Figure 2.

Perturbation of active target level and kinetics: intramolecular biosensors, or intermolecular biosensors expressed in stoichiometric excess. (a) Kinetic scheme for an intramolecular biosensor, following the nomenclature established in Fig. 1a; the scheme holds equally for an intermolecular biosensor expressed in excess ([B] ≈ constant). In the hypothetical scenario, stimulation (S = 1) is pulsed for a period of 1000 s. (b) The fractions of the total biosensor pool in the free, active target (T^∗, left) and complexed (C, right) states are plotted as a function of time. The affinity of the molecular recognition element was adjusted by progressively decreasing the dissociation rate constant k_off, as indicated: k_off = 1 s⁻¹, 0.1 s⁻¹, or 0.01 s⁻¹. (Dashed curve) Active target kinetics in the absence of complex formation (k_off = ∞). Other parameter values were fixed at k_on = 1 s⁻¹, k_act = 0.001 s⁻¹, and k_deact = 0.01 s⁻¹. (c) Same as panel b, except that the ratio of k_off/k_on was fixed at 1, with k_off = k_on = 1 s⁻¹, 0.1 s⁻¹, or 0.01 s⁻¹.