Kinetic Analysis of the Multivalent Ligand Binding Interaction between Protein A/G and IgG: A Standard System Setting

Recombinant protein A/G (PAG) has a sequence coding for eight IgG binding sites and has enhanced interspecies affinity. High-frequency sampling of a PAG titration with IgG produces concentration profiles that are sensitive to the kinetic availability of the binding sites. The full kinetic model developed here for IgG binding sequentially to PAG shows only two distinct kinetic processes, describing an initial rapid association of two antibodies to PAG with a rate constant k-fast = (1.86 ± 0.08) × 10⁶ M^-1 s^-1 and a slower antibody binding process to all remaining sites, k-slow = (1.24 ± 0.05) × 10⁴ M^-1 s^-1. At equilibrium (after 1 h), the maximum IgG occupancy of PAG is 2.8 ± 0.5, conflicting with the genetic evidence of eight binding sites and suggesting significant steric hindrance of the neighboring IgG binding sites. The phosphate-buffered saline (PBS) solution defines a standard system setting, and this may be compared with other settings. The mean association rate of PAG-IgG_n in the standard setting is 282 ± 20% higher than when PAG is tethered to a surface. A systems biology approach requires that a model parameter set that defines a system in a standard setting should be transferable to another system. The transfer of parameters between settings may be performed using activity coefficients characterizing an effective concentration of species in a system, a_i = γ_ic_i. The activity correction, γ, for the eight-site occupancy is γ = 0.35 ± 0.06, and mapping from the standard setting to the solution setting suggests γ_PAG-IgG = 0.4 ± 0.03. The role of activity coefficients and transferability of kinetic parameters between system settings is discussed.