Kinetics of glutamate binding to mGluR6. (A) Kinetic scheme for glutamate binding to mGluR6. The activated receptor was considered the doubly bound state (mGluR6*2Glu). (B) Kinetics of mGluR6 activation (i.e., doubly bound mGluR6) is shown for different glutamate-binding rates. For 1 × 10⁹, 1 × 10⁸, and 1 × 10⁷ M/s, the corresponding OFF rates for unbinding of the last glutamate molecule were 2,500/s, 250/s, and 25/s, respectively. The later portion of the decay was fit with a single exponential: 1 × 10⁹ M/s, 11.0 ms; 1 × 10⁸ M/s, 26.7 ms; 1 × 10⁷ M/s, 46.7 ms. Simulations show the average of 12 seed values run in rod 3 in the presence of 3,000 EAAT5. (C) Plot of steady-state mGluR6 activation as a function of glutamate concentration using the model parameters in A. Data were fit with a sigmoidal Hill function: EC₅₀ = 14 μM. Hill slope = 1.4.