A GPCR acts as a guanine exchange factor (GEF) for G proteins. The resulting exchange of GDP by GTP leads to the activation of the G protein; whereas a GAP hydrolyses this GTP, (3) (4) Here RA denotes the active conformation of the receptor. Using an equilibrium-binding model, these reactions lead to the following enzyme kinetic equation for the GEF and GAP activities: (5) (6) The sensitivities of the steady-state signaling output, the steady-state concentration of activated G protein, GGTP, with respect to a change in the concentration of the GEF reaction catalyst, the active conformation of the receptor (RA) equals (Kholodenko et al., 1997), (7) The ϵ coefficients are generalized kinetic orders [or elasticity coefficients (Kholodenko et al., 1997)] of the reactions: . When they are close to zero, the enzyme operates in its zero-order regimen () and zero-order ultrasensitivity occurs (Blüthgen et al., 2006). This is most noticeable when we assume both enzyme rates insensitive to their product concentrations; then , which is much greater than 1 when the ϵ coefficients are close to 0. The sensitivity of [GGTP] to a ligand of the GPCR, , can be decomposed into (Kholodenko et al., 1997): (8) In a dose-response curve of log[GGTP] as function of log S, the slope, which is the sensitivity, equals the product of the receptor sensitivity, , and the G protein sensitivity, . Therefore, "sensitivity amplification" (Goldbeter and Koshland, 1981; Koshland et al., 1982; Kholodenko et al., 1997) occurs when both r values exceed 1. |