Homoprotocatechuate 2,3-dioxygenase from Brevibacterium fuscum. A dioxygenase with catalase activity.

Homoprotocatechuate 2,3-dioxygenase (2,3-HPCD) cleaves the aromatic ring of its substrate with insertion of both atoms of oxygen from O2 to form alpha-hydroxy- delta-carboxymethyl cis-muconic semialdehyde. The enzyme has been purified from the Gram-positive bacterium Brevibacterium fuscum and characterized. The enzyme appears to have a range of quaternary structures with predominant components of alpha4 and alpha6 (alpha subunit Mr = 42500 +/- 1500) and binds approximately 1 Fe(II)/subunit. Although the substrate Km values are similar to those of other Fe(II) ring cleaving dioxygenases, the turnover number is lower by 90-97%, and the enzyme exhibits much higher stability to metal chelators and H2O2. The stability to H2O2 is shown to derive from an endogenous catalase activity of 2,3-HPCD (stoichiometry: 2 H2O2 --> 2 H2O + O2) that is novel for dioxygenases. H2O2 is a mixed-type inhibitor of the dioxygenase activity, suggesting that dioxygenase and catalase activities are both catalyzed by the enzyme, but at distinguishable sites. In contrast, catecholic substrates, including homoprotocatechuate and p-nitrocatechol, are nonessential activators of the catalase activity. The plot of 1/vi of catalase activity versus 1/[H2O2] is parabolic in the absence of catecholic substrates and linear in their presence, indicating that these reactions proceed by different mechanisms. A mechanism for catalase activity is proposed in which 2 H2O2 molecules bind simultaneously to the iron to account for the observed parabolic kinetic plot. Electron transfer between the peroxides mediated by the iron would yield 2 H2O and O2. Catecholic substrates are proposed to modify this reaction by excluding one H2O2 from the Fe(II), thereby causing the kinetic plots to appear linear. Electron donation by the catecholic substrates would facilitate O O bond cleavage of H2O2, but outer sphere electron transfer from a second H2O2 in another step would be necessary to complete the reaction. p-Nitrocatechol is shown to bind differently to 2,3-HPCD than to other Fe(II) ring cleavage dioxygenases. Possible explanations for this observation are considered in the context of the proposed catalase and normal dioxygenase mechanisms which may also have bearing on the unique catalase activity and low dioxygenase turnover number of the enzyme.