Fig. 4. The ternary complex model. The ternary complex model is shown in the  box . R represents the receptor, G represents the G protein, A represents the corresponding agonist, and GN represents the guanyl nucleotide (GDP or GTP). The constants  K , J , and  M are affinity constants, and α, β, and γ represent allosteric factors. Free, uncoupled receptors have low affinities for the agonist and G protein (represented as  K and  M ). Agonists increase the average receptor’s affinity for the G protein (from  M to α  M ) and  vice versa . The allosteric factor α is therefore greater than 1. The agonist equilibrium dissociation constants from agonist–receptor–G protein complex (ARG) are  K ^H= 1/α  K and  K ^L= 1/  K . Guanyl nucleotides inhibit the binding of agonists; αγ  K is lower than α  K , so γ must be lower than 1. The affinity of GN for ARG (βγ  J ) is therefore lower than the affinity of GN for RG (βγ): Agonists inhibit the recognition of nucleotides by receptor-coupled G proteins .