Figure 6.

Reaction scheme showing the proposed proton relay involving His216, Glu218, and Glu305 in the BC domain active site leading to the deprotonation of bicarbonate and the transfer of a phosphoryl group from bound MgATP leading to the formation of carboxyphosphate (1). Carboxyphosphate decarboxylates, and the resultant PO₄^3– deprotonates biotin while the carbon dioxide carboxylates it; carboxybiotin and P_i then leave the BC domain (2). Glu305 is deprotonated by the solvent leading to reversal of the proton relay (3). When MgADP dissociates, His216 moves away from Glu218. Note that bound Mg²⁺ ions are not illustrated for the sake of clarity.