Quinaldine 4-oxidase from Arthrohacter sp. Rub1 a, an inducible molybdenum-containing hydroxylase, was purified to homogeneity by an optimized five-step procedure. Molecular oxygen is proposed as physiological electron acceptor. Electrons are also transferred to artificial electron acceptors with Eh > -8 mV. The molybdo-ironhlfur flavoprotein regiospecifically attacks its N-heterocyclic substrates :isoquinoline and phthalazine are hydroxylated adjacent to the N-heteroatom at C1, whereas quinaldine, quinoline, cinnoline and quinazoline are hydroxylated at C4. Additionally, the aromatic aldehydes benzaldehyde, salicylaldehyde, vanillin and cinnamaldehyde are oxidized to the corresponding carboxylic acids, whereas short-chain aliphatic aldehydes are not.Quinaldine 4-oxidase is compared to the two molybdenum-containing hydroxylases quinoline 2-oxidoreductase from Pseudomonas putida 86 [Tshisuaka, B., Kappl, R., Hiittermann, J. with respect to the substrates converted and the electron-acceptor specificities. These dehydrogenases hydroxylate their N-heterocyclic substrates exclusively adjacent to the heteroatom. Whereas the aldehydes tested are scarcely oxidized by quinoline 2-oxidoreductase, isoquinoline 1 -0xidoreductase catalyzes the oxidation of the aromatic aldehydes, although being progressively inhibited. Neither quinoline 2-oxidoreductase nor isoquinoline 1 -0xidoreductase transfer electrons to oxygen. Otherwise, the spectrum of electron acceptors used by quinoline 2-oxidoreductase and quinaldine 4-oxidase is identical. However, isoquinoline I-oxidoreductase differs in its electron-acceptor specificity.Quinaldine 4-oxidase is unusual in its substrate and electron-acceptor specificity. This enzyme is able to function as oxidase or dehydrogenase, it oxidizes aldehydes, and it catalyzes the nucleophilic attack of N-containing heterocyclic compounds at two varying positions depending on the substrate.Keywords: benzodiazines ; benzopyridine derivatives ; aldehyde oxidation ; molybdenum-containing hydroxylase ; quinaldine 4-oxidase.Quinaldine 4-oxidoreductase from the gram-positive bacterium Arthrobacter sp. Rii61 a, an inducible molybdenum-containing hydroxylase, catalyzes the initial step in quinaldine (2-methylquinoline) degradation (Hund et al., 1990) : quinaldine is converted to 1 H-4-oxoquinaldine with concomitant reduction of a suitable electron acceptor. The oxygen atom incorporated into the product derives from water. Quinaldine 4-oxidoreductase Abbreviations. INT, 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazolium chloride; 11 OR, isoquinoline l-oxidoreductase; Q20R, quinoline 2-oxidoreductase; NBT, 2,2'-di-p-nitrophenyl-5,5'-diphenyl-3,3'-~3,3'-dimethoxy-4,4'-diphenylene]-dite~azolium chloride.Enzymes.
For three prokaryotic enzymes of the xanthine oxidase family, namely quinoline 2-oxidoreductase, quinaldine 4-oxidase, and isoquinoline 1-oxidoreductase, the electron transfer centers were investigated by electron paramagnetic resonance. The enzymes are containing a molybdenum-molybdopterin cytosine dinucleotide cofactor, two distinct [2Fe-2S] clusters and, apart from isoquinoline 1-oxidoreductase, a flavin adenine dinucleotide. The latter cofactor yields two different organic radical signals in quinoline 2-oxidoreductase and quinaldine 4-oxidase, typical for the neutral and anionic form, respectively. A "rapid" Mo(V) species is present in all enzymes with small differences in magnetic parameters. From spectra simulation of 95Mo-substituted quinoline 2-oxidoreductase, a deviation of 25 degrees between the maximal g and 95Mo-hyperfine tensor component was derived. The very rapid Mo(V) species was detected in small amounts upon reduction with substrates in quinoline 2-oxidoreductase and quinaldine 4-oxidase, but showed a different kinetic behavior with considerable EPR intensities in isoquinoline 1-oxidoreductase. The FeSI and FeSII centers produced different signals in all three enzymes and, in case of isoquinoline 1-oxidoreductase, revealed a dipolar interaction, from which a maximum distance of 15 A between FeSI and FeSII was estimated. The midpoint potentials of the FeS centers were surprisingly different and determined for FeSI/FeSII with -155/-195 mV in quinoline 2-oxidoreductase, -250/-70 mV in quinaldine 4-oxidase, and +65/+10 mV in isoquinoline 1-oxidoreductase. The slopes of the fitting curves for the Nernst equation are indicative for nonideal behavior. Only in quinoline 2-oxidoreductase, an averaged midpoint potential of the molybdenum redox pairs of about -390 mV could be determined. Both of the other enzymes did not produce Mo(V) signals in redox titration experiments, probably because of direct reduction of Mo(VI) to Mo(IV) in the presence of dithionite.
No abstract
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.