FIGURE 6.

A simplified energy diagram illustrates the connection between ligand binding (left) and conformational exchange between the open-in (AB) and open-out (BA) states of ligand-bound EmrE (right). The free energy of binding, ΔG_bind, was calculated from measured binding affinities. The dashed lines indicate that nothing is known about the transition state connecting the apo and bound states of EmrE. The two ligand-bound ground states (AB and BA) have equal energy and are separated by the transition state, ‡, with energetic barrier of height ΔG_conf^‡ calculated from the rates of conformational interconversion and transition-state theory. The diagram shows the tetrahedral series, MeTPP⁺ (red), EtTPP⁺ (magenta), TPP⁺ (black), MBTPP⁺ (green), and DPhTPP⁺ (blue), as well as the planar substrates DQ²⁺ (purple) and PP²⁺ (orange).