Kinetics of reduction of putidamonooxin by NADH-putidamonooxin oxidoreductase, sodium dithionite and superoxide radicals

Substrate and solvent isotope effects on the fate of the active oxygen species in substrate-modulated reactions of putidamonooxin

Using 4-methoxybenzoate monooxygenase from Pseudomonas putida, the substrate deuterium isotope effect on product formation and the solvent isotope effect on the stoichiometry of oxygen uptake, NADH oxidation, product and/or H2O2 (D2O2) formation for tight couplers, partial uncouplers, and uncouplers as substrates were measured. These studies revealed for the true, intrinsic substrate deuterium isotope effect on the oxygenation reaction a k1H/k2H ratio of < 2.0, derived from the inter- and intramolecular substrate isotope effects. This value favours a concerted oxygenation mechanism of the substrate. Deuterium substitution in a tightly coupling substrate initiated a partial uncoupling of oxygen reduction and substrate oxygenation, with release of H2O2 corresponding to 20% of the overall oxygen uptake. This H2O2 (D2O2) formation (oxidase reaction) almost completely disappeared when the oxygenase function was increased by deuterium substitution in the solvent. The electron transfer from NADH to oxygen, however, was not affected by deuterium substitution in the substrate and/or the solvent. With 4-trifluoromethylbenzoate as uncoupling substrate and D2O as solvent, a reduction (peroxidase reaction) of the active oxygen complex was initiated in consequence of its extended lifetime. These additional two electron-transfer reactions to the active oxygen complex were accompanied by a decrease of both NADH oxidation and oxygen uptake rates. These findings lead to the following conclusions: (a) under tightly coupling conditions the rate-limiting step must be the formation time and lifetime of an active transient intermediate within the ternary complex iron/peroxo/substrate, rather than an oxygenative attack on a suitable C-H bond or electron transfer from NADH to oxygen. Water is released after the monooxygenation reaction; (b) under uncoupling conditions there is competition in the detoxification of the active oxygen complex between its protonation (deuteronation), with formation of H2O2 (D2O2) and its further reduction to water. The additional two electron-transfer reactions onto the active oxygen complex then become rate limiting for the oxygen uptake rate.

Substrate-modulated reactions of putidamonooxin. The nature of the active oxygen species formed and its reaction mechanism

1. 4-Methoxybenzoate monooxygenase is fairly nonspecific. The enzyme system with putidamonooxin as its oxygen-activating component catalyses: (a) O-, S- and N-demethylation; (b) the oxygenation of 4-methylbenzoate and 4-methylmercaptobenzoate, with formation of 4-carboxybenzyl alcohol and 4-carboxyphenylmethyl sulfoxide, respectively, and (c) attack of the aromatic ring of 4- and 3-hydroxybenzoate and 4-aminobenzoate, yielding 3,4-dihydroxybenzoate and 4-amino-3-hydroxybenzoate, respectively. 2. Compounds which are bound by the active sites of putidamonooxin have two essential features in common: a planar aromatic ring system, and a free carboxyl group attached to it. 3. By a substrate-modulated reaction putidamonooxin can be induced to function not only as a monooxygenase but also as a peroxotransferase, i.e. it incorporates both atoms of the activated oxygen molecule into a substrate molecule. This finding supports the hypothesis that a mesomeric state of the iron.peroxo complex, [FeO2]+, is indeed the active oxygenating species of putidamonooxin. 4. The lifetime of the ternary complex consisting of enzyme.iron-peroxo-complex.substrate is significantly prolonged by uncoupling and partially uncoupling substrates, except when it is inactivated by protonation of the iron.peroxo complex by a proton transported into the active sites by a special kind of substrate (i.e. isomers of monoaminobenzoate), with the direct formation of H2O2. 5. The lifetime of the active oxygen species is determined by (a) the rate of the oxygenation reaction in the presence of tight-coupling substrates and (b) the rate of the oxygenation reaction as well as detoxification by the availability of a dissociable proton in the presence of partial uncoupling (and uncoupling) substrate analogues. 6. The rate of the oxygenation reaction depends on the lifetime of the active oxygen species, [FeO2]+, in the presence of partial uncoupling substrates. 7. The iron.peroxo complex attacks an aromatic ring system according to the empiric rules of electrophilic substitution, whereas the attack of aliphatic substituents at the aromatic ring is controlled by steric criteria.

Cloning and sequencing of Pseudomonas genes encoding vanillate demethylase

A 2,598-base-pair (bp) SalI-HincII DNA fragment has been cloned which codes for vanillate demethylase, the enzyme responsible for the demethylation of vanillate (3-methoxy-4-hydroxybenzoate) to protocatechuate (3,4-dihydroxybenzoate). Complementation and insertional inactivation experiments have shown that this fragment carries two genes (vanA and vanB) which are predominantly cotranscribed from a promoter upstream of vanA. Nucleotide sequencing of the SalI-HincII fragment confirmed the genetic data: two open reading frames of 987 and 942 bp were present in the transcribed orientation. These had a very high G + C content in the third base of each codon, which is characteristic of Pseudomonas chromosomal genes. Expression of the genes in Escherichia coli with the T7 RNA polymerase-promoter system gave rise to two polypeptides of 36 and 33 kilodaltons which could be identified by deletion analysis as the products of vanA and vanB, respectively. A search of the protein sequence data bank indicated that the vanB gene product was related to the ferredoxin family.

Protein-protein interactions and antigenic relationships between the components of 4-methoxybenzoate monooxygenase and of benzene 1,2-dioxygenase from Pseudomonas putida

The investigations presented in this paper were performed on two enzyme systems from Pseudomonas putida: (a) 4-methoxybenzoate monooxygenase, consisting of a NADH: putidamonooxin oxidoreductase and putidamonooxin, the oxygen-activating component, and (b) benzene 1,2-dioxygenase, a three-component enzyme system with an NADH: ferredoxin oxidoreductase, functioning together with a plant-type ferredoxin as electron-transport chain, and an oxygen-activating component similar to putidamonooxin in its active sites. The influence of temperature, ionic strength, and pH on the activities of 4-methoxybenzoate monooxygenase and of NADH: putidamonooxin oxidoreductase were investigated. The studies revealed that the activity of 4-methoxybenzoate monooxygenase is determined by the behaviour of the reductase. Spectroscopic measurements showed that the interaction between the two components of 4-methoxybenzoate monooxygenase influences the optical-absorption behaviour of one or both components. As a criterion for the affinity between the two components of 4-methoxybenzoate monooxygenase, the Km value of the reductase for putidamonooxin was determined and found to be 31 +/- 11 microM. Antibodies against both components of 4-methoxybenzoate monooxygenase were obtained from rabbits. The antibodies against putidamonooxin inhibited the O-demethylation reaction (up to 80%) and also the reduction of putidamonooxin by the reductase (up to 40%). The antibodies against putidamonooxin did not interact with the oxygen-activating component of benzene 1,2-dioxygenase. The electron-transport chains of 4-methoxybenzoate monooxygenase and benzene 1,2-dioxygenase could not be replaced by one another without a complete loss of enzyme activity.