Ring-Cleaving Dioxygenases with a Cupin Fold

Abstract

<jats:title>ABSTRACT</jats:title><jats:p>Ring-cleaving dioxygenases catalyze key reactions in the aerobic microbial degradation of aromatic compounds. Many pathways converge to catecholic intermediates, which are subject to<jats:italic>ortho</jats:italic>or<jats:italic>meta</jats:italic>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<jats:sup>II</jats:sup>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<jats:sub>2</jats:sub>, 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<jats:sub>2</jats:sub>is supported by model chemistry.</jats:p>

Identification and Characterization of a Tetramethylpyrazine Catabolic Pathway in Rhodococcus jostii TMP1

“…The analogous rearrangement of double bonds by TpdB may contribute to the destabilization of TTMP, facilitating its oxidation by TpdA. Even though the product of TpdAB-catalyzed TTMP oxidation resembles that of dioxygenases (24), similar products may also be formed via the cleavage of the aromatic ring by flavin monooxygenases, such as 2-methyl-3-hydroxypyridine-5-carboxylic acid oxygenase or 5-pyridoxic acid oxygenase (21). These monooxygenases open the aromatic ring of the substrate and incorporate only one oxygen atom from O 2 , whereas another atom comes from an H 2 O molecule.…”

Monooxygenase Substrates Mimic Flavin to Catalyze Cofactorless Oxygenations

“…In part because of the nature of their substrates, the O 2 -activating mechanisms of many cofactor-independent oxygenases have been proposed to be flavoprotein-like (3)(4)(5)(6)(7)(8). Flavoproteindependent oxidases and oxygenases are able to accelerate the rate of the flavin/O 2 reaction by 10 4 -10 6 -fold over the reaction of free flavin in aqueous solution (9,10) according to principles that are well described (summarized in Scheme 1).…”

Rubber Oxygenase and Latex Clearing Protein Cleave Rubber to Different Products and Use Different Cleavage Mechanisms

“…For instance, Lcps (ϳ40 kDa) and RoxAs (ϳ73 kDa) differ in molecular mass, and Lcp sequences do not have a binding motif for covalent attachment of heme to the protein or other motifs that could be indicative for the binding of metal ions such as a cupin fold (13). This indicates that Lcps could be members of the family of cofactor-independent oxygenases (14,15).…”