a,b) Surface representations of CDA-C1 show a tunnel through CDA-C1, with the active site in its center. c) A representation of the tunnel from CAVER 42 output. d,e) A model of the transition state of the CDA-C1 catalyzed reaction including CDA-C1, ACP, PCP1 and covalently-attached transition state. Note that the binding sites of upstream and downstream carrier proteins are not related by the same symmetry operation that relates the CAT core of the N- and C-terminal subdomains. The CAT cores are related by a ~120° rotation about an axis approximately parallel to the acceptor portion of the tunnel. Since the tunnel has a carrier protein at each end, rotation of the entire modeled complex about this axis does not interchange the positions of the carrier proteins.