TABLE 1.

Model parameters

Parameters		Values
Φ	Cell length	10 μm
ΔL	Gradient	0.02L*
Rtotal	Total receptor	80,000 molecules/cell†
KR	Ligand affinity	0.18 μM†
koff	Dissociation rate	1 s−1†
kon	Association rate	5.6 s−1μM−1†
Xtotal	Total second messenger	200,000 molecules/cell‡
EC50	§	0.01μM§
KX	§	4210 molecules/cell§
kd	X degradation rate	1.0 s−1‡
kp	X production rate	1/4210 s−1(molecules/cell)−1¶

X represents G-protein, because the second reaction is G-protein activation in Dictyostelium cells.

^*The gradient is expressed as the concentration difference between the anterior and posterior ends of a cell. For calculation of the SNR, the concentration difference at the center of anterior and posterior regions was used, and the difference, ΔL, is 0.01L (13,14).

^†Receptor number, ligand affinity, and dissociation rate were obtained experimentally (Fig. 1) (16,17,19). Association rate was obtained from K_R and k_off.

^‡The values were inferred.

^§EC₅₀ and K_X are the concentration of cAMP and active receptor where the activation of the second messenger reaches half-maximum, respectively (36). K_X was obtained from EC₅₀, K_R, and R_total, according to Eq. 8.

^¶k_p was determined by k_p = k_d/K_x.