Ring-cleaving dioxygenases with a cupin fold

Ring-cleaving dioxygenases catalyze key reactions in the aerobic microbial degradation of aromatic compounds. Many pathways converge to catecholic intermediates, which are subject to ortho or meta cleavage by intradiol or extradiol dioxygenases, respectively. However, a number of degradation pathways proceed via noncatecholic hydroxy-substituted aromatic carboxylic acids like gentisate, salicylate, 1-hydroxy-2-naphthoate, or aminohydroxybenzoates. The ring-cleaving dioxygenases active toward these compounds belong to the cupin superfamily, which is characterized by a six-stranded β-barrel fold and conserved amino acid motifs that provide the 3His or 2- or 3His-1Glu ligand environment of a divalent metal ion. Most cupin-type ring cleavage dioxygenases use an Fe(II) center for catalysis, and the proposed mechanism is very similar to that of the canonical (type I) extradiol dioxygenases. The metal ion is presumed to act as an electron conduit for single electron transfer from the metal-bound substrate anion to O(2), resulting in activation of both substrates to radical species. The family of cupin-type dioxygenases also involves quercetinase (flavonol 2,4-dioxygenase), which opens up two C-C bonds of the heterocyclic ring of quercetin, a wide-spread plant flavonol. Remarkably, bacterial quercetinases are capable of using different divalent metal ions for catalysis, suggesting that the redox properties of the metal are relatively unimportant for the catalytic reaction. The major role of the active-site metal ion could be to correctly position the substrate and to stabilize transition states and intermediates rather than to mediate electron transfer. The tentative hypothesis that quercetinase catalysis involves direct electron transfer from metal-bound flavonolate to O(2) is supported by model chemistry.