Effect of zinc on human IgG1 and its FcγR interactions

In the present study, we show that histidines 310 and 435 at the CH2-CH3 interface of the Fc portion of human IgG1 can coordinate a Zn2+ and participate in the control of the CH2-CH2 interdomain opening. Structures obtained in the absence of Zn2+ have a reduced interdomain gap that likely hamper FcγR binding. This closed conformation of the Fc is stabilized by inter-CH2 domain sugar contacts. Zinc appears to counteract the sugar mediated constriction, suggesting that zinc could be an important control factor in IgG1/FcγR interactions. The results of binding studies performed in the presence of EDTA on FcγR expressing cells supports this hypothesis. When a mutated Fc fragment, in which histidines 310 and 435 have been substituted by lysines (Fc H/K), was compared with the wild-type Fc in crystallographic studies, we found that the mutations leave the interface unaltered but have a long-range effect on the CH2 interdomain separation. Moreover, these substitutions have a differential effect on the binding of IgG1 to Fcγ receptors and their functions. Interaction with the inhibitory FcγRIIB is strongly perturbed by the mutations and mutant IgG1 H/K only weakly engages this receptor. By contrast, higher affinity FcγR are mostly unaffected.