Avidity favors a ternary complex in which the two domains of RP bind to two adjacent monomers in the PPDK multimer. (a) Illustration of the two domains of an RP enzyme binding adjacent PPDK monomers in a PPDK tetramer. The phosphoryl-addition domain binds PPDK1, whereas the phosphoryl-removal domain binds PPDK2. This double-binding results in high avidity. (b) Reactions for the formation of binary and ternary complexes of RP and PPDK. When one domain of RP binds a monomer, the rate for binding an adjacent monomer by the second domain, kon2 and kon3, is very large. (c) The fraction of RP found in a ternary complex with two adjacent monomers [PPDK1 RP PPDK2] is higher than the fraction found in binary complexes with only one monomer [PPDK1 RP] or [PPDK2 RP], over most of the range of PPDK phosphorylation levels. In this plot the parameters were normalized to koff1 and total RP levels such that kon1 = kon4 = 0.01[koff1/RP], koff4 = koff1, kon2 = kon3 = 200[koff1], koff2 = koff3 = 1[koff1], Vk = Vp = 0.01[koff1] and total PPDK/total RP = 100 [11, 13, 33]. The complex fractions were calculated with a model that takes into account the spatial configurations of PPDK subunits (see Methods).