Figure 6.
(A and B) Simulation of K M and I max for the five possible models. Parameters are as follows: dissociation constant for glutamate (K S) 5.5 μM, pK of the ionizable amino acid residue in the transporter protein 8.0, pK of glutamate 4.4. (C) Semi-logarithmic plot of the extracellular and intracellular K M values at different pH values. The solid and dashed lines represent the fits {K S(K H + [H]/[H]} to the model the (Table ), where first the proton binds to the transporter and is followed by the glutamate binding process. The parameters were set to the following values: extracellular K M values (solid square), solid line, K S 5.5 μM and K H 1.1·10−8 M, intracellular K M (open circle), dotted line, K S 28 μM and K H 3.0·10-7 M. (D) Kinetic model for glutamate transport by EAAC1. The empty transporter T first binds the proton H forming TH and afterwards binds glutamate S, under formation of the transporter-proton-glutamate complex THS. The respective states with their substrate binding sites exposed to the intracellular side are marked with an asterisk. K S and K S * are the dissociation constants of glutamate from the extra- and intracellular site of EAAC1, respectively. k f and k b are the rate constants (forward mode) for translocation of the fully loaded carrier and relocation of the K+-bound carrier, respectively.