Fig. 6.

An ordered-binding model for melibiose/Na⁺ efflux catalyzed by MelB_St (see text). The N- and C-terminal domains of MelB_St molecule are colored in green and blue, respectively. Sugar and Na⁺ are shown as black hexagons and red spheres, respectively. P_I or P_O represents the permease at an inward-facing or outward-facing conformation, respectively. P:(Na⁺:Mel) or P:(empty), cosubstrates-bound or unloaded permease at the occluded state, respectively.