Fig. 10. Proposed model of S1P/H⁺ antiport in HnSpns.

a Proton-dependent conformational dynamics are significantly enhanced on the periplasmic side, providing a pathway for ligand exchange. b In this model, the stable resting state is IF, and O is of relatively high energy. Side chains of the functionally conserved residues are represented as sticks. Asp41 protonation promotes the Glu129 deprotonation and formation of a charge pair with Arg42 (1). Subsequently, the released proton translocates to the intracellular side, where the acidic residues of the charge-relay network (e.g., Asp142) are protonated, stabilizing the IF conformation for substrate binding from the intracellular side (2). Proton transfer from Asp41 to Glu129 liberates Arg42 to bind the substrate, e.g., S1P (3). Glu129 protonation triggers closing of the intracellular side and transition to a periplasmic flexible but closed conformation (4). This enables the substrate release to the periplasmic leaflet, possibly through the putative periplasmic gate between TMs 5/8, restoration of the Arg42-Glu129 charge pair, and Glu129 deprotonation. For an electrogenic transport, Glu129 is re-protonated from the flexible periplasmic side (4 to 1). Otherwise, proton transfer from Glu129 to the intracellular charge-relay network and protonation of Asp41 in step 4 are sufficient to directly reset the transporter to the IF conformation (4 to 2).