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.

► CH2–CH3 interface of huIgG1 binds a zinc cation through histidines 310 and 435.
► Zinc binding stabilizes an open Fc conformation favoring FcγR binding.
► Absence of zinc (His/Lys mutant, use of EDTA) lowers the opening between CH2 domains.
► Absence of zinc prevents huIgG1 binding to and activation of FcγRIIB not FcγRIII.
► Binding of mAbs to various FcγR can be modulated by affecting zinc coordination.