Details of the crystal structure of wild type SDH and comparison with
CSO. A, ribbon diagram of the SDH heterodimer with the SorA and
SorB subunits colored blue and cyan, respectively, and the
redox cofactors in space-filling mode with the molybdenum atom colored
green and the iron atom colored violet. B, ribbon diagram of a
single subunit of CSO with the molybdopterin binding domain in the same
orientation as SorA in A. The cytochrome domain of CSO is clearly in
a different position with respect to the molybdenum cofactor than is seen for
the cytochrome subunit of SDH. C, SDH molybdopterin cofactor
demonstrating the geometry of the molybdenum ligands. The thiol ligands
donated by the organic component of molybdopterin and the Cys-104 side chain,
and the reactive oxygen ligand (Oeq) sit in the equatorial
plane with the axial oxygen (Oax) ligand at the
apex of a square pyramid. Atoms are colored as
follows: molybdenum (green), sulfur (orange), phosphorous
(magenta), oxygen (red), nitrogen (blue), and carbon
(yellow in the cofactor and white in the protein).
D, hydrogen bonding network around the substrate binding site. The
molybdopterin and heme cofactors are shown together with active site residues
Cys-104, Arg-55, His-57, Tyr-236, and Gln-33. Figs. 1 and
4 were prepared using Pymol
(37).