EFFECTS OF DECYL-AURACHIN-D AND REVERSED ELECTRON-TRANSFER IN CYTOCHROME BD

Decyl-aurachin D is a near-stoichiometric inhibitor of cytochrome bd f rom Azotobacter vinelandii. Interaction of decyl-aurachin D with the o xidase induces a redshift of the a-band and Soret band of a b-type cyt ochrome, probably b-558, suggesting close proximity of the inhibitor b inding site to this haem and hence to the proposed quinol binding doma in. The compound does not affect the oxygen binding site directly as j udged from unchanged CO recombination kinetics to haem d in dithionite -reduced enzyme. Although in the presence of ubiquinol-l a decyl-aurac hin D containing sample generates levels of haem reduction and catalyt ic intermediates similar to the control, the approach to this steady s tate is severely inhibited. In addition to the spectral effect on b-55 8, decyl-aurachin D raises the midpoint potential of haem b-558, but a lso lowers that of haem b-595. Consistent with the shift in midpoint p otentials, electron backflow from haem d to the b-type haems can be ob served in decyl-aurachin D inhibited samples following photolysis of t he mixed-valence CO-ligated form of the enzyme. The data show that dec yl-aurachin D acts on the donor side of haem b-558 without substantial ly affecting internal electron transfer rates or the oxygen reduction site.