Structure of ATCase. (A) The quaternary structure of aspartate transcarbamoylase as viewed from the top. The drawing in the center is a simplified representation of the relations between subunits. A single catalytic trimer [catalytic (c) chains, shown in yellow] is visible; in this view, the second trimer is hidden below the one visible. Notice that each r chain interacts with a c chain through the zinc domain. (B) A side view of the complex.

[Drawn from 1RAI.pdb.]