Figure 1.

Stigmatellin binding as a model for the enzyme–substrate complex. (Stereo pairs.) The ISP is shown as a pale green ribbon, with the 2Fe–2S cluster as space-filling spheres. cyt b is represented by the exterior surface of the protein, mapped with a 1.4-Å probe. The protein has been cut away to reveal the Q_o-site volume. The side chains of His-161 (ISP) and Glu-272 (cyt b) are shown as Corey–Pauling–Koltun (CPK) colored tube models, and heme b_L as a ball-and-stick model, with the Fe atom space-filling and C atoms in orange. (Upper) The binding of stigmatellin (C atoms yellow, O atoms red; from PDB 2bcc). (Lower) The enzyme–substrate complex: ubiquinol is shown with C atoms blue, O atoms red. A putative water chain (bottom right) occupies a channel in cyt b leading from the external aqueous phase to the heme b_L binding pocket and the Q_o pocket. See Experimental Procedures for model building and the text for discussion. These and the native and myxothiazol- and quinone-containing structures are available as supplementary material for interactive viewing through a Chime tutorial at www.pnas.org.