BK channel structure and gating mechanisms. (A) Structure of the human BK channel (Protein Database accession no. 6V38; Tao and MacKinnon, 2019). The VSD (green) and PGD (orange) of only two subunits are shown for clarity. Ca²⁺ ions bound to the CSD (red and blue) are shown as yellow dots. (B) An allosteric gating mechanism (Horrigan and Aldrich, 2002). L, J, and K are equilibrium constants, which are influenced by conformational changes in the GPD, VSD, and CSD domains and interactions among the three domains. D, C, and E are allosteric constants for these interactions.