Inhibition analysis of hydroxyquinol-cleaving dioxygenases from the chlorophenol-degrading Azotobacter sp. GP1 and Streptomyces rochei 303

Unlike other intradiol-cleaving dioxygenases, hydroxyquinol 1,2-dioxygenase (HQ-DO) from Azotobacter sp. GP1 (for this enzyme an improved purification is described) and 6-chlorohydroxyquinol 1,2-dioxygenase (CHQ-DO) from Stre ptomyces rochei 303 only convert 1,2,4-trihydroxybenzene compounds and do n ot accept catechols as substrate. Inhibition studies revealed the ability o f the two enzymes to interact with hydroxylated aromatic and chloroaromatic compounds. Thus polychlorinated catechols strongly inhibited both enzymes by fully mixed mechanism. Also for both enzymes, chlorophenols were weak or no inhibitors and methylcatechols were found to be less effective inhibito rs than the corresponding chlorocatechols. Nonchlorinated hydroxylated arom atic compounds inhibited HQ-DO but not CHQ-DO. Mono- and dichlorinated hydr oquinones and catechols were competitive inhibitors for HQ-DO but they acte d upon CHQ-DO by two different mechanisms: fully competitive and fully mixe d inhibition. Differences and similarities in inhibition effect and mechani sm are discussed with regard to the interaction of hydroxyaromatic compound s with either enzyme. A hypothesis is presented to explain why hydroxyquino l cleaving enzymes are unable to catalyze the ring fission of catechol.