Chemical Simulation of Biogenesis of the 2,4,5-Trihydroxyphenylalanine Quinone Cofactor of Copper Amine Oxidases:  Mechanistic Distinctions Point toward a Unique Role of the Active Site in the o-Quinone Water Addition Step

Abstract

The biogenesis of the 2,4,5-trihydroxyphenylalanine quinone (TPQ) cofactor from tyrosine at the active site of copper amine oxidases is believed to proceed along a pathway that includes a conjugate addition of water to the corresponding o-quinone intermediate, followed by autoxidation of the resulting benzenetriol to the hydroxyquinone cofactor. The water addition reaction has been presumed to occur not only in previous model studies reported for cofactor biogenesis starting with either catechol or o-quinone, but also for generation of the neurotoxin 6-hydroxydopamine during autoxidation of dopamine. We here report the surprising finding that water addition does not occur under solution chemistry conditions. The production of hydroxyquinone from catechol arises instead from reaction of the o-quinone with H 2 O 2 generated during autoxidation of catechol. When starting with the o-quinone itself, production of hydroxyquinone still arises from autoxidation of the catechol, generated either by reduction of the o-quinone by its decomposition products at moderate pH, or by a novel base-mediated redox disproportionation of the o-quinone at high pH. These conclusions are supported by the behavior of independently studied o-quinone intermediates, the observed effects of added catalase, and 18 O-labeling studies utilizing both [ 18 O]H 2 O and [ 18 O]O 2 . The failure to observe water addition to the o-quinone has broad implications for aqueous o-quinone chemistry, and suggests that in TPQ biogenesis, this hydration is being catalyzed at the enzyme active site, possibly by the bound copper.

Metal catalyzed oxidation of tyrosine residues by different oxidation systems of copper/hydrogen peroxide

“…Furthermore, this is the first study to indicate that a very strong neurotoxic agent like TOPA [60][61][62][63][64] could be formed from non-enzymatic oxidation of a biological substrate such as tyrosine by copper MCO. The shift in the retention time of HPLC results as shown in Table 3(a) could result from the formation of o-Topaquinone (2,4,5-trihydroxyphenylalanine quinone) from TOPA [65].…”

Chemically Modified Amino Acids in Copper Proteins That Bind or Activate Dioxygen

“…By analogy with the mechanism in Scheme , this was taken to show that the active nucleophile in this reaction is a free, rather than Cu‐bound, hydroxide ion 32. However, it has recently been demonstrated that, under these conditions, the O atom at the C‐2 position is, in fact, derived by conjugate addition of H 2 O 2 , which is produced as a by‐product of catechol oxidation (Scheme ) 34. The preference for nucleophilic attack of HO 2 − rather than OH − in vitro can be attributed to the lower p K a of H 2 O 2 compared to H 2 O.…”

AFM and UFM surface characterization of rubber‐toughened poly(methyl methacrylate) samples

“…Analysis of the isotopic pattern with Isotopica47 indicates that these patterns result from incorporation of between 0.3 and 0.5 18 O atoms. This analysis is only possible with MALDI‐TOFMS as the carbonyl 18 O will back‐exchange during LC/MS 48, 49. The observation of a single substoichiometric 18 O exchange may prove useful for the identification of protein modifications containing solvent‐exchangeable carbonyl groups even when the mass change of this modification is not known.…”