Temperature-dependent binding of human IgG 1 to a human high affinity Fc receptor

Abstract

We have measured the kinetics of binding and unbinding of human IgG 1 to a human high affinity Fc receptor (FcγI) at several temperatures. The association rate constant ( k f ) and the dissociation rate ( k r ) of this complex was determined with 125I-IgG 1 monomer and FcγI on U937 cells. At 37°C, k f = 2.7 × 10 5 M −1 s −1 and k r = 4.5 × 10 −4 s −1. Both rates decreased with decreasing temperature. However, the equilibrium association constant, K a , increased with decreasing temperature. From the temperature dependence of K a we determined that the binding of IgG 1 to FcγI is driven largely by enthalpic forces and that a small but positive entropic contribution to free energy leads to a tighter complex at lower temperature.