Figure 5.

Cartoon of the 6-state ordered kinetic model for Na⁺/glucose cotransport. States 1–3 and 4–6 face the external and internal membrane surfaces, respectively. In the absence of ligands, the transporter exists in 2 states (1 and 6). At the external surface, 2 Na⁺ ions bind to the transporter to form the complex CNa₂ (state 2). The fully loaded transporter CNa₂S (state 3) undergoes a conformational change (state 3 to 4) resulting in Na⁺/glucose cotransport. The reaction from state 2 to 5 represents the uniport (“leak”) pathway. Presteady–state currents are due to the partial reactions: 2 ⇌ 1 ⇌ 6. Note that the cysteine © at residue 457 is only accessible in state 2, and that it most probably is not within the membrane electric field.