic hormones that cannot pass the cellular membrane, activity is mediated and amplified within a cell by the action of second messenger mechanisms (molecules that relay signals from receptors on the cell surface to target molecules inside the cell in the cytoplasm or nucleus).

Most hormone receptors are G protein-coupled receptors. Upon hormone binding, the receptor undergoes a conformational change and exposes a binding site for a G-protein. The G-protein is bound to the inner membrane of the cell and consists of three subunits: alpha, beta, and gamma. Upon binding to the receptor, it releases a GTP molecule, at which point the alpha subunit of the G-protein breaks free from the beta and gamma subunits and is able to move along the inner membrane until it contacts another membrane-bound protein: the primary effector. The primary effector then has an action, which creates a signal that can diffuse within the cell. This signal is called the secondary messenger. The secondary messenger may then activate a secondary effector, whose effects depend on the particular secondary messenger system.

Key Points

• Secondary messengers relay signals from receptors on the cell surface to the target molecules.

• The secondary messenger systems bind hormones to a receptor that causes a cascade of changes that leads to actions.

Key Terms

secondary messenger: Molecules that relay signals from receptors on the cell surface to target molecules inside the cell, in the cytoplasm or nucleus.

G-proteins: a special type of proteins involved in the transmission of signals to the cells

lipophobic: phobia to lipids i.e. molecules that stay away from the lipids

G protein–coupled receptor: a protein located in the cell membrane that binds extracellular substances and transmits signals from these substances to an intracellular molecule called a G protein