Figure 1

Structure and ion-exchange reactions of NCX_Mj.

(A) Crystal structure of NCX_Mj (PDB 3V5U) in cartoon representation. Helices 1–5 (TM1-5) are orange and helices 6–10 (TM6-10) are pink. Purple and green spheres represent Na⁺ and Ca²⁺ ions, respectively. (B) Ion coordination as suggested by the crystal structure of NCX_Mj¹⁸. (C) 3Na⁺ ion coordination as suggested by molecular dynamics simulations and ion-flux assays²². (D) Ca²⁺ binding site. (E) Schematic representation of the ion-flux assay for Na⁺/⁴⁵Ca²⁺ exchange or Ca²⁺/⁴⁵Ca²⁺ exchange and the ping-pong mechanism describing the exchange reactions^10,16,17. The red, green and purple arrows represent Ca²⁺ -entry, Ca²⁺ -exit and Na⁺ -exit steps of the transport cycle, respectively. The dotted arrows represent the reactions of the transport cycle, which negligibly contribute to the observed ion-exchange reactions under the given experimental conditions. The K′_d and K”_d values represent the dissociation constants for Ca²⁺ binding to NCX_Mj at the cytosolic and extracellular sides, respectively. The rate contributions to the observed k_cat and K_m values of the exchange reactions are expressed in the equations described in Table S1.