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Abstract

₄-Hydroxybenzoyl-CoA reductase (₄-HBCR) is a member of the xanthine oxidase (XO) family of molybdenum cofactor containing enzymes and catalyzes the irreversible removal of a phenolic hydroxy group by reduction, yielding benzoyl-CoA and water. In this work the effects of various activity modulating compounds were characterized by kinetic, electron paramagnetic resonance (EPR) spectroscopic, and X-ray crystallographic studies. ₄-HBCR was readily inactivated by cyanide and by the reducing agents titanium(III) citrate and dithionite; in contrast, reduced viologens had no inhibitory effect. Cyanide inhibition occurred in both the oxidized and reduced state of ₄-HBCR. In the reduced state, cyanide-inhibited ₄-HBCR was reactivated by simple oxidation. In contrast, reactivation from the oxidized state was only achieved in the presence of sulfide. Dithionite-inhibited ₄-HBCR was reactivated by oxidation, whereas inhibition by titanium(III) citrate was irreversible. The previously reported inhibitory effect of azide could not be confirmed; instead, azide rather protected the enzyme from inactivation by titanium(III) citrate. The EPR spectra of the Mo(V) states were nearly identical in the noninhibited methyl viologen and in the dithionite-inhibited states of ₄-HBCR; they exhibited a hyperfine splitting due to magnetic coupling with two solvent-exchangeable protons. The cyanide-treated enzyme showed the typical desulfo-inhibited Mo(V) EPR signal in D₂O, whereas in H₂O the hyperfine splitting was altered but indicated no loss of Mo(V)−proton interactions. The structures of dithionite- and azide-bound ₄-HBCR were solved at 2.1 and 2.2 Å, respectively. Both dithionite and azide bound directly to equatorial ligation sites of the Mo atom. The results obtained revealed further insights into the active site of an unusual member of the XO family of molybdenum cofactor containing enzymes.