Figure 9.

Six-state kinetic model of Na⁺/glucose uptake by SGLT1 expressed in oocytes. The external Na⁺ concentration was 100 m-equiv/l and external glucose concentration was 0–100 mM. As indicated in light blue the internal Na⁺ was 5 m-equiv/l and the cytoplasm was sugar-free. For simulations, the rate constants are: WT hSGLT1: k₁₂  =  14000 M ^–2s^–1, k₂₁ = 300 s^–1, k₁₆ = 600 s^–1, k₆₁ = 25 s^–1, k₂₃  =  45000 M ^–1s^–1, k₃₂ = 20 s^–1, k₃₄ = 50 s^–1, k₄₃ = 50 s^–1, k₄₅ = 800 s^–1, k₅₄ = 7800 M^–1s^–1, k₆₅ = 4500 M^–2s^–1, k₅₆ = 10 s^–1, and k₂₅ = 0.3 s^–1. k₅₄ = 7800 M^–1s^–1 and k₅₂ = 0.001 s^–1 were determined by microscopic reversibility.¹⁵ The rate constants of mutant Q457R that accounted for its kinetics were the same as WT, except for k₂₃ = 800 M^–2s^–1, k₂₅ = 50 s^–1, k₃₄ = 0 s^–1, and k₄₃ = 0 s^–1. Again, k₅₄ = 140 M^–1s^–1 and k₅₂ = 0.2 s^–1 were determined by microscopic reversibility. u = FV/RT, α' = 0.3, δ = 0.7. α" = 0.