Background: Intradiol dioxygenases catalyze the critical ring-cleavage step in the conversion of catecholate derivatives to citric acid cycle intermediates. Catechol 1,2-dioxygenases (1, 2-CTDs) have a rudimentary design structure - a homodimer with one catalytic non-heme ferric ion per monomer, that is (alphaFe(3+))(2). This is in contrast to the archetypical intradiol dioxygenase protocatechuate 3,4-dioxygenase (3,4-PCD), which forms more diverse oligomers, such as (alphabetaFe(3+))(2-12).
Results: The crystal structure of 1,2-CTD from Acinetobacter sp. ADP1 (Ac 1,2-CTD) was solved by single isomorphous replacement and refined to 2.0 A resolution. The structures of the enzyme complexed with catechol and 4-methylcatechol were also determined at resolutions of 1.9 A and 1.8 A, respectively. While the characteristics of the iron ligands are similar, Ac 1,2-CTD differs from 3,4-PCDs in that only one subunit is used to fashion each active-site cavity. In addition, a novel 'helical zipper', consisting of five N-terminal helices from each subunit, forms the molecular dimer axis. Two phospholipids were unexpectedly found to bind within an 8 x 35 A hydrophobic tunnel along this axis.
Conclusions: The helical zipper domain of Ac 1, 2-CTD has no equivalent in other proteins of known structure. Sequence analysis suggests the domain is a common motif in all members of the 1,2-CTD family. Complexes with catechol and 4-methylcatechol are the highest resolution complex structures to date of an intradiol dioxygenase. Furthermore, they confirm several observations seen in 3,4-PCDs, including ligand displacement upon binding exogenous ligands. The structures presented here are the first of a new family of intradiol dioxygenases.