Figure 1. Trimeric G-protein cycle.

The cycle contains four dependent variables (Gα^GDPβγ, Gα^GTP, βγ and Gα^GDP) and two independent variables [Rc* (activated receptor) and GAP]. Three reactions (labelled ➀–➂ respectively) govern the cycle: association of βγ and Gα^GDP into the trimeric complex, Rc*-catalysed dissociation of the trimeric complex into βγ and Gα^GTP, and GAP-catalysed hydrolysis of GTP on Gα^GTP with formation of Gα^GDP. The rates of these reactions, V₁, V₂ and V₃, are described in eqns (2)–(4) in the Materials and methods section.