Model Studies of TTQ-Containing Amine Dehydrogenases

Abstract

The reactions of a TTQ model compound [1, 3-methyl-4-(3'-methylindol-2'-yl)indole-6,7-dione] with several amines have been investigated in organic media to obtain mechanistic information on the action of quinoprotein methylamine and aromatic amine dehydrogenases. It has been found that compound 1 acts as an efficient catalyst for the autorecycling oxidation of benzylamine by molecular oxygen in CH(3)OH. In order to evaluate the oxidation mechanism of amines by 1, the product analyses and kinetic studies have been carried out under anaerobic conditions. In the first stage of the reaction of 1 with amines, 1 is converted into an iminoquinone-type adduct (so-called substrateimine), which was isolated and characterized by using cyclopropylamine as a substrate. The observed NOE of the isolated product indicates clearly that the addition position of the amine is C-6 of the quinone. The molecular orbital calculations suggest that the thermodynamic stability of the carbinolamine intermediate is a major factor to determine such regioselectivity; the C-6 carbinolamine is more stable than the C-7 counterpart by 2.9 kcal/mol. The reactivity of several primary amines and the electronic effect of the p-substituents of benzylamine derivatives in the iminoquinone formation suggest that the addition step of the amine to the quinone is rate-determining. When amines having an acidic alpha-proton such as benzylamine derivatives are employed as substrates, formation of the iminoquinone adduct was followed by rearrangement to the productimine. The kinetic analysis has revealed that this rearrangement consists of noncatalyzed and general base-catalyzed processes. Large kinetic isotope effects of 7.8 and 9.2 were observed for both the noncatalyzed and general base-catalyzed processes, respectively, since these steps involve a proton abstraction from the alpha-position of the substrate. In the reaction with benzhydrylamine, the product imine was isolated quantitatively and well characterized by several spectroscopic data. In the case of benzylamine, the product imine is further converted into the aminophenol derivative by the imine exchange reaction with excess benzylamine. These results indicate clearly that the amine oxidation by compound 1 proceeds via a transamination mechanism as suggested for the enzymatic oxidation of amines by TTQ cofactor.

Red Pyrroloquinoline Alkaloids from the Mushroom Mycena haematopus

“…Similarly, in bacterial methylamine dehydrogenase, which uses tryptophan tryptophylquinone (TTQ) as a cofactor, the iminoquinone moiety of the cofactor is converted into the corresponding quinone moiety during the transamination of methylamine into formaldehyde. [7] Moreover, it is known that TTQ and related quinones are able to react at C-7 with primary amines to yield the corresponding imines. [7] The reaction with amines has also been used for the biomimetic synthesis of some marine pyrroloquinoline alkaloids, such as makaluvamin C. [8] Accordingly, if a tenfold excess of 3-aminopropanol was added to an aqueous solution of 1 or 4, molecular ions corresponding to 11 or 12a, respectively, could be detected in the MS (Scheme 2).…”

“…[7] Moreover, it is known that TTQ and related quinones are able to react at C-7 with primary amines to yield the corresponding imines. [7] The reaction with amines has also been used for the biomimetic synthesis of some marine pyrroloquinoline alkaloids, such as makaluvamin C. [8] Accordingly, if a tenfold excess of 3-aminopropanol was added to an aqueous solution of 1 or 4, molecular ions corresponding to 11 or 12a, respectively, could be detected in the MS (Scheme 2). Therefore, upon injury, the liberated pyrroloquinoline alkaloids also might play a role in the chemical defence of M. haematopus, as they might, for instance, react with the ε-amino group of lysine residues in proteins.…”

Modeling Novel Quinocofactors: An Overview

“…The molecular orbital calculations performed on 28, therefore, indicate that the molecular geometry of TTQ in the enzyme active site is almost adjusted to that having the minimum steric energy of the molecule. On the other hand, reduction of the quinone moiety to the quinol form of the model compound results in the increase of the dihedral angle between the two indole rings ( ϭ 58.5Њ) (110).…”

“…Subsequent to its synthesis and characterization, 28 was shown to act as a very efficient turnover catalyst for the aerobic oxidation of benzylamine in CH 3 OH (103,110,113). In-depth insight on the oxidation mechanism of amines by 28 was then obtained by performing product analyses and kinetic studies under anaerobic conditions (110).…”

“…In-depth insight on the oxidation mechanism of amines by 28 was then obtained by performing product analyses and kinetic studies under anaerobic conditions (110). Detailed 1 H NMR analysis of the iminoquinone (substrate imine) 33 (Fig.…”