Proposed catalytic cycle of CARDO on the basis of various Oxy:Fd complex structures. Statuses are as follows: (1) the resting state with the oxidized Rieske cluster and the ferrous nonheme iron (Oxy: Fdrest, PDB code 2DE5) ; (2) the reduced state (Oxy: Fdred, PDB code 2DE6) ; (2’) the CAR-bound state (Oxy: FdCAR, PDB code 2DE7) ; (3) the reduced CAR-bound state (Oxy: Fdred-CAR, PDB code 3VMG); (3’) the oxygen-bound state (Oxy: FdO2, PDB code 3VMH); (4) the CAR- and oxygen-bound state (Oxy: FdCAR-O2, PDB code 3VMI); and (5) the product-bound state. [2Fe-2 S] shows the coupling Rieske [2Fe-2 S] cluster of the neighboring Oxy subunit, and nonheme iron ligands in the active site are omitted. Solid arrows indicate the proposed main pathway in the catalytic cycle, and broken arrows show the possible pathway based on the complex structures determined in previous and present studies. Red and blue letters show the oxidized and reduced states of redox centers, respectively. The subscripts “ox” and “red” also indicate the oxidized and reduced states, respectively, of the Rieske [2Fe-2 S] cluster.