Evidence from mechanistic probes for distinct hydroperoxide rearrangement mechanisms in the intradiol and extradiol catechol dioxygenases.

Three mechanistic probes were used to investigate whether the Criegee rearrangement step of catechol 1,2-dioxygenase (CatA) from Acinetobacter sp. proceeds via a direct 1,2-acyl migration, via homolytic O-O cleavage, or via a benzene oxide-oxepin rearrangement. Incubation of CatA with 3-chloroperoxybenzoic acid led to the formation of a 9:1 mixture of 2-chlorophenol and 3-chlorophenol, via a mechanism involving O-O homolytic cleavage. Incubation of CatA with 2-hydroperoxy-2-methylcyclohexanone led to formation of 5,6-diketoheptan-1-ol, also consistent with an O-O homolytic cleavage mechanism, and not consistent with a direct 1,2-acyl migration. No reaction product was isolated from incubation of CatA with 6-hydroxymethyl-6-methylcyclohexa-2,4-dienone, an analogue that is able to undergo the benzene oxide-oxepin rearrangement, but not able to undergo O-O homolytic cleavage. In contrast, incubation of extradiol dioxygenase MhpB from Escherichia coli with 6-hydroxymethyl-6-methylcyclohexa-2,4-dienone led to the formation of a 2-tropolone ring expansion product, consistent with a direct 1,2-alkenyl migration for extradiol cleavage. Taken together, the results imply different mechanisms for the Criegee rearrangement steps of intradiol and extradiol catechol dioxygenases: a direct 1,2-alkenyl migration for extradiol cleavage and an O-O homolytic cleavage mechanism for intradiol cleavage.