Topaquinone Cofactor Research Articles

Cellular location influences copper-dependent topaquinone formation for phenylethylamine oxidase expressed in Escherichia coli K-12.

Arthrobacter globiformis amine oxidase produced by Escherichia coli cells grown in copper-depleted media was reported to undergo activation due to formation of its topaquinone cofactor in a copper-dependent autocatalytic reaction. Likewise, a mutated E. coli amine oxidase located in the cytoplasm was reported to form topaquinone autocatalytically in an EDTA-sensitive reaction. Here we show unequivocally that formation of an amine oxidase lacking topaquinone is primarily a consequence of the location of the enzyme in the cytoplasm rather than the level of copper in the growth medium. For E. coli, insertion of copper into apoamine oxidase and subsequent topaquinone formation occur after export of the apoenzyme into the periplasm.

Unusually large isotope effect for the reaction of aromatic amine dehydrogenase. A common feature of quinoproteins?

The reduction of the tryptophan tryptophylquinone [TTQ] prosthetic group of aromatic amine dehydrogenase by dopamine exhibits a deuterium kinetic isotope effect of 8.6–11.7. This effect is partially suppressed in the steady-state. These data support the proposed mechanism for the reductive half-reaction of AADH in which TTQ reduction is linked to proton abstraction from a covalent enzyme-substrate intermediate and in which release of the aldehyde product is partially rate-limiting for the overall reaction. The magnitude of this kinetic isotope effect appears to exceed the semiclassical limit for a proton abstraction. It is also nearly identical to values for the similar reactions catalyzed by methylamine dehydrogenase, which also possesses TTQ, and plasma amine oxidase, which possesses the topaquinone cofactor. These data suggest that these three quinoproteins share a very similar and possible unique mechanism for catalyzing the oxidative deamination of primary amines.

Characterization of the topa quinone cofactor in amine oxidase from Escherichia coli by resonance Raman spectroscopy.

The aromatic amine oxidase from Escherichia coli (ECAO) utilizes Cu(II) and 2,4,5-trihydroxyphenylalanine quinone (TPQ) as cofactors in enzymatic catalysis. The TPQ cofactor is clearly identified by a set of characteristic vibrational modes between 1200 and 1700 cm-1 in the resonance Raman (RR) spectrum of the native enzyme. This is the first report of a RR spectrum for an underivatized TPQ cofactor in an enzyme, showing that it is possible to study changes in the cofactor during the natural reaction cycle. The RR spectrum of ECAO closely matches that of a 2-hydroxy-1,4-benzoquinone model compound, particularly in the deprotonated state in aqueous solution. The principal in-phase C = O symmetric stretching mode of the quinone occurs at 1681 cm-1 in ECAO and at 1666 cm-1 in the model compound and, in both cases, undergoes a downshift of approximately 25 cm-1 upon substitution of one of the carbonyl oxygens with 18O. The overall similarity of the 18O and D shifts in their RR spectra shows that the TPQ cofactor and model compound have the same structure and reactivity, with oxygen exchange occurring at the carbonyl adjacent to the hydroxyl group. Substrate reduction of ECAO under anaerobic conditions leads to a stable semiquinone (lambda max at 442 and 468 nm) with a RR spectrum characteristic of an amine-substituted semiquinone.(ABSTRACT TRUNCATED AT 250 WORDS)

Copper/Topa Quinone-containing Histamine Oxidase from Arthrobacter globiformis: MOLECULAR CLONING AND SEQUENCING, OVERPRODUCTION OF PRECURSOR ENZYME, AND GENERATION OF TOPA QUINONE COFACTOR

The gene coding for histamine oxidase has been cloned and sequenced from a Coryneform bacterium Arthrobacter globiformis. The deduced amino acid sequence consists of 684 residues with a calculated molecular mass of 75,109 daltons and shows a high overall identity (58%) with that of phenethylamine oxidase derived from the same bacterial strain. Although the sequence similarities are rather low when compared with copper amine oxidases from other organisms, the consensus Asn-Tyr-Asp/Glu sequence, in which the middle Tyr is the precursor to the quinone cofactor (the quinone of 2,4,5-trihydroxyphenylalanine, topa) covalently bound to this class of enzymes, is also conserved in the histamine oxidase sequence. To identify the quinone cofactor, an overexpression plasmid has been constructed for the recombinant histamine oxidase. The inactive enzyme purified from the transformed Escherichia coli cells grown in a copper-depleted medium gained maximal activity upon stoichiometric binding of cupric ions. Concomitantly with the enzyme activation by copper, a brownish pink compound was generated in the enzyme, which was identified as the quinone of topa by absorption and resonance Raman spectroscopies of the p-nitrophenylhydrazine-derivatized enzyme and found at the position corresponding to the precursor Tyr (Tyr-402). Therefore, the copper-dependent autoxidation of a specific tyrosyl residue operates on the formation of the topa quinone cofactor in this enzyme, as recently demonstrated with the precursor form of phenethylamine oxidase (Matsuzaki, R., Fukui, T., Sato, H., Ozaki, Y., and Tanizawa, K. (1994) FEBS Lett. 351, 360-364).

Active-site covalent modifications of quinoprotein amine oxidases from Aspergillus niger. Evidence for binding of the mechanism-based inhibitor, 1,4-diamino-2-butyne, to residue Lys356 involved in the catalytic cycle.

Interactions of two distinct quinoprotein amine oxidases from Aspergillus niger, AO-I and AO-II, with active-site covalent modifiers have been investigated. Both enzymes are inhibited similarly by phenylhydrazine or p-nitrophenylhydrazine, forming an orange Schiff base with a carbonyl group of topaquinone cofactor. Modification of histidyl and tyrosyl residues by diethylpyrocarbonate and sulfhydryl groups by 5,5'-dithio-bis-(2-nitrobenzoic acid) and 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole have been described. A substrate analog, 1,4-diamino-2-butyne, was found to function as a mechanism-based inhibitor. It shows both substrate saturation kinetics and time-dependent irreversible inhibition caused by formation of pyrrole bound to the active site. The pyrrole formation was confirmed spectrophotometrically by reaction with Ehrlich's reagent at 525 nm. Inhibition by 1,4-diamino-2-butyne produces a new maximum in the absorption spectra of AO-I and AO-II at 310 nm and 306 nm, respectively. Inactivated AO-I was digested by proteases; labeled peptides were purified by C18 HPLC and sequenced by Edman degradation. Data reveal the evidence that 1,4-diamino-2-butyne reacts with the epsilon-amino group of the Lys356 residue in the sequence Lys-Met-Pro-Asn-Ala of Aspergillus niger amine oxidase AO-I.

Generation of the topa quinone cofactor in bacterial monoamine oxidase by cupric ion-dependent autooxidation of a specific tyrosyl residue

The quinone of 2,4,5-trihydroxyphenylalanine (topa), recently identified as the covalently bound redox cofactor in copper amine oxidases, is encoded by a specific tyrosine codon. To elucidate the mechanism of its formation, the recombinant phenylethylamine oxidase of Arthrobacter globiformis has been overproduced in Escherichia coli and purified in a Cu 2+-deficient form. The inactive precursor enzyme thus obtained was dramatically activated upon incubation with Cu 2+, concomitantly with the formation of the topa quinone at the position corresponding to Tyr 382, occurring in the tetrapeptide sequence highly conserved in this class of enzymes. The topa quinone was produced only under aerobic conditions, but its formation required no external enzymatic systems. These findings demonstrate the Cu 2+-dependent autooxidation of a specific tyrosyl residue to generate the topa quinone cofactor.

Characterization of topa quinone cofactor

Electrochemical characterization of topa quinone (6-hydroxydopa quinone), the organic cofactor of copper-containing amine oxidases, has been performed with the aid of spectroscopy and ab initio energy minimization technique. Topa quinone exhibits a totally reversible cyclic voltammogram at a mercury electrode, which is ascribed to a two-step one-electron conversion between topa quinone and topa via topa semiquinone intermediate. Digital simulation of the reversible wave has afforded the separated estimation of each one-electron redox potential. The acid-dissociation constants of the phenolic hydroxyl groups of topa quinone, topa semiquinone and topa have been evaluated electrochemically and supported by electronic and electron spin resonance spectra. At pH 7.0, topa quinone is acid-dissociated and has two-electron redox potential of 0.079 V vs. NHE coupled with a three-proton transfer. Redox catalytic activity of topa quinone for the oxidation of amines and NADH was not observed over conventional voltammetric time periods. Energy minimization calculation of acid-dissociated topa quinone anion indicates an intermediate electronic structure between the p- and o-quinone types with three almost equivalent carbonyl groups. The lack of the redox catalytic activity of free topa quinone appears to be attributable to the partial contribution of the p-quinone-type structure.

Ruthenium(II) coordination to a model for the topasemiquinone cofactor of amine oxidases. Resolution of1H and99,101Ru EPR hyperfine structure

The paramagnetic complex (L2–˙) RuII(bpy)2, where L2–˙= 5-methyl-2-oxido-1,4-benzosemiquinone and bpy = 2,2′-bipyridine, was studied because the quinonoid form LH is a model of the topa-quinone (TPQ) cofactor of certain copper-dependent amine oxidases. For the metal complex the potential ortho- and para-quinonoid character of the LHn system was probed by high resolution EPR spectroscopy of the semiquinone state which could be obtained through oxidation of the basic catecholate form.