Distinct SPD³⁺ and SPM⁴⁺ coordination at the cytosolic end of the MthK pore. Positions of each polyamine nitrogen (in shades of blue) for SPD³⁺ (top panels) and SPM⁴⁺ (bottom panels) as a function of time, superimposed on positions of glutamate side chain oxygens (corresponding to centers of mass for the pairs of oxygens) and hydroxyl oxygen of T59 are shown in shades of red. With strong depolarization (panels at right), SPD³⁺ N atoms may simultaneously interact with T59-OH and E92, whereas SPM⁴⁺ N atoms may interact additionally with E96.