2.A.4.7.1
Zn²⁺/Cd²⁺/Hg²⁺/Fe²⁺:H⁺ antiporter, YiiP or FieF (Chao and Fu, 2004b; Grass et al., 2005; Wei et al., 2004; Wei and Fu, 2006). The structure (3.8 Å resolution) reveals a homodimer interconnected at the cytoplasmic domain through four Zn²⁺ ions. A 6 TMS bundle features of a tetrahedral Zn²⁺ binding site (Lu and Fu, 2007). The gated water access to the transport site enables a stationary proton gradient to facilitate the conversion of zinc-binding energy to the kinetic power stroke of a vectorial zinc transport (Gupta et al. 2014). Of the two cavities of the dimer, only one was accessible from the cytosol during transport. Zinc(II) binding to D49 of the transport site triggered a rearrangement of the TMS that closed the accessible cavity. The free-energy profiles of metal transit in the channel suggested the existence of a high barrier preventing release from the transport site (Sala et al. 2019). The mechanism of zinc transport through YiiP has been proposed (Sharma and Merz 2022).