Kinase activity for different Tar-modification levels.

(A) Kinase activity for Tar receptors in QEQE (black), QEQQ (blue) and QQQQ (green) modification states as a function of MeAsp concentration. The curves are normalized with respect to QQQQ activity at concentration c = 10−4mM. (B) All model curves share the same receptor density ρ (light green), which reflects the average complex size, depicted next to the corresponding Hill coefficients nH (dark green). Parameter N (yellow) of the classical MWC model (taken from [23]) is shown for comparison. (C) The chemical potential μ, shown as a function of ligand concentration for the three modification levels, is adjusted throughout the simulation to ensure constant ρ at all concentrations. Color coding is the same as in panel A. The vertical dashed lines indicate half-maximum concentrations cH from the corresponding Hill fits. While for c cH μ is approximately constant, the curves follow a logarithmic function in the regime of c > cH. The dotted blue line shows f(c) = ln c + 5.5 for comparison. Model parameters: Δϵ(QEQE) = −1.12, Δϵ(QEQQ) = −2.16, Δϵ(QQQQ) = −3.03, , , ρ = 10.30, , , and J = −3.81. Values for , , and J are shared with curves shown in Figs 4 and 5. The superscript 0 indicates wild-type expression levels for CheA/CheW.