Temperature-dependent binding of IgG1 to a human high affinity Fc receptor

We have measured the kinetics of binding and unbinding of human IgG1 to a human high affinity Fc receptor (FcgammaI) at several temperatures. The association rate constant (kappaf) and the dissociation rate (kr) of this complex was determined with 125I-IgG1 monomer and FcgammaI on U937 cells. At 37 degrees C, kappaf = 2.7 x 10(5) M(-1) s(-1) and kappar = 4.5 x 10(-4) s(-1). Both rates decreased with decreasing temperature. However, the equilibrium association constant, Ka, increased with decreasing temperature. From the temperature dependence of Ka we determined that the binding of IgF1 to FcgammaI 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.