The pMMO subunits. (a) Stereoview of a single protomer with pmoB shown in magenta, pmoA shown in yellow, and pmoC shown in blue. Copper ions are shown as cyan spheres (the arrow points to the binuclear copper site), and a zinc ion is shown as a grey sphere. (b) The pmoB subunit viewed looking down the membrane normal. The N-terminal β-barrel is in the middle. (c) The pmoA subunit, and (d) the pmoC subunit. (Reprinted with permission from Macmillan Publishers Ltd: Nature Ref. 685, copyright 2005.)