Figure 1.

TRAP is a biosensor with 11 equivalent binding sites for tryptophan. (A) Crystal structure of Trp-bound TRAP₁₁ (26); bound Trp are shown as spheres. (B) Closeup of two neighboring protomers in the crystal structure (red and blue) showing proximity of bound tryptophan molecules. (C) NN-a model of cooperativity between sites, in which binding free energy is the sum of an intrinsic free energy ΔG_i plus coupling to either empty (ΔG_α) or occupied (ΔG_β) neighboring sites. (D) NN-na model, in which binding to a site flanked by two empty sites has a reference ΔG₀, modified by coupling free energies when the site is flanked by one or two occupied neighbors (ΔG_oe, ΔG_oo), respectively. To see this figure in color, go online.