Final answer:
The active form of a G protein is when it is bound to GTP instead of GDP, which occurs after a G-protein-coupled receptor is activated by a signaling molecule. The G protein then dissociates and can activate other proteins in the cell, with the cycle ending when GTP is hydrolyzed to GDP.
Step-by-step explanation:
The active form of a G protein (trimeric GTP-binding protein) is present when the G protein is bound to guanosine triphosphate (GTP) rather than guanosine diphosphate (GDP). This activation process begins when a signaling molecule binds to a G-protein-coupled receptor (GPCR) on the plasma membrane.
As the receptor undergoes a conformational change, it activates the G protein through the exchange of GDP for GTP on the α subunit. The G protein, composed of α, β, and γ subunits, then dissociates into two parts: the α subunit with the bound GTP and the βγ dimer. Either or both of these parts can then go on to activate other proteins within the cell, resulting in a biological response.
This activation is terminated when the GTP on the α subunit is hydrolyzed back to GDP, and the βγ dimer is deactivated, leading to reassociation of the G protein subunits into their inactive form.