What change occurs in Step 2 of the epinephrine signal transduction pathway?

In Step 2 of the epinephrine signal transduction pathway, the critical change involves the replacement of GDP bound to the α-subunit of the G protein with GTP. This transition is significant because it activates the G protein, which is essential for propagating the signal initiated by the binding of epinephrine to its receptor (a G protein-coupled receptor, or GPCR).

When epinephrine binds to the GPCR, a conformational change is induced in the receptor. This causes the associated G protein to exchange GDP for GTP on the α-subunit. The binding of GTP not only activates the G protein but also triggers the dissociation of the α-subunit from the β and γ subunits. The α-GTP complex can then interact with other downstream effectors in the pathway, such as adenylate cyclase, leading to the production of cyclic AMP (cAMP) and amplification of the cellular response to epinephrine.

This process is foundational to how cellular signaling works, illustrating the dynamic role of G proteins in mediating signals from external stimuli to produce internal cellular responses.