FIGURE 5.

A proposed two-metal model at the single-molecule level. The regulatory site (top right circle on endo V) may be activated by either Ca²⁺ or Mg²⁺, the catalysis site or the catalytic site (top left circle on endo V) is only activated by Mg², but may be occupied by Ca²⁺ or Mg²⁺. The time lapses T₁, T₂, and T₃ corresponding to the single-molecule association, recognition, and dissociation events are illustrated for the three cases (Ca²⁺, Mg²⁺, and Ca²⁺-Mg²⁺), respectively. The regulatory site in case 2 could be occupied by Mg²⁺ or left empty, as indicated by the gray circle.