Thermodynamic cycle for the formation of the melibiose–Na⁺–MelB_St ternary complex at pH 7.5. Symbols for MelB_St, melibiose or Na⁺ are indicated. State [A], cartoon for MelB_St from the crystal structure (Protein Data Bank accession no. 4M64) represents a periplasmic side-open apo state. States [B] and [C] represent a Na⁺-bound or melibiose-bound binary complex, respectively. State [D] represents a Na⁺–melibiose–MelB_St ternary complex. ΔG values are listed for each binding step. The thermodynamic coupling free energy (ΔΔG) reflects the effect of one substrate on the binding of the other. ΔG^sum, the sums of free energies for the path [A→B→D] or [A→C→D]. The difference in the ΔG from paths [A→B→D] and [A→C→D] is close to zero.