Kinetic and spectroscopic studies on the quercetin 2,3-dioxygenase from Bacillus subtilis

Quercetin 2,3-dioxygenase from Bacillus subtilis (QueD) converts the flavonol quercetin and molecular oxygen to 2- protocatechuoylphloroglucinolcarboxylic acid and carbon monoxide. QueD, the only known quercetin 2,3-dioxygenase from a prokaryotic organism, has been described as an Fe²⁺-dependent bicupin dioxygenase. Metal-substituted QueDs were generated by expressing the enzyme in Escherichia coli grown on minimal media in the presence of a number of divalent metals. The addition of Mn ²⁺, Co²⁺, and Cu²⁺ generated active enzymes, but the addition of Zn²⁺, Fe²⁺, and Cd²⁺ did not increase quercetinase activity to any significant level over a control in which no divalent ions were added to the media. The Mn²⁺- and Co ²⁺-containing QueDs were purified, characterized by metal analysis and EPR spectroscopy, and studied by steady-state kinetics. Mn²⁺ was found to be incorporated nearly stoichiometrically to the two cupin motifs. The hyperfine coupling constant of the g = 2 signal in the EPR spectra of the Mn²⁺-containing enzyme showed that the two Mn²⁺ ions are ligated in an octahedral coordination. The turnover number of this enzyme was found to be in the order of 25 s^-1, nearly 40-fold higher than that of the Fe²⁺-containing enzyme and similar in magnitude to that of the Cu²⁺-containing quercertin 2,3-dioxygenase from Aspergillus japonicus. In addition, kinetic and spectroscopic data suggest that the catalytic mechanism of QueD is different from that of the Aspergillus quercetinases but similar to that proposed for the extradiol catechol dioxygenases. This study provides evidence that Mn²⁺ might be the preferred cofactor for this enzyme and identifies QueD as a new member of the manganese dioxygenase family.