Figure 2 - Topologic model of the dimeric plant AOX in the inner mitochondrial membrane. Highly conserved residues T, Y, and E, located in the two transmembrane helices, are proposed to be a potential quinone-binding site. The four-helix bundle (postulated binuclear iron center) is located in the C-terminal hydrophilic domain of the protein on the matrix side of the inner mitochondrial membrane. On the same side of the membrane, the relative positions of the two conserved cysteine residues are shown. One, located in the N-terminal hydrophilic domain closest to the membrane (near the postulated diiron site and the postulated quinone-binding site), may be the binding site for pyruvate (allosteric effector). Second, the more N-terminal cysteine residue may be the redox-active group involved in conversion between the more active noncovalently associated reduced state of the enzyme and the less active oxidized state. Based on Siedow and Umbach (10), Moore et al. (18), and Umbach and Siedow (33).