<i>Cis</i>−<i>Trans</i> Isomerization of a Cyclopropyl Radical Trap Catalyzed by Extradiol Catechol Dioxygenases:  Evidence for a Semiquinone Intermediate

Abstract

Substrate analogues cis - and trans -2-(2,3-dihydroxyphenyl)cyclopropane-1-carboxylic acid were synthesized as probes for a semiquinone radical intermediate in the (2,3-dihydroxyphenyl)propionate 1,2-dioxygenase reaction. These analogues were found to be substrates for oxidative cleavage by extradiol dioxygenases from Escherichia coli and Alcaligenes eutrophus . The stereochemistry of the ring fission products was analyzed by conversion to cyclopropane-1,2-dicarboxylic acids using the ensuing hydrolase enzyme MhpC, followed by GCMS analysis. This analysis revealed 85−94% trans product and 6−15% cis products, implying that cis/trans isomerization of the cyclopropyl ring substituents had taken place during the enzymatic conversion. These results are consistent with a reversible opening of the cyclopropyl ring, and hence consistent with the intermediacy of a semiquinone radical intermediate in the extradiol catechol dioxygenase reaction.