Fig. 6

Comparison of the cytoplasmic and apical substrate-binding sites of Dcytb and AtCytb₅₆₁. a Ascorbate is bound to the cytoplasmic side of Dcytb by interaction with three conserved residues (K79, K83, and R152). F142 interacts with ascorbate by van der Waals contact. b Similarly, ascorbate is bound to the cytoplasmic side of AtCytb₅₆₁ by three conserved cationic residues, and Y140 replaces F142 of Dcytb. c Zn²⁺-ascorbate is bound to the pocket on the apical side of Dcytb. Zn²⁺ is coordinated by H108 and two-hydroxyl groups of ascorbate. H108 is conserved in all members of Cytb₅₆₁. Ascorbate is in van der Waals contact with F184. The dihydroxyethyl moiety of ascorbate is free from interaction with the protein. d Ascorbate is bound to the same site on the noncytoplasmic side of AtCytb₅₆₁. Ascorbate is in van der Waals contact with H106 and F182 and forms H-bonds with Y115 and N186