Fig. 3.

Ton box dynamics in FhuA, BtuB, and FecA. (a) X-band EPR spectra of single sites labeled with R1 (Fig. 2a) along the FhuA Ton box (residues 7–13) without (black line) and with (red line) the substrate ferrichrome. In every case the spectra from apo and ligand-bound states overlap. (b) A plot of the M_s obtained from the EPR spectra of R1 when placed along the Ton box in the absence (●, black) and presence (▴, red) of substrate for BtuB, FecA, and FhuA. The M_s is defined as M_s = (δ⁻¹ − δ_i⁻¹)/(δ_m⁻¹ − δ_i⁻¹), where δ is the central resonance linewidth and δ_i and δ_m are the least and most mobile R1 side chains found in proteins (42). Values for δ_i and δ_m were taken as 8.5 and 1.8 G, respectively.