Effects of metal ions on the electronic, redox, and catalytic properties of cofactor TTQ of quinoprotien amine dehydrogenases

Model compounds of novel organic cofactor TTQ (tryptophan tryptophylquinone) of quinoprotein methylamine and aromatic amine dehydrogenases have been shown to interact with a series of metal ions in anhydrous organic media. Spectroscopic analyses including UV-vis, NMR, and resonance Raman indicate that the metal ion binds to the TTQ model compounds at their o-quinone moiety, the binding mode of which is similar to that proposed for the interaction between cofactor TTQ and a cationic species in the native enzymes. The binding constants K_ML for the metal ion complexes of TTQ model compounds have been determined from the spectral changes in UV-vis due to the complex formation. Remarkable enhancement of the oxidizing ability of the TTQ model compounds by the complexation with metal ions has been demonstrated as a large positive shift in the one-electron reduction potentials E⁰_red of the complexes as compared to those of the TTQ model compounds in metal free form (e.g. ΔE⁰_red = 1.17 V for the Mg²⁺ complex and ΔE⁰_red = 1.16 V for the SC³⁺ complex). The complexes can oxidize not only benzylamine but also aliphatic amines in anhydrous acetonitrile, whereas no reaction takes place in the absence of the metal ion under otherwise the same experimental conditions. Kinetic studies have revealed that the reaction proceeds via a transamination mechanism involving iminoquinone and product imine intermediates to yield the oxidized products and the reduced TTQ. The ESR spectra of the half-reduced species of TTQ model compounds, i.e. semiquinone radical anions, are detected successfully. The spin distribution derived from the hyperfine structures indicates that the spin is partially delocalized on the indole ring connected at the 4-position of the quinone skeleton. This indicates that the indole group plays an important role as a part of the electron-transfer pathway from the reduced TTQ to a blue copper protein in biological systems.